Everything about Isomer totally explained
» This article is about the chemical concept. For "isomerism" of atomic nuclei, see nuclear isomer.
In
chemistry,
isomers are compounds with the same
molecular formula but different
structural formulae Isomers don't necessarily share similar properties unless they also have the same
functional groups. This shouldn't be confused with a
nuclear isomer, which involves a nucleus at different states of excitement. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical isomers, et cetera (see graph below).
A simple example of
isomerism is given by
propanol: it has the formula
C3H8O (or C
3H
7OH) and occurs as 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's attached to an end
carbon in the first isomer, and to the center carbon in the second.
There is, however, another isomer of
C3H8O which has significantly different properties:
methoxyethane (methyl-ethyl-ether;
III). Unlike the isomers of propanol, methoxyethane has an oxygen atom that's connected to two carbons rather than to one carbon and one hydrogen. This makes it an
ether, not an alcohol, as it lacks a
hydroxyl group, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.
An example of isomers having more subtly different properties can be found in certain
xanthines.
Theobromine, found in
chocolate, is a
vasodilator with some effects in common with
caffeine, but if one of the two
methyl groups is moved to a different position on the two-ring core, the isomer is
theophylline, which has a variety of effects, including
bronchodilation and
anti-inflammatory action.
Allene and
propyne are examples of isomers containing different bond types. Allene contains two
double bonds, while propyne contains one
triple bond.
Classification
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 non-superimposable mirror-images of each other, and
diastereomers when they're not. Diastereomerism is again subdivided into
conformational isomerism (conformers) when isomers can interconvert by chemical bond rotations and
cis-trans isomerism when this isn't 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
skeletal isomers the main carbon chain is different between the two isomers. This type of isomerism is most identifiable in secondary and tertiary alcohol isomers.
Tautomers are structural isomers of the same chemical substance that spontaneously interconvert with each other, even when pure. They have different chemical properties, and consequently, distinct reactions characteristic to each form are observed. If the interconversion reaction is fast enough, tautomers can't be isolated from each other. An example is when they differ by the position of a proton, such as in keto/enol tautomerism, where the proton is alternately on the carbon or oxygen.
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 academic 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 the potentially harmful or biologically inactive enantiomer.
While structural isomers typically have different chemical properties, stereoisomers behave identically in most chemical reactions, except in their reaction with other stereoisomers.
Enzymes however can distinguish between different enantiomers of a compound, and organisms often prefer one isomer 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.
History
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'd 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,
Jöns Jakob 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.
Further Information
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