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
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
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
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
Author: Fred Senese email@example.com