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Naming organic molecules


Historically, the names of many organic compounds were derived from the sources in which they were found. As more organic compounds were discovered, a systematic naming method became necessary. The systematic method used here was developed by the International Union of Pure and Applied Chemistry, IUPAC.

The following example is from Chap 22 of Modern Chemistry (Holt Rinehart Winston)

carbon atom chain prefixes

from Modern Chemistry (HRW)

To name an unbranched alkane, find the prefix that corresponds to the number of carbon atoms in the chain of the hydrocarbon. Then add the suffix -ane to the prefix. An example is shown below.

Heptane alkane

The molecule has a chain seven carbon atoms long, so the prefix hept- is added to the suffix -ane to form heptane.

Straight chain alkane groups

Branched-Chain Alkane Nomenclature

The naming of branched-chain alkanes also follows a systematic method. The hydrocarbon branches of alkanes are alkyl groups.

Alkyl groups are groups of atoms that are formed when one hydrogen atom is removed from an alkane molecule. Alkyl groups are named by replacing the suffix -ane of the parent alkane with the suffix -yl.

Some examples of alkyl groups are shown in Figure 2.3.

Alkyl group names are used when naming branched- chain alkanes. We will present only the method for naming simple branched-chain alkanes with only straight-chain alkyl groups. Consider the following molecule.

Branched-Chain Alkane

To name this molecule, locate the parent hydrocarbon. The parent hydrocarbon is the longest continuous chain that contains the most straight-chain branches.

In this molecule, there are two chains that are 8 carbon atoms long. The parent hydrocarbon is the chain that contains the most straight-chain branches.

Do not be tricked by the way the molecule is drawn. The longest chain may be shown bent.

branched chain alkane naming example

To name the parent hydrocarbon, add the suffix -ane to the prefix oct- (for a carbon-atom chain with eight carbon atoms) to form octane.

Now identify and name the alkyl groups.

ethyl trimethyloctane

The three —CH3 groups are methyl groups.

The —CH2—CH3 group is an ethyl group.

Arrange the names in alphabetical order in front of the name of the parent hydrocarbon.

ethyl methyloctane

To show that there are three methyl groups present, attach the prefix tri- to the name methyl to form trimethyl.

ethyl trimethyloctane

Now, we need to show the locations of the alkyl groups on the parent hydrocarbon.

Number the octane chain so that the alkyl groups have the lowest numbers possible.

ethyl trimethyloctane naming

Place the location number of each of the alkyl groups in front of its name. Separate the numbers from the names of the alkyl groups with hyphens.

The ethyl group is on carbon 3.

3-ethyl trimethyloctane

Because there are three methyl groups, there will be three numbers, separated by commas, in front of trimethyl.


The full name is 3-ethyl-2,4,5-trimethyloctane.


Alkane Nomenclature

1. Name the parent hydrocarbon. Find the longest continuous chain of carbon atoms that have straight-chain branches. Add the suffix -ane to the prefix corresponding to the number of carbon atoms in the chain.

2. Add the names of the alkyl groups. Add the names of the alkyl groups in front of the name of the parent hydrocarbon in alphabeti- cal order. When there is more than one branch of the same alkyl group present, attach the appropriate numerical prefix to the name:

di = 2, tri = 3, tetra = 4, and so on.

Do so after the names have been put in alphabetical order.

3. Number the carbon atoms in the parent hydrocarbon. If one or more alkyl groups are present, number the carbon atoms in the continuous chain to give the lowest numbers possible in the name. If there are two equivalent lowest positions with two different alkyl groups, give the lowest number to the alkyl group that comes first in the name. (This will be the alkyl group that is first in alphabetical order, before any prefixes are attached.)

4. Insert position numbers. Put the position numbers of each alkyl group in front of the name of that alkyl group.

5. Punctuate the name. Use hyphens to separate the position numbers from the names. If there is more than one number in front of a name, use commas to separate the numbers.

Sample problem

Name the following simple branched-chain alkane:


trimethylhexane 1

trimethylhexane 2

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