Beginning Organic Chemistry (BOC)
5. Structural Isomers.

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Isomers.

To this point all compounds have had structures in which their carbon atoms are linked in a continuous chain. Naturally, this need not be the case!

Compounds which have the same molecular formula but different atomic arrangements within the molecula are called isomers and there are several different types of isomers. On this page we look at structural or constitutional isomers.

Structural or constitutional isomerism occurs when isomers differ in the order in which the atoms are bonded together.

Four Carbon Alkanes.

When 4 carbons are considered, there are two different ways to organize the carbons. There are two structural isomers of C4H10:

Line Structure Name Model
butane
methylpropane

Five Carbons

When 5 carbons are considered, there are three different ways to organize the carbons. There are three structural isomers of C5H12

Line Structure Name Model
pentane
methylbutane
dimethylpropane

Six Carbons

There are five structural isomers of hexane:

Line Structure Name Model
hexane
2-methylpentane
3-methylpentane
2,2-dimethylbutane
2,3-dimethylbutane

Number of Isomers of the Alkanes

The number of possible structural isomers increases rapidly with the number of carbons:

Number of Carbon Atoms 4 5 6 7 8 9 10 12 15 20
Number of Structural Isomers 2 3 5 9 18 35 75 355 4,347 366,319

As you might expect, not all of the possible isomers are known compounds.

A listing of the names of all alkane isomers with up to 10 carbon atoms is available.

Date created: 2005 06 11.