Isomerism

As a result of the discussion of Yu. Liebig and F. Wöhler in 1823, it was established that there are two sharply different in their properties AgCNO compounds - cyano-acid ( AgNCO ) and explosive ( AgONC ) silver . Another example was tartaric and grape acids, after the study of which J. Berzelius introduced the term "isomerism" in 1830 and suggested that the differences arise due to the "different distribution of simple atoms in a complex atom" (that is, in modern terms, molecule).

A true explanation of isomerism was received only in the 2nd half of the 19th century on the basis of the theory of the chemical structure of A. M. Butlerov (structural isomerism) and the stereochemical teachings of J. G. Vant-Hoff (spatial isomerism).

Structural isomerism is the result of differences in chemical structure. This type includes:

Isomerism of the carbon chain (carbon skeleton)

The isomerism of the carbon skeleton, due to the different order of bonding of carbon atoms. The simplest example is butane CH ₃ —CH ₂ —CH ₂ —CH ₃ and isobutane (CH ₃ ) ₃ CH. Other examples are anthracene and phenanthrene (formulas I and II, respectively), cyclobutane and methylcyclopropane (III and IV).

Valence Isomerism

Valence isomerism is a special type of structural isomerism in which isomers can be converted into each other only due to the redistribution of bonds. For example, the valence isomers of benzene (V) are bicyclo [2.2.0] hexa-2,5-diene (VI, “Dewar benzene”), prism (VII, “Ladenburg benzene”), benzwalen (VIII).

Functional Isomerism (Interclass Isomerism)

It differs in the nature of the functional group; for example, ethanol (CH ₃ —CH ₂ —OH) and dimethyl ether (CH ₃ —O — CH ₃ ).

Isomerism of position

A type of structural isomerism characterized by a difference in the position of identical functional groups or multiple bonds with the same carbon skeleton. Example: 2-chlorobutanoic acid and 4-chlorobutanoic acid.

Metamerism

Metamerism is a type of structural isomerism characterized by a different distribution of carbon atoms between several hydrocarbon radicals separated in a molecule by a heteroatom . Metamerism is known in the ranks of aliphatic ethers , esters , thioalcohols and amines . Currently, the term is rarely used.

This type of isomerism was also pointed out by A. M. Butlerov, calling it the "isomerism of non-integral structures."

Example: CH ₃ CH ₂ OCH ₂ CH ₃ - diethyl ether and CH ₃ OCH ₂ CH ₂ CH ₃ - methylpropyl ether

Spatial isomerism (stereoisomerism) arises as a result of differences in the spatial configuration of molecules having the same chemical structure. To denote the spatial isomers of various types, a stereochemical nomenclature has been developed, collected in section E of the IUPAC nomenclature rules for chemistry .

This type of isomerism is divided into enantiomerism (optical isomerism) and diastereomerism .

Enantiomerism (optical isomerism)

Enantiomers (optical isomers, mirror isomers) are pairs of optical antipodes - substances characterized by opposite in sign and identical magnitude rotations of the plane of polarization of light with the identity of all other physical and chemical properties (except for reactions with other optically active substances and physical properties in a chiral environment ) A necessary and sufficient reason for the occurrence of optical antipodes is that the molecule belongs to one of the following point groups of symmetry : C _n , D _n , T, O, or I ( chirality ). Most often we are talking about an asymmetric carbon atom, that is, an atom bonded to four different substituents.

Other atoms can be asymmetric, for example, silicon , nitrogen , phosphorus , and sulfur atoms. The presence of an asymmetric atom is not the only cause of enantiomerism. Thus, derivatives of adamantane (IX), ferrocene (X), 1,3-diphenylallylene (XI), 6,6'-dinitro-2,2'-diphenic acid (XII) have optical antipodes. The reason for the optical activity of the latter compound is atropoisomerism , that is, spatial isomerism caused by the lack of rotation around a simple bond. Enantiomerism also manifests itself in the spiral conformations of proteins , nucleic acids , in hexagelicene (XIII).

Diastereomerism

Diastereomeric consider any combination of spatial isomers that do not constitute a pair of optical antipodes. Distinguish between σ and π diastereomers.

σ — diastereomerism

σ-diastereomers differ from each other in the configuration of part of the elements of chirality present in them. So, diastereomers are (+) - tartaric acid and meso-tartaric acid, D-glucose and D-mannose, for example:

π — diastereomerism (geometric isomerism)

π-diastereomers, also called geometric isomers , differ from each other in different spatial arrangement of the substituents relative to the plane of the double bond (most often C = C and C = N) or cycle. These include, for example, maleic and fumaric acids (formulas XIV and XV, respectively), (E) and (Z) -benzaldoximes (XVI and XVII), cis- and trans-1,2-dimethylcyclopentanes (XVIII and XIX).

Chemical transformations, as a result of which structural isomers are converted into each other, is called isomerization . Such processes are important in industry. So, for example, normal alkanes are isomerized into isoalkanes to increase the octane number of motor fuels; pentane isomerized to isopentane for subsequent dehydrogenation to isoprene . Intramolecular rearrangements are also isomerization, of which, for example, Beckmann rearrangement - the conversion of cyclohexanone oxime to caprolactam (raw material for the production of capron ) is of great importance.

The process of mutual conversion of enantiomers is called racemization : it leads to the disappearance of optical activity as a result of the formation of an equimolar mixture of (-) - and (+) - forms, that is, the racemate. Interconversion of diastereomers leads to the formation of a mixture in which a thermodynamically more stable form predominates. In the case of π-diastereomers, this is usually a trans form. Interconversion of conformational isomers is called conformational equilibrium.

The phenomenon of isomerism greatly contributes to an increase in the number of known (and even more so, the number of potential) compounds. Thus, the possible number of structurally isomeric decyl alcohols is more than 500 (of which about 70 are known), the spatial isomers here are more than 1,500.

In the theoretical analysis of the problems of isomerism, topological methods are becoming more widespread; mathematical formulas are derived to calculate the number of isomers.

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• Great Encyclopedic Dictionary. Chemistry. Publisher: Big Russian Encyclopedia, 2003, ISBN 5-85270-253-6

• Tautomerism
• Chirality
• Asymmetric atom
• Orbital Models
• Crystallization of enantiomers