practice problems on alkane nomenclature

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Organic Chemistry

Alkanes and cycloalkanes.

In today’s post, we will talk about the IUPAC rules of nomenclature for naming alkanes and alkyl halides.

The first thing you need to do before learning the IUPAC rules for systematic nomenclature is making sure you know the names of the first ten alkanes:

practice problems on alkane nomenclature

Assuming you have already mastered those, let’s draw a structure and name it simply based on the molecular formula:

The compound has five carbons with no multiple bonds, therefore the formula is C 5 H 12, and based on the common names, we can see that it is pentane .

The complications and need for rules arise when the molecules get branched out. For example, what if we add a methyl (CH 3 ) group to pentane?

This group is considered a substituent ; an additional group that is on the “main part” of the molecule called the parent chain .

So, remember, we distinguish two units; the “main part” of the molecule, called the parent chain , and the additional group(s) known as substituents.

There are certain rules for determining the parent chain and the substituent(s) so let’s discuss them one-by-one and name this molecule (let’ name it molecule A ) in the course of doing that.

Substituents and Alkyl groups

The substituent can be a carbon fragment, and these are called alkyl groups , or any other functional group such as a halide, an OH, a nitro group, etc.

Alkyl groups are formed by removing one hydrogen from the corresponding alkane and are named based on this alkane by simply changing the ending from – ane to -yl .

Sec- , tert- and iso- prefixes

Just like the constitutional isomers , it is possible to have different alkyl groups with the same chemical formula.

For example, aside from the propyl group, there is also iso propyl. The difference is that in the isopropyl, a hydrogen, connected to a secondary carbon atom, is removed and it is this secondary carbon that is connected to the parent chain.

Likewise, the butyl group can also be primary, secondary, and tertiary. It will be very helpful to memorize all these groups and below is a general scheme to visualize how the names of these alkyl groups are derived:

You can also read this post about primary, secondary, and tertiary carbon atoms .

Identifying the Parent Chain

The parent chain is determined based on the longest continuous carbon chain that is present in the molecule. As an example, let’s consider molecule A mentioned earlier:

If we start numbering the carbon atoms from the methyl substituent, we can only have a continuous chain of four carbons. However, starting from any end allows making a five-carbon chain which is preferred since it makes a longer parent chain.

Therefore, the parent chain is pentane and the substituent is a methyl group.

If you run into a situation where there are two chains of equal length, then choose the one with the greater number of substituents :

When a ring is present, the parent chain is determined based on the number of carbons. If the ring has more carbons than the chain, then it is the parent chain:

Notice that the carbons in the ring belong to the ring only. I.e. you cannot count the carbon twice or include it in the carbon chain.

If the ring and the chain have an equal number of carbon atoms, the ring gets a priority and is considered as the parent chain.

Putting the parent chain and substituents together

When naming a compound, the alkyl groups are listed first followed by the parent chain. No sign or space needed to separate two words .

However, notice also that a number to specify the position of the alkyl groups is included in the final name. And this an important piece of information.

To illustrate this, let’s look at this example.

The following two compounds are both methylpentanes but they are clearly not identical:

And, in order to distinguish them, we need to specify the location of the methyl group. For this, the parent chain is numbered, and the rule here is to always do it such that the alkyl group gets the lowest possible number :

Starting from the left or the right side of the parent chain, we get two names and out of these, 2-methylpentane is better than 4-methylpentane. Therefore, 2-methylpentane is the correct IUPAC name of this compound.

Notice that for the second compound, it does not matter where we start the numbering, since it is a symmetric molecule, and either way the methyl group gets number 3:

A few additional details to point out when writing the name of a compound:

1) Numbers and words are separated by a dashed line

2) Words are not separated by any sing or a space

Parent chain with two substituents

Now, let’s add another methyl group next to the first one:

Again, you have two options for numbering the parent chain. One gives the 3,4 and the other one 2,3 locants for the two methyl groups and 2,3 clearly beats 3,4.

Therefore, the final name of our compound is going to be 2,3- di methylpentane.

Notice that numbers are separated by commas and because there are two methyl groups, we need to use the prefix “di” before the name of the alkyl groups.

In general, if two or more identical substituents are present, the corresponding prefixes are used to indicate their number:

Two – di Three – tri Four – tetra Five – penta Six – hexa Seven – hepta Eight – octa

Let’s also consider the other option of having two methyl groups on pentane:

Notice that, in this case, regardless of where we start the numbering, the first methyl gets locant 2, and the second one gets 4. Therefore, it is 2,4-dimethylpentane.

Now, the question comes – what if there is a third substituent and it does matter where to start numbering?

To answer this, let’s consider heptane with three methyl groups:

Starting from left or right makes no difference as far as having the location of the first substituent. Either way, it is 2. However, if you start from the left, you are getting 2,3,6-trimethylheptane, while starting from the right, gives 2,5,6-trimethylheptane.

And because 2,3,6 is better than 2,5,6, the correct name of this molecule is 2,3,6-trimethylheptane.

To summarize this observation, when there is a tie for the location of the first substituent, compare the second one, then the third till you find a tiebreak if there is one.

If numbering the alkyl groups does not break the tie, then the substituent with the alphabetical priority gets the lower locant:

For example, having a bromine and chlorine on both ends of hexane brings up the need of prioritizing the substituents based on their alphabetical order .

In this case, 1-bromo-6-chlorohexane beats 6-bromo-1-chlorohexane:

If none of the rules discussed above give a tiebreak, then it is a symmetrical molecule and it does not matter where you start numbering the parent chain – as long as you do find the correct parent chain.

Alphabetical order in IUPAC naming

So far, we have considered having identical alkyl groups. Now, suppose we need to name the following compound:

Step 1. Find the parent chain. The longest possible chain here consists of nine carbons, so the parent chain is nonane.

Step 2. Find the substituents. In this case, we have a methyl and an ethyl group.

Step 3. Number the parent chain giving the lowest possible numbers to the substituents:

Out of the two options, 2-methyl is better than 4-ethyl.

Step 4. Put the parent chain and substituents together by placing the substituents in alphabetical order!

This means that even though the methyl group is at position 2, the ethyl group with the locant 6 is still placed before it:

The alphabetical priority of prefixes

None of the prefixes such as di, tri, tetra, sec- , tert- are considered for alphabetical priority except the -iso.

For example:

Still looking forward to finding out why -iso is privileged…

Naming complex substituents

Sometimes, we run out of the common names for the substituents such as sec -butyl, tert -butyl, iso -butyl but we still need to name a substituent that is larger than usual.

For example, in the following molecule, it is easy to spot the ethyl group but a naming the second substituents needs to follow certain rules.

The good news is that these rules are no different than what we use when naming a compound. Essentially, you need to look at the complex substituent as a separate molecule and find its “parent chain” and the alkyl groups on it.

To do this, start numbering from the carbon directly connected to the actual parent chain of the molecule and list the alkyl groups alphabetically:

Notice that at the end, the quasi parent chain gets the -yl suffix since it is still a substituent and the actual parent chain is placed at the end.

This systematic approach for naming alkyl groups can also be applied for the ones with common names and you will likely need to know both options.

In the following practice problems, we will go over naming alkanes using the IUPAC nomenclature rules which include finding the parent chain, numbering it to have the substituents in the correct positions, and finally putting all of this together to name the compound.

The next exercise will teach you to draw the structure based on the IUPAC name.

Identify and name the parent in each of the following compounds:

Provide a systematic name for each of the following compounds:

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Draw the bond-line structure for each of the following molecues:

a) 4-ethyl-5-isopropyl-3-methyloctane

b) 1-ethyl-4-methyl-2-propylcyclopentane

c) 5-( sec -butyl)-8-isopropyl-3,4,9-trimethylundecane

d) 6-( sec -butyl)-7-ethyl-3,4-dimethyldecane

e) 3-ethyl-4,5-dimethylheptane

f) 4-ethyl-3-isopropyl-1,1-dimethylcyclohexane

g) 6,6-diethyl-3,5,5-trimethylnonane

h) 4-( sec -butyl)-3,3,5,5-tetramethylheptane

Check Also:

Naming Bicyclic Compounds
Naming Bicyclic Compounds-Practice Problems
How to Name a Compound with Multiple Functional Groups
Primary Secondary and Tertiary Carbon Atoms in Organic Chemistry
Constitutional or Structural Isomers with Practice Problems
Degrees of Unsaturation or Index of Hydrogen Deficiency
Newman Projections with Practice Problems
Gauche Conformation, Steric, Torsional Strain Energy Practice Problems
Drawing the Chair Conformation of Cyclohexane
Ring Flip: Drawing Both Chair Conformations with Practice Problems
1,3-Diaxial Interactions and A value for Cyclohexanes
Ring-Flip: Comparing the Stability of Chair Conformations with Practice Problems
Cis and Trans Decalin

6 thoughts on “Naming Alkanes with Practice Problems”

Can you please tell me the proper name of this compound: (CH3 )3 C-CH2-CH(Cl)-(CH2)3-C(CH3)2-CH(NH2)-C(C2H5)(CH3)2

You need to first convert this into a bond-line structure to find the parent chain and substituents.

9-amine-4-chloro-2,2,8,8,10,10-hexamethyl n-dodecane

4-amine-9-chloro-3,3,5,5,11,11-hexadodecane I guess

Sorry’one mistake its 4-amine-9-chloro-3,3,5,5,11,11-hexamehtyldodecane

This is the correct 👆

What is the bond line structure of the above?

2.5: Practice - Alkane Nomenclature

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I. The parent compound must have the longest chain of carbon atoms

Learn: methane (1 carbon atom), ethane (2 carbon atoms), propane (3 carbon atoms), butane (4 carbon atoms), pentane (5 carbon atoms), hexane (6 carbon atoms), heptane (7 carbon atoms), octane (8 carbon atoms), nonane (9 carbon atoms) and decane (10 carbon atoms).

Try to name the following compounds:

CH 3 -CH 2 -CH 2 -CH 3
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

Try to draw structures for the following compounds:

II. The parent chain is numbered to give substituents the lowest possible numbers

Substituent names are methyl, ethyl, propyl, butyl, etc. The number showing the point of attachment to the parent chain precedes the substituent name. If you have more than one substituent with the same name, a number of attachment must be given for each substituent and the number of the substituents must be designated with di-, tri-, etc.

Try to name the following compounds...

Try to draw structures for the following compounds...

2,2-dimethylpropane
3-methylheptane
4,5-diethylnonane

III. Substituents are named in alphabetical order

Remember that the numbering of the parent chain must give all substituents the lowest possible numbers regardless of their names.

Try to name the following compound...

IV. A large substituent is numbered to give its point of attachment

The point of attachment is number one and any other smaller groups are named as substituent groups on the larger group. This numbering is independent of the numbering of the parent chain. Try to name the following compounds...

4-(1-methylethyl)heptane
5-(1,1-dimethylethyl)nonane

V. Ring compounds are designated with a cyclo- prefix and are numbered to give multiple substituents the lowest possible numbers

A single substituent does not need to be numbered. A ring can also be named as a substituent. Try to name the following compounds:

ethylcyclobutane
1-ethyl-4-methylcyclohexane
cyclobutylcyclooctane

VI. Common Names

isopropyl = 1-methylethyl
isobutyl = 2-methylpropyl
sec -butyl = 1-methylpropyl
tert -butyl = 1,1-dimethylethyl
neo -pentyl = 2,2-dimethylpropyl
iso -pentyl = 3-methylbutyl

Try to name the following compounds using common names...

4-isopropyloctane
isopentylcyclohexane
CH 3 -CH 2 -CH 3
CH 3 -CH 2 -CH 2 -CH2-CH 3
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
2-methylbutane
3-ethylhexane
3-methyloctane (Remember that you must number the parent chain to give the substituent the lowest possible number.)
4-ethyl-3-methylheptane

4-(1-methylethyl)octane

5-(1-methylpropyl)decane

1-ethyl-2-methylcyclohexane

methylcyclopentane (You do not need a number since there is only one substituent and it is assumed to be on the first carbon regardless of how the ring compound is drawn.)

cyclopropylcyclopentane

neo -pentylcycloheptane (1-(2,2-dimethyl-1-propyl)cycloheptane is the proper name.)

isopropylcyclohexane

Contributors

Richard Banks ( Boise State University )

Select the correct structure for the IUPAC names given below:

Part 1. 4-methylheptane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 2. 2,4-dimethylhexane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 3. 2,3-dimethylpentane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 4. 2,2-dimethylhexane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 5. 2-ethyl-1,1,3,3-tetramethylcyclopentane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 6. 3-ethyl-2-methylpentane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L

CHEM 221/222: Organic Chemistry: Alkanes

Acids & Bases
Aldehydes & Ketones
Alkyl Halides
Aromatic Compounds
Bonding & Lewis Structures
Nomenclature
C-C Bond Forming
Carboxylic Acids
Radical Reactions
Spectroscopy
Stereochemistry / Isomerism
Synthesis Strategies
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