Isomer
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. That is to say, they have different
structural formulae. Many isomers share similar if not identical properties in most chemical contexts. This should not be confused with a
nuclear isomer, which involves a nucleus at different states of excitement.
A simple example of
isomerism is given by
propanol: it has the formula
C3H8O (or
C3H7OH) and two isomers Propan-1-ol (n-propyl alcohol;
I) and Propan-2-ol (isopropyl alcohol;
II)
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. The number of possible isomers increases rapidly as the number of atoms increases; for example the next largest alcohol, named
butanol (C
4H
10O), 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 (
III). 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
hydroxyl 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.
Another example of isomers having very different properties can be found in certain
xanthines.
Theobromine is found in
chocolate, but if one of the two
methyl groups is moved to a different position on the two-ring core, the isomer is
theophylline, used as a
bronchodilator.
 |
The different types of isomers |
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
enantiomers where different isomers are mirror-images of each other, and
diastereomers when they are not. Diastereomerism is again subdivided into
conformational isomerism (conformers) when isomers can interconvert by chemical bond rotations and
cis-trans isomerism when this is not possible. Note that although conformers can be referred to as having a diastereomeric relationship, the isomers over all are not diastereomers, since bonds in conformers can be rotated to make them mirror images.
In
food chemistry,
medicinal chemistry and
biochemistry,
cis-trans isomerism is always considered. In
medicinal chemistry and
biochemistry,
enantiomers are of particular interest since most changes in these types of isomers are now known to be meaningful in living organisms. Pharmaceutical and university-level researchers have found
chromatographical methods to reliably separate these from each other. On an industrial scale, however, these methods are rather costly and are mostly used to filter out potentially harmful isomers.
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.
Other types of isomerism exist outside this scope.
Topological isomers called
topoisomers are generally large molecules that wind about and form different shaped knots or loops. Molecules with topoisomers include
catenanes and
DNA.
Topoisomerase enzymes can knot DNA and thus change its topology. There are also
isotopomers or
isotopic isomers that have the same numbers of each type of isotopic substitution but in chemically different positions. In
nuclear physics,
nuclear isomers are excited states of atomic nuclei.
Isomerism was first noticed in 1827, 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
chemical compounds could be different only when 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 atomic composition as the chemically distinct ammonium cyanate,
Berzelius introduced the term
isomerism to describe the phenomenon.
In 1849,
Louis Pasteur separated tiny crystals of
tartaric acid into their two
mirror-image forms. The individual molecules of each were the left and right optical
stereoisomers, solutions of which rotate the plane of
polarized light in opposite directions.
*
List of publications in chemistry
