assign r or s configuration

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Stereochemistry, what is the r and s configuration and why do we need it.

If we name these two alkyl halides based on the IUPAC nomenclature rules, we get the name as 2-chlorobutanbe for both:

However, they don’t look exactly the same as the Cl atom points in different directions – wedge and dash. These molecules are not the same compound – they are non-superimposable mirror images which are known as enantiomers :

The problem with the wedge and dash notation is that it is not a universal approach and quickly loses validity when we simply look at the molecule from the opposite direction:

So, we need an extra piece of information to distinguish enantiomers (and other stereoisomers) by their names properly addressing the stereochemistry as well.

Cahn, Ingold, and Prelog developed a system that, regardless of the direction we are looking at the molecule, will always give the same name ( unlike the wedge and dash notation ).

And that is why this is also known as the absolute Configuration or most commonly referred to as the R and S system.

Let’s see how it works by looking first at the following molecule and we will get back to the 2-chlorobutane after that:

Assigning R and S Configuration: Steps and Rules

To assign the absolute configuration, we need to first locate the carbon(s) with four different groups (atoms) connected to it. These are called chirality centers (chiral center, stereogenic center).

In our molecule, we only have one carbon with four different groups and that is the one with the bromine and we are going to assign the absolute configuration of this chiral center.

For this, you need to follow the steps and rules of the Cahn-Ingold-Prelog system.

Give each atom connected to the chiral center a priority based on its atomic number . The higher the atomic number, the higher the priority.

So, based on this, bromine gets priority one, the oxygen gets priority two, the methyl carbon is the third and the hydrogen is the lowest priority-four:

Draw an arrow starting from priority one and going to priority two and then to priority 3:

If the arrow goes clockwise , like in this case, the absolute configuration is R .

As opposed to this, if the arrow goes counterclockwise then the absolute configuration is S .

As an example, in the following molecule, the priorities go Cl > N > C > H and the counterclockwise direction of the arrow indicates an S absolute configuration:

So, remember: Clockwise – R , Counterclockwise – S .

Now, let’s see what would be the absolute configuration of the enantiomer:

The priorities are still the same since all the groups around the carbon are the same. Starting from the bromine and going to the oxygen and then the carbon, we can see that this time the arrow goes counterclockwise. If the arrow goes counterclockwise , the absolute configuration is S .

And this is another important thing to remember:

All the chirality centers in enantiomers are inverted (every R is S , every S is R in the enantiomer).

So, we discussed the roles of priorities 1, 2, and 3 but what about the lowest priority? We did not mention anything about the arrow going to it. Is it part of the game and how do you use it?

The lowest priority does not affect the direction of the arrow. However, this is very important, and it is a requirement when assigning the R and S configuration, that;

The lowest priority must point away from the viewer .

In other words, the lowest priority must be a dashed line to assign the R and S based on the direction of the arrow as we just did:

With that in mind, how can we assign the absolute configuration of this molecule where the hydrogen is a wedge line pointing towards us?

R and S When the lowest priority is a wedge

You have two options here:

Option one. Turn the molecule 180 o such that the hydroxyl is now pointing towards you and the hydrogen is pointing away. This allows to have the molecule drawn as needed – the lowest priority pointing backward as it is supposed to be for determining the R and S configuration:

Next, assign the priorities; chlorine-number one, oxygen-two, carbon-three and the H as number four.

The arrow goes clockwise , therefore the absolute configuration is R .

The problem with this approach is that sometimes you will work with larger molecules and it is impractical to redraw the entire molecule and swap every single chirality center.

For example, look at biotin with all these hydrogens pointing forward. Not the best option to redraw this molecule changing all the hydrogens and keeping the rest of the molecule as it should be.

This is why we have the second approach which is what everyone normally follows.

Here, you leave the molecule as it is with the hydrogen pointing towards you . Continue as you would normally do by assigning the priorities and drawing the arrow.

The only thing you have to do at the end is change the result from R to S or from S to R .

In this case, the arrow goes counterclockwise but because the hydrogen is pointing towards us, we change the result from S to R .

Of course, either approach should give the same result as this is the same molecule drawn differently.

R and S When Group #4 is not a Wedge or a Dash

There is a third possibility for the position of group 4 and that is when it is neither pointing away or towards you. This means we cannot determine the configuration as easily as if the lowest priority was pointing towards or away from us, and then switch it at the end as we did when group 4 was a wedge line.

As an example, what would be the configuration of this molecule?

For this, there is this simple yet such a useful trick making life a lot easier. Remember it:

Swapping any two groups on a chiral center inverts its absolute configuration ( R to S , S to R ):

Notice that these are different molecules. We are not talking about rotating about an axis or a single bond, in which case the absolute configuration(s) must stay the same. We are actually converting to a different molecule by swapping the groups to make it easier determining the R and S configuration.

Let’s do this on the molecule mentioned above:

The lowest priority group is in the drawing plane , so what we can do is swap it with the one that is pointing away from us (Br). After determining the R and S we switch the result since swapping means changing the absolute configuration and we need to switch back again.

The arrow goes counterclockwise indicating S  configuration and this means in the original molecule it is R.

Alternatively, which is more time-consuming, you can draw the Newman projection of the molecule looking from the angle that places group 4 in the back (pointing away from the viewer):

The lowest priority group is pointing and therefore, the clockwise direction of the arrow indicates an R configuration.

These two articles will be very helpful when dealing with stereochemistry in Newman projectiopns:

• R and S configuration on Newman projections
• Converting Bond-Line, Newman Projection, and Fischer Projections

R and S when Atoms (groups) are the same

Sometimes it happens that two or more atoms connected to the chiral center are the same and it is not possible to assign the priorities right away.

For example, let’s go back to the 2-chlorobutane starting with the wedge chlorine:

Chlorine is the first priority, then we have two carbons and a hydrogen which gets the lowest priority. We need to determine the second priority comparing two carbon atoms and there is a tie since they both (obviously) have the same atomic number.

What do you do? You need to look at the atoms connected to the ones you compare:

The carbon on the left (CH 3 ) is connected to three hydrogens, while the one on the right is connected to two hydrogens and one carbon. This extra carbon gives the second priority to the CH 2 and the CH 3 gets priority three.

The arrow goes clockwise, so this is the ( R )-2-chlorobutane.

And if these atoms were identical as well, we’d have to move farther away from the chiral center and repeat the process until we get to the first point of difference.

It is like layers: the first layer is the atoms connected to the chiral center and you are comparing those and only move to the second layer if there is a tie.

You should never compare any atom of the second layer to a first layer atom regardless of its atomic number. For example, in the following molecule, layer 1 is a tie so we proceed to layer 2 which gives the priority to the carbon connected to the chiral center on the left since it has oxygen connected to it.

So, we do not compare layer 2 and 3 which would’ve given the priority to the carbon with a Br since Br has a higher atomic number than oxygen. Because the oxygen is connected to a carbon closer to the chiral center, it gives the prioirty to that carbon regardless of what is connected to the carbon atoms on the next layer:

Double and triple bonds in the R and S configurations

  Let’s do the R and S for this molecule:

Bromine is the priority and the hydrogen is number four. Carbon “a” is connected to one oxygen and two hydrogens. Carbon “b” is connected to one oxygen and one hydrogen. However, because of the double bond , carbon “b” is treated as if it is connected to two oxygens . The same rule is applied for any other double or triple bond. So, when you see a double bond count it as two single bonds when you see a triple bond cut it as three single bonds .

The arrow goes clockwise, however, the absolute configuration is S , because the hydrogen is pointing towards us.

More Tricks in the R and S configurations

• What if you are comparing two carbons; one connected to three high-atomic number elements, and the other one with two hydrogens and a heteroatom . Which one gets a higher priority?

Let’s see this with this molecule:

Even if only one atom has a higher atomic number than the highest one on the other carbon, the group gets higher priority.

So, one S beats N, O, F because it has a higher atomic number than the others individually.

• Carbon is not the only atom designated by  R  and  S .  In theory, any atom with four different groups is chiral and can be described by the R and S system. For example, phosphorous and sulfur chiral centers are often assigned as  R  or  S .

• Hydrogen is not always the lowest priority. A lone pair of electrons is lower.

• Carbanions are achiral because the lone pair rapidly flips from one side to another unless at very low temperatures:

• R and S do not apply to the nitrogen in amines for the same reason as for carbanions. Quaternary ammonium groups, however, can be chiral.

• The same element can get different priorities based on its isotopes . For example, tritium atom has a higher priority than deuterium: T > D > H

A recent article in Nature ( https://www.nature.com/articles/s41586-023-05719-z ) shows the first synthesis of compounds containing a chiral oxygen which unlike nitrogen, phosphorus, and sulfur does not undergo pyramidal inversion:

The presented triaryl oxonium ions are unique structures as the framework of the rings prevents the inversion of the oxygen lone pair through geometric restriction.

And this should cover most possibilities that I can think of about R and S configurations.

Let me know in the comments if there are any other tips and tricks you would like to be mentioned. 

Practicing R and S is never too much. This 1.5-hour video is a collection of examples taken from the multiple choice quizzes determining the R and S configuration in the context of naming compounds, determining the relationship between compounds, and chemical reactions. 

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Check Also in Stereochemistry:

• The R and S Configuration Practice Problems
• Chirality and Enantiomers
• Diastereomers-Introduction and Practice Problems
• Cis and Trans Stereoisomerism in Alkenes
• E and Z Alkene Configuration with Practice Problems
• Enantiomers Diastereomers the Same or Constitutional Isomers with Practice Problems
• Optical Activity
• Enantiomeric Excess (ee): Percentage of Enantiomers from Specific Rotation with Practice Problems
• Calculating Enantiomeric Excess from Optical Activity
• Symmetry and Chirality. Meso Compounds
• Fischer Projections with Practice Problems
• R and S Configuration in the Fischer Projection
• Converting Bond-Line, Newman Projection, and Fischer Projections
• Resolution of Enantiomers: Separate Enantiomers by Converting to Diastereomers

Stereochemistry Practice Problems Quiz

Identify all the chiral centers in each molecule and determine the absolute configuration as R or S :

Identify all the chiral centers in each Fischer projection  and determine the absolute configuration as R or S :

For each of the following pairs of compounds, determine the relationship between the two compounds: Are they enantiomers or the same compound drawn differently? If you hesitate, determine the absolute configuration of chiral centers (if any:  R or S ).

Determine the absolute configuration of each chiral center in the following Newman projections:

15 thoughts on “How to Determine the R and S configuration”

Thanks for sharing this useful article for finding out the Absolute configuartion

This was such a good article to explain things! A big thanku

Thanks for the kind words

Thanks for sharing this article we got a lot of help thanks❤️❤️❤️

Great to hear that, Ali.

I was wondering how the different layers were given priority, as my teacher did not cover that part but still expected us to know R or S configuration. Thanks for explaining it so well visually!

Glad it was helpful, Aron.

Thank you, it really help and Interesting

R and S do not apply to the nitrogen in amines for the same reason as for carbanions. Quaternary ammonium groups, however, can be chiral ( in the last example ). In this section the lower priority group is in the plane , it should be below the plane . Then will it be a R- configuration? It may be S-configuration , do check please.

Correct, the lower priority is the methyl group and it is in the plane. One option to make it appear in the back is to look at the molecule from the opposite direction of where the methyl is pointing – 10 o’clock. From there, the arrow would look like going clockwise based on the priorities assigned shown in the example, so the absolute configuration is R.

Thanks so much for the in-depth article and thorough treatment of techniques to approach chirality! It seems that our textbook just loves to share the basic principles but fails to help when things get sticky–leaving me to flounder on the trick exam questions! I feel much more confident about my knowledge of chirality after reading through your extremely thoughtful article.

Thank you for your feedback. It does take time to write the articles around the situations where I see students getting stuck in the class, and it is great to see it is helpful.

This is so understandable and straight forward. Thanks for the help, it’s really useful.

Thanks for the good feedback!

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Home / Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

Stereochemistry and Chirality

By James Ashenhurst

• Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

Last updated: November 6th, 2023 |

Assigning  R and  S Configurations With the Cahn-Ingold-Prelog (CIP) Rules

Table of Contents

• Chiral Centers And The Problem Of Naming
• The Cahn-Ingold-Prelog System For Naming Chiral Centers
• Oh No! What Do We Do When #4 Is Not In The Back?
• Assigning R/S When #4 Is In The Front: A Short Cut
• Assigning R/S When #4 Is In The Plane Of The Page
• Breaking Ties With The “Dot Technique”
• Conclusion: Assigning R and S With CIP

(Advanced) References and Further Reading

This post was co-authored by Matt Pierce of Organic Chemistry Solutions .  Ask Matt about scheduling an online tutoring session here .

1. Chiral Centers And The Problem Of Naming

Previously on MOC we’ve described enantiomers : molecules that are non-superimposable mirror images of each other. Perhaps the most memorable example is these “enantiocats”.

Each of these cats is said to be “chiral”: they lack a plane of symmetry.

What causes molecules to have chirality?

The most common source of chirality is a “chiral centre”: typically a  tetrahedral  carbon attached to four different “groups”, or “substituents”.  For each chiral centre there are  two  (and only two!) different ways of arranging the 4 different substituents, which gives rise to two different configurations.  [If you don’t believe there are only two, see Single Swap Rule ].

The purpose of this post is to introduce and describe the nomenclature we use to describe these configurations: the (R)/(S) notation, or Cahn-Ingold Prelog Rules.

Let’s look at a simple example.

Both of these molecules are 1-bromo-1-chloroethane. But they are not  exactly the same molecule, in the same way that your left shoe is not exactly the same as your right. They are non-superimposable mirror images of each other. How do we communicate this difference?

One way would be to describe their physical properties. For example, although these two molecules have the same boiling point, melting point, and share many other physical properties, they rotate plane-polarized light in equal and opposite directions, a property called  optical rotatio ( See Optical Rotation and Optical Activity ) We could use (+)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the right (clockwise, or “dextrorotatory”) and use (-)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the left (counterclockwise, or “levorotatory”).

However this nomenclature suffers from a serious problem. There is no simple correlation between the arrangement of substituents around a chiral centre and the direction in which polarized light is rotated. Another solution is needed.

2. The Cahn Ingold Prelog (CIP) System For Naming Chiral Centers

A solution to this quandary was proposed by Robert Cahn, Chris Ingold, and Vladimir Prelog in 1966. The resulting “CIP” protocol works as follows:

• Prioritize the four groups around a chiral center according to atomic number . The highest atomic number is assigned priority #1, and the lowest atomic number is assigned priority #4.
• Orient the chiral centre such that the #4 priority substituent is pointing away from the viewer. For our purposes, it’s enough for it merely to be attached to a “dashed” bond.
• If the path traced from 1-2-3   is clockwise , the chiral center is assigned ( R ) (from Latin,  rectus )
• If the path traced is counter clockwise , the chiral center is assigned ( S ) (from the Latin  sinister)
• Now we have a better way to describe molecules [A] and [B] shown above. Molecule [A] is named ( R )-1-bromo-1-chloroethane, and molecule [B] is named ( S )-1-bromo-1-chloroethane:

We should reiterate that the designations (R) and (S) bear no relationship to whether a molecule rotates plane-polarized light clockwise (+) or counterclockwise (-). For example the most common naturally occurring configuration of the amino acid alanine is (S), but its optical rotation (in aqueous acid solution) is (+).

3. What About When #4 Is Not In The Back?

That seems simple enough! “Is that it?”, you might ask.

Uh, no. As it happens, there’s a few bumps in the road toward determining (R)/(S) once we get beyond the simple example above.

These “trickier cases” fall into three main categories.

• What if the #4 substituent is not helpfully pointing away from the viewer , as it was in our example above? What if it’s in the “front” (i.e. attached to a “wedged” bond) or, heaven forbid, in the plane of the page?
• Assigning priorities in complex situations . What do we do in situations where a chiral centre has two or more identical atoms attached? In other words, how do we break ties? 
• What do we do if the molecule is drawn a peculiar way , such as in  Fischer or Newman projections ?

We’re not going to be able to fully address all of these issues in this post. But we can certainly deal with #1 and make some headway with #2. We’ll deal with #3 in a future post.

4. Determining R/S When The #4 Substituent Is In Front (i.e. on a “Wedge”): A Short Cut

Let’s first consider the molecule below. The name of this molecule is ( R )-1-fluoroethanol. It is listed below with priorities assigned based on atomic number. In this case F>O>C>H. So F is #1 and H is #4. The tricky part here is that the #4 priority is pointing out of the page (on a “wedge”).

How do we determine (R)/(S) in this case?  There are two ways to do it.

Many instructors will tell you to “simply” rotate the molecule in your head so that the #4 priority is on a dash. Then you can assign R or S in the traditional way. This “simple” advice is not always an easy task for beginners.

Thankfully, it is technically unnecessary to perform such a mental rotation.

Here’s  a way around this. When the #4 priority is on a wedge you can just reverse the rules. So now we have two sets of rules:

If the #4 priority is on a dash :

• Clockwise = R
• Counterclockwise = S

If the #4 priority is on a wedge , reverse the typical rules:

• Clockwise = S
• Counterclockwise = R

R and S can easily be assigned to either picture of the molecule. I still encourage you to use a model kit and learn how to do so, however. Organic chemistry is much easier to understand, and much more beautiful, if you can master how to visualize a tetrahedral carbon atom.

5. Determining R/S When The #4 Group Is In The Plane Of The Page?

What if the #4 priority is in the plane of the paper, for example on a line? In this case it’s impossible to assign R and S in the traditional way. You’d have a 50:50 shot of getting it correct: not good odds. Again, if you can redraw the molecule in your head, then it’s better to just do that. If you can’t do this reliably then you need to learn the “single swap” concept.

Here’s how it works.  Swapping any two groups on a chiral centre will flip the configuration of the chiral centre from R to S (and vice versa). [ We previously talked about the “single swap rule” here ]

Knowing this, we can do a nifty trick.

• Take the #4 substituent (in the plane of the page) and  swap it with the substituent in the back [If the configuration is R, this will switch it to S. If the configuration is S, this will flip it to R. We’ll need to account for this in step #3].
• Redraw the chiral centre, and determine R/S on the new chiral centre which now has group #4 in the back.
•  Whatever result you got,  flip it to its opposite to account for the fact that you did a single swap in step #1.

Here’s an example. Note that here  I first switched #4 and #3, but the main point is to switch two groups so that #4 is out of the plane of the paper.

This method always works, assuming you’ve determined the four priorities accurately. (It also works for cases when #4 is on a wedge).

However, sometimes we’re not in the position of dealing with 4 different atoms attached to a chiral carbon. For instance, it’s possible to have chiral carbons which are attached to 4  carbons . So how do we break the ties in these cases?

6. Determining CIP Priorities: Breaking “Ties” With The “Dot Technique”

The quick answer is to use the “dot technique”. Here’s how it works. Let’s do it for 4-ethyl-4-methyloctane, above.

• Go outward from the chiral centre to each of the surrounding 4 atoms and assign priorities (based on atomic number) to each of these atoms. [Sometimes it’s helpful to draw  dots on each of these atoms.]

3. Compare each list, atom by atom. In our example, since C>H, (C,H,H) takes priority over (H,H,H) so the CH 3 group is assigned priority #4.

4. If there is still a tie, move the dots to the highest ranking atom in the list (i.e. the atom with highest atomic number). The dots are helpful because they help you to keep track of where you are, which can be important in complex examples.

5. In this case, we keep moving along the chain. By the way, if you ever reach the end of the chain without determining a difference, that means that the groups are identical and it isn’t a chiral centre after all.

6. By this point we have enough information to assign (R)/(S). Since priority #4 is in the front, we can also break out our “opposite rule” for good measure:

7. Conclusion: The Cahn-Ingold-Prelog Rules For Assigning R and S Configurations

In the next post we’ll go into some trickier examples with determining R/S, including how to deal with double bonds, rings, and isotopes. In a future post, we’ll get into determining R/S in the Fischer and Newman projections.

Thanks to Matt Pierce for making major contributions to this article.  

Ask Matt about scheduling an online tutoring session  here .

Related Articles

• Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) – The Method of Dots
• How To Determine R and S Configurations On A Fischer Projection
• Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)
• The Single Swap Rule
• Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
• On Cats, Part 4: Enantiocats
• On Cats, Part 6: Stereocenters
• Stereochemistry Practice Problems and Quizzes (MOC Membership)
• How To Draw A Bond Rotation
• Specification of Molecular Chirality R. S. Cahn, Sir Christopher Ingold, V. Prelog Angew. Chem. Int. Ed. 1966, 5 (4), 385-415 DOI: 10.1002/anie.196603851 This is not the first paper on the topic by the authors (see Refs. 4 and 5), but it is a major publication and an attempt to consolidate all the information on chirality in a single place. This paper discusses the various types of chirality possible in chemistry (not just at tetrahedral carbons!) and how to assign chirality unambiguously.
• Basic Principles of the CIP‐System and Proposals for a Revision Dr. Vladlmir Prelog and Prof. Dr. Günter Helmchen Angew. Chem. Int. Ed. 1982 , 21 (8), 567-583 DOI: 10.1002/anie.198205671 An update to Ref. #1, which addresses a lot of edge cases that may come up in complex stereochemical assignments.
• CHIRALITY IN CHEMISTRY Vladimir Prelog Nobel Lecture, 1975 Prelog’s Nobel Lecture. Nobel Lectures are fascinating to read as they give insight into the life of scientists and the path to discovery, which is rarely linear.
• “Absolutely” simple stereochemistry Philip S. Beauchamp Journal of Chemical Education 1984, 61 (8), 666 DOI : 10.1021/ed061p666 This paper describes a simple method for determining stereochemistry of tetrahedral carbons using the hands, suitable for undergraduate students of organic chemistry.
• A simple hand method for Cahn-Ingold-Prelog assignment of R and S configuration to chiral carbons Martin P. Aalund and James A. Pincock Journal of Chemical Education 1986, 63 (7), 600 DOI : 10.1021/ed063p600 A follow-up to the previous paper (Ref #4), but sadly it is incomplete!
• A Web-Based Stereochemistry Tool to Improve Students’ Ability to Draw Newman Projections and Chair Conformations and Assign R/S Labels Nimesh Mistry, Ravi Singh, and Jamie Ridley Journal of Chemical Education 2020, 97 (4), 1157-1161 DOI : 10.1021/acs.jchemed.9b00688 This paper discusses a web-based tool that helps students with visualization of chiral compounds and assignment of stereochemistry as per the Cahn-Ingold-Prelog (CIL) rules. See ref. 34 in the paper for the link.

00 General Chemistry Review

• Lewis Structures
• Ionic and Covalent Bonding
• Chemical Kinetics
• Chemical Equilibria
• Valence Electrons of the First Row Elements
• How Concepts Build Up In Org 1 ("The Pyramid")

01 Bonding, Structure, and Resonance

• How Do We Know Methane (CH4) Is Tetrahedral?
• Hybrid Orbitals and Hybridization
• How To Determine Hybridization: A Shortcut
• Orbital Hybridization And Bond Strengths
• Sigma bonds come in six varieties: Pi bonds come in one
• A Key Skill: How to Calculate Formal Charge
• Partial Charges Give Clues About Electron Flow
• The Four Intermolecular Forces and How They Affect Boiling Points
• 3 Trends That Affect Boiling Points
• How To Use Electronegativity To Determine Electron Density (and why NOT to trust formal charge)
• Introduction to Resonance
• How To Use Curved Arrows To Interchange Resonance Forms
• Evaluating Resonance Forms (1) - The Rule of Least Charges
• How To Find The Best Resonance Structure By Applying Electronegativity
• Evaluating Resonance Structures With Negative Charges
• Evaluating Resonance Structures With Positive Charge
• Exploring Resonance: Pi-Donation
• Exploring Resonance: Pi-acceptors
• In Summary: Evaluating Resonance Structures
• Drawing Resonance Structures: 3 Common Mistakes To Avoid
• How to apply electronegativity and resonance to understand reactivity
• Bond Hybridization Practice
• Structure and Bonding Practice Quizzes
• Resonance Structures Practice

02 Acid Base Reactions

• Introduction to Acid-Base Reactions
• Acid Base Reactions In Organic Chemistry
• The Stronger The Acid, The Weaker The Conjugate Base
• Walkthrough of Acid-Base Reactions (3) - Acidity Trends
• Five Key Factors That Influence Acidity
• Acid-Base Reactions: Introducing Ka and pKa
• How to Use a pKa Table
• The pKa Table Is Your Friend
• A Handy Rule of Thumb for Acid-Base Reactions
• Acid Base Reactions Are Fast
• pKa Values Span 60 Orders Of Magnitude
• How Protonation and Deprotonation Affect Reactivity
• Acid Base Practice Problems

03 Alkanes and Nomenclature

• Meet the (Most Important) Functional Groups
• Condensed Formulas: Deciphering What the Brackets Mean
• Hidden Hydrogens, Hidden Lone Pairs, Hidden Counterions
• Don't Be Futyl, Learn The Butyls
• Primary, Secondary, Tertiary, Quaternary In Organic Chemistry
• Branching, and Its Affect On Melting and Boiling Points
• The Many, Many Ways of Drawing Butane
• Wedge And Dash Convention For Tetrahedral Carbon
• Common Mistakes in Organic Chemistry: Pentavalent Carbon
• Table of Functional Group Priorities for Nomenclature
• Summary Sheet - Alkane Nomenclature
• Organic Chemistry IUPAC Nomenclature Demystified With A Simple Puzzle Piece Approach
• Boiling Point Quizzes
• Organic Chemistry Nomenclature Quizzes

04 Conformations and Cycloalkanes

• Staggered vs Eclipsed Conformations of Ethane
• Conformational Isomers of Propane
• Newman Projection of Butane (and Gauche Conformation)
• Introduction to Cycloalkanes (1)
• Geometric Isomers In Small Rings: Cis And Trans Cycloalkanes
• Calculation of Ring Strain In Cycloalkanes
• Cycloalkanes - Ring Strain In Cyclopropane And Cyclobutane
• Cyclohexane Conformations
• Cyclohexane Chair Conformation: An Aerial Tour
• How To Draw The Cyclohexane Chair Conformation
• The Cyclohexane Chair Flip
• The Cyclohexane Chair Flip - Energy Diagram
• Substituted Cyclohexanes - Axial vs Equatorial
• Ranking The Bulkiness Of Substituents On Cyclohexanes: "A-Values"
• The Ups and Downs of Cyclohexanes
• Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?
• Fused Rings - Cis-Decalin and Trans-Decalin
• Naming Bicyclic Compounds - Fused, Bridged, and Spiro
• Bredt's Rule (And Summary of Cycloalkanes)
• Newman Projection Practice
• Cycloalkanes Practice Problems

05 A Primer On Organic Reactions

• The Most Important Question To Ask When Learning a New Reaction
• The 4 Major Classes of Reactions in Org 1
• Learning New Reactions: How Do The Electrons Move?
• How (and why) electrons flow
• The Third Most Important Question to Ask When Learning A New Reaction
• 7 Factors that stabilize negative charge in organic chemistry
• 7 Factors That Stabilize Positive Charge in Organic Chemistry
• Common Mistakes: Formal Charges Can Mislead
• Nucleophiles and Electrophiles
• Curved Arrows (for reactions)
• Curved Arrows (2): Initial Tails and Final Heads
• Nucleophilicity vs. Basicity
• The Three Classes of Nucleophiles
• What Makes A Good Nucleophile?
• What makes a good leaving group?
• 3 Factors That Stabilize Carbocations
• Equilibrium and Energy Relationships
• What's a Transition State?
• Hammond's Postulate
• Grossman's Rule
• Draw The Ugly Version First
• Learning Organic Chemistry Reactions: A Checklist (PDF)
• Introduction to Addition Reactions
• Introduction to Elimination Reactions
• Introduction to Free Radical Substitution Reactions
• Introduction to Oxidative Cleavage Reactions

06 Free Radical Reactions

• Bond Dissociation Energies = Homolytic Cleavage
• Free Radical Reactions
• 3 Factors That Stabilize Free Radicals
• What Factors Destabilize Free Radicals?
• Bond Strengths And Radical Stability
• Free Radical Initiation: Why Is "Light" Or "Heat" Required?
• Initiation, Propagation, Termination
• Monochlorination Products Of Propane, Pentane, And Other Alkanes
• Selectivity In Free Radical Reactions
• Selectivity in Free Radical Reactions: Bromination vs. Chlorination
• Halogenation At Tiffany's
• Allylic Bromination
• Bonus Topic: Allylic Rearrangements
• In Summary: Free Radicals
• Synthesis (2) - Reactions of Alkanes
• Free Radicals Practice Quizzes

07 Stereochemistry and Chirality

• How To Draw The Enantiomer Of A Chiral Molecule
• Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
• Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
• The Meso Trap
• Optical Rotation, Optical Activity, and Specific Rotation
• Optical Purity and Enantiomeric Excess
• What's a Racemic Mixture?
• Chiral Allenes And Chiral Axes
• Stereochemistry Practice Problems and Quizzes

08 Substitution Reactions

• Introduction to Nucleophilic Substitution Reactions
• Walkthrough of Substitution Reactions (1) - Introduction
• Two Types of Nucleophilic Substitution Reactions
• The SN2 Mechanism
• Why the SN2 Reaction Is Powerful
• The SN1 Mechanism
• The Conjugate Acid Is A Better Leaving Group
• Comparing the SN1 and SN2 Reactions
• Polar Protic? Polar Aprotic? Nonpolar? All About Solvents
• Steric Hindrance is Like a Fat Goalie
• Common Blind Spot: Intramolecular Reactions
• The Conjugate Base is Always a Stronger Nucleophile
• Substitution Practice - SN1
• Substitution Practice - SN2

09 Elimination Reactions

• Elimination Reactions (1): Introduction And The Key Pattern
• Elimination Reactions (2): The Zaitsev Rule
• Elimination Reactions Are Favored By Heat
• Two Elimination Reaction Patterns
• The E1 Reaction
• The E2 Mechanism
• E1 vs E2: Comparing the E1 and E2 Reactions
• Antiperiplanar Relationships: The E2 Reaction and Cyclohexane Rings
• Bulky Bases in Elimination Reactions
• Comparing the E1 vs SN1 Reactions
• Elimination (E1) Reactions With Rearrangements
• E1cB - Elimination (Unimolecular) Conjugate Base
• Elimination (E1) Practice Problems And Solutions
• Elimination (E2) Practice Problems and Solutions

10 Rearrangements

• Introduction to Rearrangement Reactions
• Rearrangement Reactions (1) - Hydride Shifts
• Carbocation Rearrangement Reactions (2) - Alkyl Shifts
• Pinacol Rearrangement
• The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate

11 SN1/SN2/E1/E2 Decision

• Identifying Where Substitution and Elimination Reactions Happen
• Deciding SN1/SN2/E1/E2 (1) - The Substrate
• Deciding SN1/SN2/E1/E2 (2) - The Nucleophile/Base
• Deciding SN1/SN2/E1/E2 (3) - The Solvent
• Deciding SN1/SN2/E1/E2 (4) - The Temperature
• Wrapup: The Quick N' Dirty Guide To SN1/SN2/E1/E2
• Alkyl Halide Reaction Map And Summary
• SN1 SN2 E1 E2 Practice Problems

12 Alkene Reactions

• E and Z Notation For Alkenes (+ Cis/Trans)
• Alkene Stability
• Addition Reactions: Elimination's Opposite
• Selective vs. Specific
• Regioselectivity In Alkene Addition Reactions
• Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition
• Hydrohalogenation of Alkenes and Markovnikov's Rule
• Hydration of Alkenes With Aqueous Acid
• Rearrangements in Alkene Addition Reactions
• Addition Pattern #1: The "Carbocation Pathway"
• Halogenation of Alkenes and Halohydrin Formation
• Oxymercuration Demercuration of Alkenes
• Alkene Addition Pattern #2: The "Three-Membered Ring" Pathway
• Hydroboration Oxidation of Alkenes
• m-CPBA (meta-chloroperoxybenzoic acid)
• OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
• Palladium on Carbon (Pd/C) for Catalytic Hydrogenation
• Cyclopropanation of Alkenes
• Alkene Addition Pattern #3: The "Concerted" Pathway
• A Fourth Alkene Addition Pattern - Free Radical Addition
• Alkene Reactions: Ozonolysis
• Summary: Three Key Families Of Alkene Reaction Mechanisms
• Synthesis (4) - Alkene Reaction Map, Including Alkyl Halide Reactions
• Alkene Reactions Practice Problems

13 Alkyne Reactions

• Acetylides from Alkynes, And Substitution Reactions of Acetylides
• Partial Reduction of Alkynes With Lindlar's Catalyst or Na/NH3 To Obtain Cis or Trans Alkenes
• Hydroboration and Oxymercuration of Alkynes
• Alkyne Reaction Patterns - Hydrohalogenation - Carbocation Pathway
• Alkyne Halogenation: Bromination, Chlorination, and Iodination of Alkynes
• Alkyne Reactions - The "Concerted" Pathway
• Alkenes To Alkynes Via Halogenation And Elimination Reactions
• Alkynes Are A Blank Canvas
• Synthesis (5) - Reactions of Alkynes
• Alkyne Reactions Practice Problems With Answers

14 Alcohols, Epoxides and Ethers

• Alcohols - Nomenclature and Properties
• Alcohols Can Act As Acids Or Bases (And Why It Matters)
• Alcohols - Acidity and Basicity
• The Williamson Ether Synthesis
• Ethers From Alkenes, Tertiary Alkyl Halides and Alkoxymercuration
• Alcohols To Ethers via Acid Catalysis
• Cleavage Of Ethers With Acid
• Epoxides - The Outlier Of The Ether Family
• Opening of Epoxides With Acid
• Epoxide Ring Opening With Base
• Making Alkyl Halides From Alcohols
• Tosylates And Mesylates
• PBr3 and SOCl2
• Elimination Reactions of Alcohols
• Elimination of Alcohols To Alkenes With POCl3
• Alcohol Oxidation: "Strong" and "Weak" Oxidants
• Demystifying The Mechanisms of Alcohol Oxidations
• Protecting Groups For Alcohols
• Thiols And Thioethers
• Calculating the oxidation state of a carbon
• Oxidation and Reduction in Organic Chemistry
• Oxidation Ladders
• SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi
• Alcohol Reactions Roadmap (PDF)
• Alcohol Reaction Practice Problems
• Epoxide Reaction Quizzes
• Oxidation and Reduction Practice Quizzes

15 Organometallics

• What's An Organometallic?
• Formation of Grignard and Organolithium Reagents
• Organometallics Are Strong Bases
• Reactions of Grignard Reagents
• Protecting Groups In Grignard Reactions
• Synthesis Problems Involving Grignard Reagents
• Grignard Reactions And Synthesis (2)
• Organocuprates (Gilman Reagents): How They're Made
• Gilman Reagents (Organocuprates): What They're Used For
• The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
• Reaction Map: Reactions of Organometallics
• Grignard Practice Problems

16 Spectroscopy

• Degrees of Unsaturation (or IHD, Index of Hydrogen Deficiency)
• Conjugation And Color (+ How Bleach Works)
• Introduction To UV-Vis Spectroscopy
• UV-Vis Spectroscopy: Absorbance of Carbonyls
• UV-Vis Spectroscopy: Practice Questions
• Bond Vibrations, Infrared Spectroscopy, and the "Ball and Spring" Model
• Infrared Spectroscopy: A Quick Primer On Interpreting Spectra
• IR Spectroscopy: 4 Practice Problems
• 1H NMR: How Many Signals?
• Homotopic, Enantiotopic, Diastereotopic
• Diastereotopic Protons in 1H NMR Spectroscopy: Examples
• C13 NMR - How Many Signals
• Liquid Gold: Pheromones In Doe Urine
• Natural Product Isolation (1) - Extraction
• Natural Product Isolation (2) - Purification Techniques, An Overview
• Structure Determination Case Study: Deer Tarsal Gland Pheromone

17 Dienes and MO Theory

• What To Expect In Organic Chemistry 2
• Are these molecules conjugated?
• Conjugation And Resonance In Organic Chemistry
• Bonding And Antibonding Pi Orbitals
• Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion
• Pi Molecular Orbitals of Butadiene
• Reactions of Dienes: 1,2 and 1,4 Addition
• Thermodynamic and Kinetic Products
• More On 1,2 and 1,4 Additions To Dienes
• s-cis and s-trans
• The Diels-Alder Reaction
• Cyclic Dienes and Dienophiles in the Diels-Alder Reaction
• Stereochemistry of the Diels-Alder Reaction
• Exo vs Endo Products In The Diels Alder: How To Tell Them Apart
• HOMO and LUMO In the Diels Alder Reaction
• Why Are Endo vs Exo Products Favored in the Diels-Alder Reaction?
• Diels-Alder Reaction: Kinetic and Thermodynamic Control
• The Retro Diels-Alder Reaction
• The Intramolecular Diels Alder Reaction
• Regiochemistry In The Diels-Alder Reaction
• The Cope and Claisen Rearrangements
• Electrocyclic Reactions
• Electrocyclic Ring Opening And Closure (2) - Six (or Eight) Pi Electrons
• Diels Alder Practice Problems
• Molecular Orbital Theory Practice

18 Aromaticity

• Introduction To Aromaticity
• Rules For Aromaticity
• Huckel's Rule: What Does 4n+2 Mean?
• Aromatic, Non-Aromatic, or Antiaromatic? Some Practice Problems
• Antiaromatic Compounds and Antiaromaticity
• The Pi Molecular Orbitals of Benzene
• The Pi Molecular Orbitals of Cyclobutadiene
• Frost Circles
• Aromaticity Practice Quizzes

19 Reactions of Aromatic Molecules

• Electrophilic Aromatic Substitution: Introduction
• Activating and Deactivating Groups In Electrophilic Aromatic Substitution
• Electrophilic Aromatic Substitution - The Mechanism
• Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
• Understanding Ortho, Para, and Meta Directors
• Why are halogens ortho- para- directors?
• Disubstituted Benzenes: The Strongest Electron-Donor "Wins"
• Electrophilic Aromatic Substitutions (1) - Halogenation of Benzene
• Electrophilic Aromatic Substitutions (2) - Nitration and Sulfonation
• EAS Reactions (3) - Friedel-Crafts Acylation and Friedel-Crafts Alkylation
• Intramolecular Friedel-Crafts Reactions
• Nucleophilic Aromatic Substitution (NAS)
• Nucleophilic Aromatic Substitution (2) - The Benzyne Mechanism
• Reactions on the "Benzylic" Carbon: Bromination And Oxidation
• The Wolff-Kishner, Clemmensen, And Other Carbonyl Reductions
• More Reactions on the Aromatic Sidechain: Reduction of Nitro Groups and the Baeyer Villiger
• Aromatic Synthesis (1) - "Order Of Operations"
• Synthesis of Benzene Derivatives (2) - Polarity Reversal
• Aromatic Synthesis (3) - Sulfonyl Blocking Groups
• Birch Reduction
• Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds
• Aromatic Reactions and Synthesis Practice
• Electrophilic Aromatic Substitution Practice Problems

20 Aldehydes and Ketones

• What's The Alpha Carbon In Carbonyl Compounds?
• Nucleophilic Addition To Carbonyls
• Aldehydes and Ketones: 14 Reactions With The Same Mechanism
• Sodium Borohydride (NaBH4) Reduction of Aldehydes and Ketones
• Grignard Reagents For Addition To Aldehydes and Ketones
• Wittig Reaction
• Hydrates, Hemiacetals, and Acetals
• Imines - Properties, Formation, Reactions, and Mechanisms
• All About Enamines
• Breaking Down Carbonyl Reaction Mechanisms: Reactions of Anionic Nucleophiles (Part 2)
• Aldehydes Ketones Reaction Practice

21 Carboxylic Acid Derivatives

• Nucleophilic Acyl Substitution (With Negatively Charged Nucleophiles)
• Addition-Elimination Mechanisms With Neutral Nucleophiles (Including Acid Catalysis)
• Basic Hydrolysis of Esters - Saponification
• Transesterification
• Proton Transfer
• Fischer Esterification - Carboxylic Acid to Ester Under Acidic Conditions
• Lithium Aluminum Hydride (LiAlH4) For Reduction of Carboxylic Acid Derivatives
• LiAlH[Ot-Bu]3 For The Reduction of Acid Halides To Aldehydes
• Di-isobutyl Aluminum Hydride (DIBAL) For The Partial Reduction of Esters and Nitriles
• Amide Hydrolysis
• Thionyl Chloride (SOCl2)
• Diazomethane (CH2N2)
• Carbonyl Chemistry: Learn Six Mechanisms For the Price Of One
• Making Music With Mechanisms (PADPED)
• Carboxylic Acid Derivatives Practice Questions

22 Enols and Enolates

• Keto-Enol Tautomerism
• Enolates - Formation, Stability, and Simple Reactions
• Kinetic Versus Thermodynamic Enolates
• Aldol Addition and Condensation Reactions
• Reactions of Enols - Acid-Catalyzed Aldol, Halogenation, and Mannich Reactions
• Claisen Condensation and Dieckmann Condensation
• Decarboxylation
• The Malonic Ester and Acetoacetic Ester Synthesis
• The Michael Addition Reaction and Conjugate Addition
• The Robinson Annulation
• Haloform Reaction
• The Hell–Volhard–Zelinsky Reaction
• Enols and Enolates Practice Quizzes
• The Amide Functional Group: Properties, Synthesis, and Nomenclature
• Basicity of Amines And pKaH
• 5 Key Basicity Trends of Amines
• The Mesomeric Effect And Aromatic Amines
• Nucleophilicity of Amines
• Alkylation of Amines (Sucks!)
• Reductive Amination
• The Gabriel Synthesis
• Some Reactions of Azides
• The Hofmann Elimination
• The Hofmann and Curtius Rearrangements
• The Cope Elimination
• Protecting Groups for Amines - Carbamates
• The Strecker Synthesis of Amino Acids
• Introduction to Peptide Synthesis
• Reactions of Diazonium Salts: Sandmeyer and Related Reactions
• Amine Practice Questions

24 Carbohydrates

• D and L Notation For Sugars
• Pyranoses and Furanoses: Ring-Chain Tautomerism In Sugars
• What is Mutarotation?
• Reducing Sugars
• The Big Damn Post Of Carbohydrate-Related Chemistry Definitions
• The Haworth Projection
• Converting a Fischer Projection To A Haworth (And Vice Versa)
• Reactions of Sugars: Glycosylation and Protection
• The Ruff Degradation and Kiliani-Fischer Synthesis
• Isoelectric Points of Amino Acids (and How To Calculate Them)
• Carbohydrates Practice
• Amino Acid Quizzes

25 Fun and Miscellaneous

• Organic Chemistry GIFS - Resonance Forms
• Organic Chemistry and the New MCAT
• A Gallery of Some Interesting Molecules From Nature
• The Organic Chemistry Behind "The Pill"
• Maybe they should call them, "Formal Wins" ?
• Intramolecular Reactions of Alcohols and Ethers
• Planning Organic Synthesis With "Reaction Maps"
• Organic Chemistry Is Shit
• The 8 Types of Arrows In Organic Chemistry, Explained
• The Most Annoying Exceptions in Org 1 (Part 1)
• The Most Annoying Exceptions in Org 1 (Part 2)
• Reproducibility In Organic Chemistry
• Screw Organic Chemistry, I'm Just Going To Write About Cats
• On Cats, Part 1: Conformations and Configurations
• On Cats, Part 2: Cat Line Diagrams
• The Marriage May Be Bad, But the Divorce Still Costs Money
• Why Do Organic Chemists Use Kilocalories?
• What Holds The Nucleus Together?
• 9 Nomenclature Conventions To Know
• How Reactions Are Like Music

Comment section

22 thoughts on “ introduction to assigning (r) and (s): the cahn-ingold-prelog rules ”.

In a chiral molecule, two groups are attached to it with the normal line bond ,the third is shown through a wedge and hydrogen is not shown..can I conclude that the hydrogen is a dash ?

Yes! The dashed hydrogen is implied!

Thanks. Move the dots. Could not find this before.

Glad you found it useful James!

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During my studies for 11th grade and 12th grade, we had a brilliant Organic Chemistry teacher who taught the concepts beautifully. In addition, I had a passion (more of a “study crush”) on Chemistry in general and Organic Chemistry in particular. To such an extent that this topic of R and S enantiomers is still ingrained in memory. Though I am in a completely different area now of Machine Learning and Analytics in the Healthcare space in Industry, primarily a Software Engg job. Out of sheer curiosity, I googled “Chirality Detection Machine Learning” and voila !! such cool, intereesting papers I came across where they combine Bayesian Learning and Convolutional Neural Networks (Advanced ML Theory) to detect chirality in Nanoparticles. So application of ML in cutting edge Physics. Amazing stuff :!

Most people don’t learn chirality until 2nd year university in north america, so you are ahead of the curve

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I just only want to know the CIP system of Nomenclature

Man this website proved to be a boon for me in quarantine…keep it up🔥🔥 The best content of organic chem I could get in such an incredible way

Thank you so much!! :) This was a great refresher on chirality and you explained it in such a straightforward manner. Appreciate it!

What to do if the compound is not denoted using the dash and wedge but simple bond line notation or expanded notation ?

Can you show an example? There has to be some kind of indicator. If all four bonds from the chiral center are shown as simple line notation there is no way to tell if it is R or S. It’s ambiguous.

Thank you so much, you are a true life saver???

I have a lot of trouble rotating molecules in my head, so these tips feel like magic to me!!! Thank you soooo much :DDDD Btw I also go to McGill!

The molecule used to explain the dot technique is labelled as 3-ethyl-3-methyloctane, however shouldn’t the molecule be named as 4-ethyl-4-methyloctane? The branches are on the fourth carbon…

Shoot. You are right. Thanks for the catch. Fixed!

Thank You so much :)

Thanks!! You saved my org chem exam

I was having trouble with this when 4 was in the plane of the page. This technique is so easy. Thanks

Kindly take my work into consideration in your website.

Abstract:- “The Keval’s Method” is developed for the determination of absolute configuration of a chiral carbon in a Fisher Projection and Wedge-Dash Projection just by simple calculations. This method is easily applicable over both Fisher as well as Wedge-Dash Projection. Various methods for determining absolute configuration have been developed and published till now, some of them used fingers and hands and other used exchanging elements. “Keval’s Method” is the first method in which a chiral carbon is taken to be an origin and the branches to axes, also it is purely calculation based method where absolute configuration is found based on the nature of calculated answer without using fingers and hands and also without exchanging elements.

Your’ Thankfully Keval Chetanbhai Purohit 5th-Computer Engineering, Vishwakarma Government Engineering College, Mo- 7226953531

Thank you very much, I now understand the R/S, its not easy to rotate a compound in your mind……

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How to Determine R and S Configuration (Stereochemistry)

What is R or S Configurations?

R and S (rectus and sinister respectively in Latin) are terms that describe the stereochemistry configuration of organic molecules. Stereochemistry is important because it changes the shape of a molecule, and even the reactions that it participates in. In order to effectively communicate a version (or isomer) of molecule, chemists invented a system for specifying these differences. In this article, we will show you how to classify chiral molecules as R or S.

Recommended Reading

• Enantiomers
• Molecular Weight
• Organic Naming

Looking For Chiral Centers

In order for a molecule to be chiral, it needs to have a chiral center. A chiral center in an organic molecule is a carbon atom with four different substituents. Keep in mind that when structure are drawn, hydrogen atoms are not always shown; you will sometimes need to infer them. See if you can find all the chiral centers in the atoms below:

Assigning Priority to Substituents

Now that we have found our chiral center(s), we need to assign the configuration to each. First, we need to assign priority to the four different substituents at each of the chiral centers. This is achievable using the Cahn-Ingold-Prelog system . The steps are:

• Starting at the chiral center, the atom with the highest atomic mass bonded to the chiral center has the highest priority.
• If two atoms are tied for priority, look at the next atom away from the chiral center. Determine priority of the substituent based on this atom. Always move towards the heaviest atom if possible.
• Repeat until a difference in atoms is observed.
• A double bond or a triple bond can be treated as a bond to two or three separate carbon atoms for this purpose.

Lets try some examples:

Determining the Configuration

Unfortunately, this can be the tricky step as it requires you to think in 3D. We need to rotate the molecule in our heads so that the lowest priority substituent is facing away from us (into the page) and so the first priority substituent is pointed up. However, its not that bad; let’s practice with a few examples:

Now, observe the position of the 1st, 2nd, and 3rd priority substituents. If they appear in order clockwise, you have the R isomer of the molecule. If they appear in order counter clockwise, you have the S isomer of the molecule.

Using Dashed and Wedged Bonds in Determining R/S Configuration

Wedged and Dashed bonds give information about the 3D shape of a molecule, but it is important to understand how these wedged and hashed bonds translate into a R or S configuration. Wedged (solid) bonds indicated that a bond is pointing towards you (out of the page), while a dashed bond indicates a bond that is pointing away from you (into the page).

If the lowest priority substituent has a dashed bond, you are in luck, it is already pointing to the back. All you need to do is rotate the molecule so that the first priority substituent is pointing up, and then determine whether the substituents count up clockwise or counterclockwise to get the stereochemistry.

If the lowest priority substituent has a wedged bond, you will need to rotate the molecule so that the wedged bond is now pointing into the page (like a dashed bond). Then assess the stereochemistry like normal.

If any other bond is a dashed or wedged bonds, be cognizant of which way the bond will point when you rotate the molecule so that the lowest priority substituent points back. Then, number the substituents like normal and assign the R/S configuration.

For More Help, Watch Our Interactive Video Explaining R and S Configurations!

R/s configuration practice problems.

Try the following Examples:

Answers: R,R,S

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5.2 Absolute Configurations: How to Assign R and S

Chad's organic chemistry videos.

• 1.1 Lewis Structures
• 1.2 Formal Charges
• 1.3 Valence Bond Theory and Hybridization
• 1.4 Molecular Orbital Theory
• 1.5 Polarity
• 1.6 Intermolecular Forces
• 2.1 Condensed Structures
• 2.2 Bond Line Structures
• 2.3 Functional Groups
• 2.4 Resonance
• 3.1 Introduction to Acids and Bases
• 3.2 Ranking Acids and Bases
• 4.1 Naming Alkanes
• 4.2 Naming Complex Substituents
• 4.3 Naming Bicyclic Compounds
• 4.4 Drawing Constitutional Isomers
• 4.5 Newman Projections
• 4.6 Cycloalkanes and Cyclohexane Chair Conformations
• 5.1 Overview of Isomerism
• 5.2 Absolute Configurations | How to Assign R and S
• 5.3 Molecules with Multiple Chiral Centers
• 5.4 Fischer Projections
• 5.5 Isomeric Relationships Between Molecules
• 5.6 Amine Inversion and Chiral Molecules Without Chiral Centers
• 5.7 Optical Activity
• 6.1 Reaction Enthalpies and Bond Dissociation Energies
• 6.2 Entropy, Gibbs Free Energy, and the Equilibrium Constant
• 6.3 The Kinetics of Organic Reactions
• 6.4 Nucleophiles, Electrophiles, and Intermediates
• 6.5 Reaction Mechanisms and Curved Arrow Pushing
• 7.1 SN2 Reactions
• 7.2 SN1 Reactions
• 7.3 SN1 vs SN2
• 7.4 Introduction to Elimination Reactions [Zaitsev’s Rule and the Stability of Alkenes]
• 7.5 E2 Reactions
• 7.6 E1 Reactions and E1 vs E2
• 7.7 Distinguishing Between SN1/SN2/E1/E2
• 8.0 Naming Alkenes
• 8.1 Introduction to Alkene Addition Reactions
• 8.2 Hydrohalogenation
• 8.3 Hydration of Alkenes
• 8.4 Addition of Alcohols
• 8.5 Catalytic Hydrogenation
• 8.6 Halogenation of Alkenes and Halohydrin Formation
• 8.7 Epoxidation, Anti Dihydroxylation, and Syn Dihydroxylation
• 8.8 Predicting the Products of Alkene Addition Reactions
• 8.9 Oxidative Cleavage Ozonolysis and Permanganate Cleavage
• 9.1 Naming Alkynes
• 9.2 Acidity of Alkynes
• 9.3 Synthesis of Alkynes
• 9.4 Reduction of Alkynes
• 9.5 Hydrohalogenation of Alkynes
• 9.6 Halogenation of Alkynes
• 9.7 Hydration of Alkynes
• 9.8 Ozonolysis of Alkynes
• 9.9 Alkylation of Acetylide Ions
• 10.1 Free Radical Halogenation
• 10.2 The Free Radical Halogenation Mechanism
• 10.3 Allylic and Benzylic Bromination with NBS
• 10.4 Addition of HBr and Peroxide
• 11.1 Introduction to Organic Synthesis
• 11.2 Common Patterns in Organic Synthesis (Involving Alkenes)
• 11.3 Common Patterns in Organic Synthesis (involving Alkynes)
• 12.1 Naming Alcohols
• 12.2 Properties of Alcohols
• 12.3 Synthesis of Alcohols
• 12.4 Grignard Reagents
• 12.5 Protecting Alcohols
• 12.6 Substitution Reactions of Alcohols
• 12.7 Elimination Reactions (Dehydration) of Alcohols
• 12.8 Oxidation of Alcohols
• 12.9 Organic Synthesis with Alcohols
• 13.1 Naming Ethers
• 13.2 Synthesis of Ethers
• 13.3 Reactions of Ethers
• 13.4 Nomenclature of Epoxides
• 13.5 Synthesis of Epoxides
• 13.6 Ring Opening of Epoxides
• 13.7 Nomenclature, Synthesis, and Reactions of Thiols
• 13.8 Nomenclature, Synthesis, and Reactions of Sulfides
• 13.9 Organic Synthesis with Ethers and Epoxides
• 14.1 Introduction to IR Spectroscopy
• 14.2a IR Spectra of Carbonyl Compounds
• 14.2b The Effect of Conjugation on the Carbonyl Stretching Frequency
• 14.3 Interpreting More IR Spectra
• 14.4 Introduction to Mass Spectrometry
• 14.5 Isotope Effects in Mass Spectrometry
• 14.6a Fragmentation Patterns of Alkanes, Alkenes, and Aromatic Compounds
• 14.6b Fragmentation Patterns of Alkyl Halides, Alcohols, and Amines
• 14.6c Fragmentation Patterns of Ketones and Aldehydes
• 15.1 Introduction to NMR
• 15.2 The Number of Signals in C 13 NMR
• 15.3 The Number of Signals in Proton NMR
• 15.4 Homotopic vs Enantiotopic vs Diastereotopic
• 15.5a The Chemical Shift in C 13 and Proton NMR
• 15.5b The Integration or Area Under a Signal in Proton NMR
• 15.5c The Splitting or Multiplicity in Proton NMR
• 15.6a Interpreting NMR Example 1
• 15.6b Interpreting NMR Example 2
• 15.6c Interpreting NMR Example 3
• 15.6d Structural Determination From All Spectra Example 4
• 15.6e Structural Determination From All Spectra Example 5
• 15.7 Complex Splitting
• 16.1 Introduction to Conjugated Systems and Heats of Hydrogenation
• 16.2a Pi Molecular Orbitals 1,3 Butadiene
• 16.2b Pi Molecular Orbitals the Allyl System
• 16.2c Pi Molecular Orbitals 1,3,5 Hexatriene
• 16.3 UV Vis Spectroscopy
• 16.4 Electrophilic Addition to Conjugated Dienes
• 16.5 Diels Alder Reactions
• 16.6 Cycloaddition Reactions
• 16.7 Electrocyclic Reactions
• 16.8 Sigmatropic Rearrangements
• 17.1 Naming Benzenes
• 17.2 Aromatic vs Nonaromatic vs Antiaromatic
• 17.3 The Effects of Aromaticity on Reactivity
• 17.4 Pi Molecular Orbitals of Benzene
• 18.1 Electrophilic Aromatic Substitution
• 18.2 Friedel Crafts Alkylation and Acylation
• 18.3 Activating and Deactivating Groups | Ortho/Para vs Meta Directors
• 18.4 Catalytic Hydrogenation and the Birch Reduction
• 18.5 Side-Chain Reactions of Benzenes
• 18.6 Nucleophilic Aromatic Substitution
• 18.7 Retrosynthesis with Aromatic Compounds
• 19.1 Nomenclature of Ketones and Aldehydes
• 19.2 Synthesis of Ketones and Aldehydes
• 19.3 Introduction to Nucleophilic Addition Reactions of Ketones and Aldehydes
• 19.4a Formation of Hemiacetals and Acetals (Addition of Alcohols)
• 19.4b Cyclic Acetals as Protecting Groups
• 19.5 Formation of Imines and Enamines (Addition of Amines)
• 19.6 Reduction of Aldehydes and Ketones
• 19.7a Addition of Carbon Nucleophiles (Acetylide Ions, Grignard Reagents,etc.)
• 19.7b The Wittig Reaction
• 19.8 Baeyer Villiger Oxidation
• 19.9 Retrosynthesis with Aldehydes and Ketones
• 20.1 Naming Carboxylic Acids and Carboxylic Acid Derivatives
• 20.2 Nucleophilic Acyl Substitution
• 20.3 The Mechanisms of Nucleophilic Acyl Substitution
• 20.4 Reaction with Organometallic Reagents
• 20.5 Hydride Reduction Reactions
• 20.6 Synthesis and Reactions of Acid Halides
• 20.7 Synthesis and Reactions of Acid Anhydrides
• 20.8 Synthesis and Reactions of Esters
• 20.9 Properties, Synthesis, and Reactions of Carboxylic Acids
• 20.10 Synthesis and Reactions of Amides
• 20.11 Synthesis and Reactions of Nitriles
• 20.12 Retrosynthesis with Carboxylic Acids / Acid Derivatives
• 21.1 Acidity of the Alpha Hydrogen
• 21.2 Mechanisms of Alpha Substitution Reactions
• 21.3 Alpha Halogenation
• 21.4 Alpha Alkylation (including the Stork Reaction)
• 21.5 Aldol Reactions
• 21.6 Claisen Condensation Reactions
• 21.7 The Malonic Ester Synthesis and the Acetoacetic Ester Synthesis
• 21.8 Michael Reactions
• 21.9 The Robinson Annulation
• 21.10 Retrosynthesis with Enolates and Enols
• 22.1 Naming Amines
• 22.2 Basicity of Amines
• 22.3 Synthesis of Amines
• 22.4 Hofmann Elimination and Cope Elimination
• 22.5 Sandmeyer Reactions
• 22.6 EAS Reactions with Nitrogen Heterocycles
• 22.7 Retrosynthesis with Amines

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5.1 Overview of Isomers

5.3 Molecules with...

Forgot what a chiral center is or how to identify them? Check out 5.1 Overview of Isomerism and Stereoisomers .

Trying to assign R and S with Fischer Projections? Check out 5.4 Fischer Projections .

The Cahn-Ingold-Prelog Rules

Assigning r and s.

The Cahn-Ingold-Prelog system is a set of rules for assigning R and S (absolute configurations) to chiral centers.  The idea is to assign each of the 4 atoms attached to a chiral center a priority, 1 through 4, based primarily on atomic number.  Once the priorities are assigned the spacial arrangement of these 4 atoms will be in one of two configurations: R or S.  The complete rules are summarized in the table above.

To specifically determine the spacial arrangement the rules state that when the #4 priority group is attached by a dashed bond (facing away from you) then a right-handed turn (clockwise) indicates the R configuration, and a left-handed turn (counter-clockwise) indicates the S configuration.

If the #4 priority group is attached by a dashed bond then you're set.  But what if it's not.  Below we'll show examples of how to assign R and S

1) When the #4 priority group is attached by a dashed bond

2) When the #4 priority group is attached by a wedged bond

3) When the #4 priority group is attached by a bond in the plane

In the example above, the chiral centers are attached to 4 different atoms.  This is the easiest possible scenario for assigning priorities as priorities are simply determined by atomic number.  The following shows the 4 atoms arranged in decreasing order of atomic number:

Br > Cl > C > H

When the #4 Priority Group is Attached by a Dashed Bond

This is the easiest of the three scenarios.  When the #4 priority group is attached by a dashed bond (facing away from you) then a right-handed turn (clockwise) indicates the R configuration, and a left-handed turn (counter-clockwise) indicates the S configuration.  In this example, as we move from priority #1 to #2 to #3 we make a right-handed turn which indicates the molecule is in the R configuration.

When the #4 Priority Group is Attached by a Wedged Bond

When the #4 priority group is attached by a wedged bond (facing toward you) you are looking at the molecule from exactly the opposite perspective described by the Cahn-Ingold-Prelog rules.  A right-handed turn from this opposite perspective would be a left-handed turn if looked at from the correct perspective, and a left-handed turn from this opposite perspective would be a right-handed turn from the correct perspective.  The simple solution is to move from priority #1 to #2 to #3 and make your 'turn,' and to just know that the molecule is in the opposite configuration as to what you would determine if the #4 priority group had had a dashed bond.  In this example, as we move from priority #1 to #2 to #3 we make a right-handed turn (which would normally mean R) which indicates the molecule is in the S configuration.

When the #4 Priority Group is Attached by a Bond in the Plane

When the #4 priority group is attached by a bond in the plane you should have yourself a good cry before attempting to assign its configuration as this is the most challenging of the 3 scenarios.  With the bond in the plane, you are not looking at the molecule from the correct perspective but neither is it the exact opposite of the correct perspective either. If you move from priority #1 to #2 to #3 and assign it as is you'll be correct 50% of the time on average.  If you simply make it the opposite you'll once again be correct 50% of the time on average.

There are 3 ways to approach correctly assigning R and S in such a scenario:

1) Try to visualize the molecule in your mind from the correct perspective.

2) Rotate the other bond in the plane until the #4 priority group is in a position (dashed or wedged) from which you can more easily assign R and S.

3) Switch the position of the #4 priority group with the group that has a dashed bond.  By switching two groups you get the opposite configuration of the original molecule.  But with the #4 priority now in a dashed position it will be straightforward to assign R and S.  Once you've assigned R or S to this molecule, know that the original was in the opposite configuration.

While all of these approaches work, it has been my experience that undergraduate students tend to make fewer errors using the 3rd method which is why it's the method I present in the video lecture for this lesson.

How to Assign Priorities to Groups with the Same Attached Atom

In the above examples all of the atoms attached to the chiral center were different which made assigning priorities relatively easy.  But that won't be the case with most of the examples you're likely to come across.  Rule #2 in the Cahn-Ingold-Prelog System deals with assigning priorities in such cases.  For organic molecules most of the examples you'll see will have chiral centers attached to more than one carbon atom.  To distinguish between carbon atoms you next look at what 3 additional atoms these carbons are bonded to.  In the next example the chiral center is bonded to a Br (#1 priority), an H (#4 priority) and two carbon atoms.

The carbon on the left is the carbon of a methyl group and is simply bonded to 3 additional hydrogen atoms ( H H H ).  The carbon on the right is the carbon of an ethyl group and is bonded to 1 carbon atom and 2 hydrogen atoms ( C H H ).  When listing the 3 bonded atoms you list them in descending order of atomic number.  The priority is determined in the first place you see a difference by atomic number; the higher the atomic number the higher priority.

In this example the first atom the carbon of the ethyl group is attached to is a carbon, whereas the first atom the carbon of the methyl is attached to is a hydrogen, thus the carbon of the ethyl group will have a higher priority (#2) than the carbon of the methyl group (#3).

The #4 priority group has a dashed bond and as we move from priority #1 to #2 to #3 we make a right-handed turn indicating the configuration of this chiral center is R.

How to Assign Priorities Involving Double and Triple Bonds

A special case occurs when the atoms (usually carbon) attached to the chiral center have double or triple bonds.  When listing the bonded atoms you will list atoms with a double bond twice and atoms with a triple bond three times.  In the next example a the chiral center to an oxygen atom (#1 priority), a hydrogen atom (priority #4) and two carbon atoms.

The carbon on the left is bonded to a sulfur atom and two hydrogen atoms ( S H H ).  The carbon on the right has a double bond to oxygen and one bond to a hydrogen atom ( O O H ).  The first difference is sulfur vs oxygen.  As sulfur has a higher atomic number the carbon on the left is assigned the higher priority (#2) than the carbon on the right (#3).

Note that the comparison here was the first point of difference in the bonded atoms.  On just such an example I will have students ask me what to give a higher priority, S HH or O OH .  These students are trying to compare all three bonded atoms at the same time, but the proper comparison is simply the first point of difference, S vs O in this case.

Finally, the lowest priority is bonded with a wedged bond (facing toward you) and so the left-handed turn formed when proceeding from priority #1 to #2 to #3 indicates the R configuration rather than the S.

Assigning Priorities Involving Double and Triple Bonds: 2nd Example

Another special case occurs when the atoms attached to the chiral center have double or triple bonds and have the same 3 bonded atoms listed.  In such a case you have to continue on to the next atoms and list the 3 atoms they are attached to.  However, for an atom that has a double or triple bond from the previous atom in the sequence, you count the pi bonds back to the previous atom when listing the 3 atoms for the most recent atom in the sequence.  Consider the following example:

The chiral center is bonded to an oxygen atom (#1) and a hydrogen atom (#4) and two carbon atoms.  The carbon on the left is bonded to 3 methyl groups ( C C C ), and the carbon on the right is triple bonded to a carbon atom (also C C C ).  So up to this point we have not found a difference, so now we'll evaluate the next set of atoms attached to the above-listed carbon atoms.

The 3 carbon atoms on the left-hand side are all a part of methyl groups and the bonded atoms are listed as H H H.  The triple-bonded carbon on the right is only bonded to one additional atom, a single hydrogen.  It is here that we count the two pi bonds back to the previous carbon atom so that bonded atoms are listed as C C H.  The comparison here comes down to C vs H and carbon has a higher atomic number, therefore the carbon on the right of the chiral center as the higher priority (#2).

Finally, the lowest priority is bonded with a dashed bond (facing away from you) and so the right-handed turn formed when proceeding from priority #1 to #2 to #3 indicates the R configuration.

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Organic chemistry

Course: organic chemistry   >   unit 4.

• Drawing enantiomers
• Cahn-Ingold-Prelog system for naming enantiomers
• R,S (Cahn-Ingold-Prelog) naming system example 2
• R,S system practice
• More R,S practice
• Fischer projection introduction
• Fischer projection practice
• Optical activity
• Optical activity calculations

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Video transcript