General Characteristics

Isomers have the same number of atoms of each element in them and the same atomic weight but differ in other properties. For example, there are two compounds with the molecular formula C₂H₆O. One is ethanol (also called ethyl alcohol), CH₃CH₂OH, a colorless liquid alcohol; the other is dimethyl ether, CH₃OCH₃, a colorless gaseous ether. Among their different properties, ethanol has a boiling point of 78.5°C; and a freezing point of -117°C;; dimethyl ether has a boiling point of -25°C; and a freezing point of -138°C;. Ethanol and dimethyl ether are isomers because they differ in the way the atoms are joined together in their molecules:

Isomers are classified as structural isomers, which have the same number of atoms of each element and molecular weight but different bonding patterns (see chemical bond), or as stereoisomers, which have the same number of atoms of each element, molecular weight, and bonding pattern but in which the atoms have different spatial relationships. Tautomers are structural isomers that readily convert from one isomeric form to another and therefore exist in equilibrium.

Structural Isomers

Structural isomers are subdivided as chain, position, and functional group. Chain isomers occur among the alkanes. For example, there are two chain isomers of butane, C₄H₁₀. In n-butane, CH₃CH₂CH₂CH₃, the carbon atoms are joined in a so-called straight, or unbranched, chain. In isobutane, CH₃CH(CH₃)₂, the carbon atoms are joined in a branched chain; the isobutane molecule can be visualized as a carbon atom bonded to one hydrogen atom and to three methyl (CH₃) groups.

Position isomers occur among substituted alkanes and other compounds. For example, 1-propanol, CH₃CH₂CH₂OH, and 2-propanol, CH₃CH(OH)CH₃, are position isomers, as are 1-butene, CH₂--CHCH₂CH₃, and 2-butene, CH₃CH--CHCH₃. Position isomers have similar chemical properties since they differ only in the location of the functional group (e.g., the OH in an alcohol or the double bond in an alkene).

Functional group isomers, on the other hand, have very different chemical properties because differences in their structure give rise to different functional groups. Ethanol and dimethyl ether (see the example, above) are functional group isomers.

Stereoisomers

Stereoisomerism occurs when two or more molecules have the same basic arrangement of atoms in their molecules but differ in the way the atoms are arranged in space. There are two types of stereoisomerism. The first type, geometric isomerism, may occur when a compound contains a double bond or some other feature that gives the molecule a certain amount of structural rigidity. Geometric isomers differ in physical properties such as melting point and boiling point. For example, there are two geometric isomers of 2-butene, CH₃CH--CHCH₃:The prefix cis- means "same side" and trans- means "opposite side"; they are used when the groups on either side of the double bond are identical or closely related, e.g., methyl and ethyl. Syn- and anti- have similar meanings but are used when the groups are not identical or closely related.

The second type of stereoisomerism is optical isomerism. When plane-polarized light is passed through an optical isomer it is rotated into a different plane of polarization. Optical isomers exhibit this optical activity in varying degrees. Optical isomers of a given compound are often identical in all physical properties except the direction in which they rotate light. The molecules of optical isomers are asymmetrical. The simplest optical isomers have a single "asymmetrical carbon atom" in their molecules. An asymmetrical carbon atom has four different atoms or radicals bonded to it, arranged approximately at the corners of a tetrahedron centered on the carbon atom. For example, there are two optical isomers of lactic acid:The atom and radical to either side of the carbon atom are visualized as being above the plane of the paper, the central carbon atom in the plane of the paper, and the radicals above and below the central carbon atom below the plane of the paper. Thus it is seen that the two molecules are mirror images of each other and, each being asymmetrical, cannot be superposed on each other. The d- and l- prefixes stand for dextro (right) and levo (left). Two optical isomers, such as these, whose molecules are asymmetrical and are mirror images of each other, are called enantiomorphs. When equal amounts of d- and l-enantiomorphs are mixed, the mixture has no effect on polarized light; such a mixture is called racemic.

When there is more than one asymmetrical carbon atom, there may be more than two optical isomers. For example, tartaric acid has two asymmetrical carbon atoms and three optical isomers:The d- and l-tartaric acids are enantiomorphs; each molecule is asymmetrical and is the mirror image of the other. There are two asymmetrical carbon atoms in meso-tartaric acid, but the molecule is symmetrical and does not exhibit optical activity; the optical activity is internally compensated, the effect of one asymmetrical carbon atom balancing the effect of the other. A pair of optical isomers such as d-tartaric acid and meso-tartaric acid, which are not enantiomorphs, are called diastereoisomers. Molecular disymmetry in optical isomers may come from some source other than an asymmetrical carbon atom, e.g., structural rigidity resulting from double bonds or ring structures within a molecule.

Stereoisomers are important in metabolism; in many cases only one of several isomeric forms of a compound can take part in biochemical reactions. For example, there are 16 stereoisomers of a simple sugar whose molecular formula is C₆H₁₂O₄. Of these, only d-glucose is readily utilized in human metabolism.

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 do not necessarily share similar properties unless they also have the same functional groups. This should not 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 C₃H₇OH) 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 is 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 is 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.

Examples of isomers having different medical properties can be easily found. For example, in the placement of methyl groups. In substituted 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. Another example of this occurs in the phenethylamine-based stimulant drugs. Phentermine is a non-chiral compound with a weaker effect than amphetamine. It is used as an appetite reducing medication and has mild or no stimulant properties. However, a different rearrangement of the same atoms gives dextromethamphetamine, which is more potent than amphetamine; it is a very strong stimulant.

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 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 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 cannot 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. Spin isomers have differing distributions of spin among their constituent atoms.

History

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.

See also

• Structural isomerism
• Cis-trans isomerism
• Isotopomers
• Isotopologue
• Chirality (chemistry)

References