In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. Many isomers share similar if not identical properties in most chemical contexts.
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and the isomers
Propan-1-ol (n-propyl alcohol; left) Propan-2-ol (isopropyl alcohol; right)
H H H H H H
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H H H H O 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 center carbon in the second. It can be readily shown that the number of possible isomers increases rapidly as the number of atoms increase; for example the next largest alcohol, named butanol (C4H10O), has five 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.
There is, however, another isomer of C3H8O which has significantly different properties: methyl ethyl ether:
H H H
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H H H
Notice that unlike the top two examples, the oxygen is connected to two carbons rather than to one carbon and one hydrogen. As it lacks a hydroxl group the above molecule is no longer considered an alcohol but is classified as an ether, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.
Isomerism was first noticed in 1825, when Friedrich Woehler prepared cyanic acid and noted that although its elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different. This finding challenged the prevailing chemical understanding of the time, which held that different compounds were different because they had different elemental compositions. After additional discoveries of the same sort were made, such as Woehler's 1828 discovery that urea had the same composition as, yet was not, ammonium cyanate, Berzelius introduced the term isomerism to describe the phenomenon.
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 rotate polarized light.
There also exist topological isomers called topoisomers. Molecules with topoisomers include catenanes and DNA. Topoisomerase enzymes can knot DNA and thus change its topology.
In nuclear physics, isomers are excited states of atomic nuclei; see nuclear isomer.