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In chemistry, isomers are compounds with the same molecular formula but different arrangement of atoms.

A simple example of isomerism is given by propyl alcohol[?]: it has the formula C3H8O and the isomers

     n-propyl alcohol            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

Note that the critical difference is the position of the oxygen atom - it is attached to an end carbon in the one isomer, and to the centre carbon in the other isomer. 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.

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.

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