Molecular Fragments: Alkane Isomers

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Occasionally I get a request to name the isomers of a particular alkane. Sometimes that alkane is decane and I can't help wondering what the instructor asking such a problem had in mind. Perhaps the instructor was not aware of the immensity of the task. Anyway, to settle this once and for all, here then is a list of the names of all the alkane isomers up to and including decane.

First: What is Isomerism?

1. Isomers are covalent compounds which have exactly the same number and kind of atoms in them.

2. There are two major types of isomers:

(i) Those compounds which have their atoms connected together in different ways.
Compounds in group (i) are called constitutional isomers
(ii) Those compounds which have their atoms connected together in exactly the same way, but which differ in the way that those atoms are distributed in three-dimensional space. .
Compounds in group (ii) are subdivided:
- Conformational isomers. These structures are usually readily interchanged by rotation about C,C single bonds. At room temperature molecular conformations are constantly interchanging and isomers usually cannot be isolated.
- Stereoisomers. There are two types of stereoisomers.
  - (a) Enantiomers consist of a pair of stereoisomers which are nonsuperposible mirror images.
  - (b) Diastereoisomers are stereoisomers which are not enantiomers.

Now: A Table of Alkane Isomers.

First in this section: How many constitutional isomers do alkanes have?

Number of Constitutional Isomers of Alkanes.

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

So as you can see, as the number of atoms in the alkane molecule increases, so does the number of different ways of arranging them. Not too surprising! Except, perhaps, how fast the numbers of isomers increases.

As a consequence, the table next will only look at alkanes with 10 or fewer carbons.

Second in this section: A Table of Alkane Isomers

In this table streoisomers are indicated thusly:

Enantiomers separated by /, e.g. (R/S)

Diastereoisomers separated by //, e.g. (SS//RS)

Available models are referenced.

C₄H₁₀ (2 isomers)
Butane	Methylpropane

C₅H₁₂ (3 isomers)
Pentane	Methylbutane	Dimethylpropane

C₆H₁₄ (5 isomers)
Hexane	2-Methylpentane	3-Methylpentane
2,2-Dimethylbutane	2,3-Dimethylbutane

C₇H₁₆ (9 isomers)
Heptane	2-Methylhexane	3-Methylhexane (R/S)
2,2-Dimethylpentane	2,3-Dimethylpentane (R/S)	2,4-Dimethylpentane
3,3-Dimethylpentane	3-Ethylpentane	2,2,3-Trimethylbutane

C₈H₁₈ (18 isomers)
Octane	2-Methylheptane	3-Methylheptane (R/S)
4-Methylheptane	2,2-Dimethylhexane	2,3-Dimethylhexane (R/S)
2,4-Dimethylhexane (R/S)	2,5-Dimethylhexane	3,3-Dimethylhexane
3,4-Dimethylhexane (RR/SS//meso)	3-Ethylhexane	2,2,3-Trimethylpentane (R/S)
2,2,4-Trimethylpentane	2,3,3-Trimethylpentane	2,3,4-Trimethylpentane
3-Ethyl-2-methylpentane	3-Ethyl-3-methylpentane	2,2,3,3-Tetramethylbutane

C₉H₂₀ (35 isomers)
Nonane	2-Methyloctane	3-Methyloctane (R/S)
4-Methyloctane (R/S)	2,2-Dimethylheptane	2,3-Dimethylheptane (R/S)
2,4-Dimethylheptane (R/S)	2,5-Dimethylheptane (R/S)	2,6-Dimethylheptane
3,3-Dimethylheptane	3,4-Dimethylheptane (RR/SS//meso)	3,5-Dimethylheptane
4,4-Dimethylheptane	3-Ethylheptane	4-Ethylheptane
2,2,3-Trimethylhexane (R/S)	2,2,4-Trimethylhexane (R/S)	2,2,5-Trimethylhexane
2,3,3-Trimethylhexane	2,3,4-Trimethylhexane (RR/SS//RS/SR)	2,3,5-Trimethylhexane (R/S)
2,4,4-Trimethylhexane	3,3,4-Trimethylhexane	3-Ethyl-2-methylhexane (R/S)
3-Ethyl-3-methylhexane	3-Ethyl-4-methylhexane (R/S)	4-Ethyl-2-methylhexane
2,2,3,3-Tetramethylpentane	2,2,4,4-Tetramethylpentane	2,2,3,4-Tetramethylpentane (R/S)
2,3,3,4-Tetramethylpentane	3-Ethyl-2,3-dimethylpentane	3-Ethyl-2,2-dimethylpentane
3-Ethyl-2,4-dimethylpentane	3,3-Diethylpentane

C₁₀H₂₂ (75 isomers)
Decane	2-Methylnonane	3-Methylnonane (R/S)
4-Methylnonane (R/S)	5-Methylnonane	2,2-Dimethyloctane
2,3-Dimethyloctane (R/S)	2,4-Dimethyloctane (R/S)	2,5-Dimethyloctane (R/S)
2,6-Dimethyloctane (R/S)	2,7-Dimethyloctane	3,3-Dimethyloctane
3,4-Dimethyloctane (RR/SS//RS/SR)	3,5-Dimethyloctane (RR/SS//RS/SR)	3,6-Dimethyloctane (RR/SS//meso)
4,4-Dimethyloctane	4,5-Dimethyloctane (RR/SS//meso)	3-Ethyloctane
4-Ethyloctane (R/S)	2,2,3-Trimethylheptane (R/S)	2,2,4-Trimethylheptane (R/S)
2,2,5-Trimethylheptane (R/S)	2,2,6-Trimethylheptane	2,3,3-Trimethylheptane
2,3,4-Trimethylheptane (RR/SS//RS/SR)	2,3,5-Trimethylheptane (RR/SS//RS/SR)	2,3,6-Trimethylheptane (R/S)
2,4,4-Trimethylheptane	2,4,5-Trimethylheptane (RR/SS//RS/SR)	2,4,6-Trimethylheptane
2,5,5-Trimethylheptane	3,3,4-Trimethylheptane (R/S)	3,3,5-Trimethylheptane (R/S)
3,4,4-Trimethylheptane (R/S)	3,4,5-Trimethylheptane (RR/SS//meso)	3-Ethyl-2-methylheptane
3-Ethyl-3-methylheptane	3-Ethyl-4-methylheptane (R/S)	3-Ethyl-5-methylheptane (R/S)
4-Ethyl-2-methylheptane (R/S)	4-Ethyl-3-methylheptane (RR/SS//RS/SR)	4-Ethyl-4-methylheptane
5-Ethyl-2-methylheptane	4-Propylheptane	4-Isopropylheptane
2,2,3,3-Tetramethylhexane	2,2,3,4-Tetramethylhexane (RR/SS//RS/SR)	2,2,3,5-Tetramethylhexane (R/S)
2,2,4,4-Tetramethylhexane	2,2,4,5-Tetramethylhexane (R/S)	2,2,5,5-Tetramethylhexane
2,3,3,4-Tetramethylhexane (R/S)	2,3,3,5-Tetramethylhexane	2,3,4,4-Tetramethylhexane (R/S)
2,3,4,5-Tetramethylhexane (RR/SS//meso)	3,3,4,4-Tetramethylhexane	3-Ethyl-2,2-dimethylhexane (R/S)
3-Ethyl-2,3-dimethylhexane (R/S)	3-Ethyl-2,4-dimethylhexane (RR/SS//RS/SR)	3-Ethyl-2,5-dimethylhexane (R/S)
3-Ethyl-3,4-dimethylhexane (R/S)	4-Ethyl-2,2-dimethylhexane	4-Ethyl-2,3-dimethylhexane (R/S)
4-Ethyl-3,3-dimethylhexane	4-Ethyl-2,4-dimethylhexane	3,3-Diethylhexane
3,4-Diethylhexane	3-Isopropyl-2-methylhexane	2,2,3,3,4-Pentamethylpentane
2,2,3,4,4-Pentamethylpentane	3-Ethyl-2,2,4-trimethylpentane (R/S)	3-Ethyl-2,3,4-trimethylpentane
3-Ethyl-2,2,3-trimethylpentane	3,3-Diethyl-2-methylpentane	3-Isopropyl-2,4-dimethylpentane

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