Isomer





In chemistry, isomers are molecules with the same chemical formula and with
the same kinds of bonds between atoms, but in which the atoms are arranged
differently. Isomers typically share similar if not identical properties in
most chemical contexts.

A simple example of isomerism is given by propanol: it has the formula C3H8O
and the isomers

Propan-1-ol (n-propyl alcohol) Propan-2-ol (isopropyl alcohol)

         H H H                        H H H
         | | |                        | | |
       H-C-C-C-O-H                  H-C-C-C-H
         | | |                        | | |
         H H H                        H O H
                                        |
                                        H

Note that the position of the oxygen atom differs between the two: it is
attached to an end carbon in the first isomer, and to the centre carbon in
the second. It can be readily shown that the number of possible isomers
rapidly increases as the number of atoms increase; for example the next
largest alcohol, named butanol (C4H10O), has four different isomers.

In the example above it should also be noted that in both isomers all the
bonds are single bonds; there is no type of bond that appears in one isomer
and not in the other. Also the number of bonds is the same. From the
structures of the two molecules it could be deduced that their chemical
stabilities are liable to be identical or nearly so.

Different forms of isomerism

There are two main forms of isomerism: structural isomerism and
stereoisomerism.

In structural isomers, the atoms and functional groups are joined together
in different ways, as in the example of propyl alcohol above. This group
includes chain isomerism whereby hydrocarbon chains have variable amounts of
branching; position isomerism which deals with the position of a functional
group on a chain; and functional group isomerism in which one functional
group is split up into different ones.

In stereoisomers the bond structure is the same, but the geometrical
positioning of atoms and functional groups in space differs. This class
includes optical isomerism where different isomers are mirror-images of each
other, and geometric isomerism where functional groups at the end of a chain
can be twisted in different ways.

While structural isomers typically have different chemical properties,
stereoisomers behave identically in most chemical reactions. Enzymes however
can distinguish between different stereoisomers of a compound, and organisms
often prefer one stereoisomer over the other. Some stereoisomers also differ
in the way they twist polarized light.