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How can I list all the isomers of an alkane?

Structural isomers are compounds with the same molecular formula but different molecular structures. To generate a complete list of isomers for a particular molecular formula, a systematic approach is useful. If the compound is part of a series or family of related compounds (as your examples are) it's useful to start with a simpler compound in the series and add to it. Be careful to recognize and discard isomers that may appear different but are actually identical.

I'll list all structural isomers of pentane and hexane for you- you should then be able to do heptane and octane for yourself. You can also build up a complete list of octane isomers using the Isomer Construction Set.

Pentane is C5H12. Starting with the two isomers of butane, you can generate all possible isomers by attaching a carbon atom at every unique position.

Notice that for the n-butane molecule, there are only two unique positions: carbons on the end of the chain, and carbons next to the end of the chain. You can't distinguish the two middle carbons from each other; flipping the molecule over will turn the second carbon into the third and vice versa. Similarly, you can't distinguish the end carbons from each other, since flipping the molecule over will turn the first carbon into the last carbon, and vice versa. The isobutane molecule has two unique positions: the central carbon, and the end carbon. The three carbons around the central carbon are equivalent, since you can turn or flip the molecule to turn one into another.

Once you've generated a list of isomers from all the butane isomers, you must go through the list and discard isomers that are identical. notice that isopentane appears twice; there are actually only three unique isomers of pentane in the list.

Hexane is C6H14. Starting with the isomers of pentane, add a carbon to every unique position in the pentane isomer:

The n-pentane isomer has 3 unique positions: end carbons, next-to-end carbons, and center carbons. You get three unique hexane isomers by attaching a carbon at each of these positions.

The isopentane isomer has 4 unique positions: the carbons at the short ends of the Y are equivalent to each other. The first and last hexane isomers are greyed because they already have been generated from the n-pentane isomer.

The hexanes generated from the neopentane isomer aren't shown because they all appear in the n-pentane and isopentane lists.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How can I list all the isomers of an alkane?

Copyright © 1997-2010 by Fred Senese
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Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/organic/faq/listing-isomers.shtml