Diastereomers are stereoisomers that are not mirror images of each other [1] . Diastereomerism occurs when a compound has several stereo centers . If two stereoisomers have opposite configurations of all corresponding stereo centers, then they are enantiomers . However, if the configuration differs only in some (but not all) stereo centers, then such stereoisomers are diastereomers [2] . If diastereomers differ in the configuration of only one stereo center, then they are called epimers [3] .
Diastereomers also include compounds with different configurations of the double bond ( geometric isomers ).
Unlike enantiomers, diastereomers differ in physical properties and reactivity.
Content
Types of diastereomers
σ-Diastereomers
The existence of σ-diastereomers is due to the presence of several chiral centers in the molecule (or, in the general case, several chiral elements). The total number of stereoisomers of the molecule ( N ) containing n stereo centers can be calculated by the formula
Some pairs of stereoisomers will have opposite configurations of all the corresponding stereo centers and will thus be enantiomers . All other paired combinations of stereoisomers will differ only in some configurations, not being mirror images of each other. Such pairs are diastereomers [4] .
π-Diastereomers
The π-diastereomers, also called geometric isomers , differ from each other in the spatial arrangement of the substituents relative to the double bond plane. For example, maleic and fumaric acids differ from each other by the mutual arrangement of carboxyl groups at the double bond [2] .
Nomenclature
R / S Nomenclature
The full name of the diastereomer can be formed using the designations R / S , characterizing the absolute configuration of each stereo center of the molecule. Thus, the diastereomer will receive an exhaustive name by which it can subsequently be restored to the accuracy of all stereo centers.
Erythro / threo- nomenclature
To indicate the relative configuration of diastereomers, erythro - and threo - prefixes are used. If the diastereomer is depicted in the form of a Fisher projection and at the same time both identical (or randomly selected) substituents are located on one side of the carbon skeleton, then such a diastereoisomer is called erythro -, if on different sides - treo -. This nomenclature comes from the name of two diastereomeric aldoses - erythrose and threose .
Sin / anti- nomenclature
Diastereomers depicted in the form of zigzag structures are more conveniently called by syn / anti- nomenclature, while if both substituents are located on one side of the carbon skeleton plane, then the diastereomer is called syn -, if on opposite sides, then anti -. Two types of nomenclatures are interconnected, since any molecule can be depicted both in the form of a Fisher projection, and in a zigzag shape, and both representations clearly correspond to each other. The use of both nomenclatures is illustrated by the example of stereoisomeric chlorine-substituted malic acids [2] .
E / Z Nomenclature
The configuration of π-diastereomers varies depending on the location of the substituents at the double bond. In this case, the double bond receives the designation E ( German: entgegen - on the contrary) if the two highest substituents at each carbon atom of the double bond are on opposite sides of this double bond. If the senior substituents are located on one side of the double bond plane, then such a double bond has the Z configuration ( German zusammen - together). The seniority of deputies is determined according to the rules of Kan - Ingold - Prelog .
Cis / trans nomenclature
This type of nomenclature is used to indicate the configuration of π-diastereomers. It is almost completely equivalent to the E / Z nomenclature, but is mainly used for cyclic diastereomers, while IUPAC recommends the use of the E / Z nomenclature for alkenes [5] . If two senior substituents at each carbon atom of a double bond or a cyclic fragment are located on different sides of this double bond or a cyclic fragment, then the diastereomer is called trans -, if on one side, then cis -.
Physical Properties of Diastereomers
Unlike enantiomers, diastereomers differ in physical properties, including optical activity . This is due to the fact that the enantiomers are identical in all scalar properties, that is, the distance between any two atoms in them is the same. Diastereomers in this sense are not identical, therefore their properties differ [4] .
| Stereoisomer [2] | T. pl., ° C | [ α ] D , ° |
|---|---|---|
| erythro - (-) - 3-chloro-malic acid | 173 | –31.3 |
| erythro - (+) - 3-chlorobutyric acid | 173 | +31.3 |
| threo - (-) - 3-chlorobutyric acid | 167 | –9.4 |
| threo - (+) - 3-chlorobutyric acid | 167 | +9.4 |
See also
- Enantiomers
- Meso compounds
- Chirality
- Configuration (chemistry)
- Rules of Caen - Ingold - Prelog
Notes
- ↑ IUPAC Gold Book - diastereoisomerism . Date of treatment February 7, 2013. Archived February 14, 2013.
- ↑ 1 2 3 4 Potapov, 1988 , p. sixteen.
- ↑ IUPAC Gold Book - epimers . Date of treatment February 8, 2013. Archived February 14, 2013.
- ↑ 1 2 Iliel et al., 2007 , p. 48-49.
- ↑ IUPAC Gold Book - cis, trans . Date of treatment February 9, 2013. Archived February 14, 2013.
Literature
- Potapov V.M. Stereochemistry. - M: Chemistry, 1988. - ISBN 5-7245-0376-X .
- Kahn R., Dermer O. Introduction to Chemical Nomenclature = Introduction to Chemical Nomenclature / Per. from English N. N. Shcherbinovskaya, under the editorship of V. M. Potapova, R. A. Lidina. - M: Chemistry, 1983.- 224 p.
- Iliel E., Wylen S., Doyle M. Fundamentals of Organic Stereochemistry = Basic Organic Stereochemistry / Transl. from English Z. A. Bredikhina, ed. A.A. Bredikhina. - M .: Binom. Laboratory of Knowledge, 2007. - 703 p. - ISBN 978-5-94774-370-8 .