Diastereomers are stereoisomers ( chemical compounds of the same constitution but different configuration ) which - in contrast to enantiomers - do not behave like image and mirror image. Diastereomers can be both chiral and achiral. As a rule, diastereomers differ from one another in physical ( melting point , boiling point , solubility , NMR spectrum , IR spectrum , etc.) and chemical properties.
Diastereomerism due to multiple stereocenters
There is diastereomerism between two molecular compounds with the same constitution and several stereocenters if they differ in at least one, but not in all stereocenters (example: the sugars D - glucose and L - galactose , which differ in three of four stereocenters).
- If the two compounds differ in exactly one stereocenter, this diastereomerism is also called epimerism (example: D - glucose and D - galactose ).
- If the configuration is different in all stereocenters between the compounds, there is enantiomerism that is not counted as part of the diastereomerism (example: D- glucose and L- glucose).
Cause of chirality in one molecule is usually substituted carbon -atoms, the four different radicals carry. Due to the tetrahedral structure of the carbon atom, two configurations are possible at such a stereocenter , which cannot be brought into congruence even by rotation. If a compound contains n carbon atoms substituted in this way (i.e. n stereocenters), a maximum of 2 n configuration isomers can exist. Configuration isomers in which all asymmetric carbon atoms differ are enantiomers of which there are 2 n / 2 pairs. If the stereocenters in a molecule are of the same type, then there is a mirror plane in the molecule and the number of configuration isomers is smaller, since meso compounds are also present (see second figure). Diastereomers are mostly optically active , but meso compounds are achiral.
Unlike diastereomers, enantiomers have identical physical properties, but differ in the sign of their rotation value [(+) or (-)], in their chemical properties compared to other chiral molecules and thus also in their physiological properties.
Diastereomerism due to double bonds
( Z ) - ( E ) isomerism (also cis - trans isomerism) is part of diastereomerism and occurs due to the rigidity of double bonds due to π bonds . Double bonds and thus the bound groups cannot rotate freely. There are different spatial arrangements. Examples are the cis - trans isomerism of alkenes .
Diastereomerism in cyclic compounds
Also in unbridged bicyclic hydrocarbons (e.g., at. Cis - and trans -Decalin ) and substituted monocyclic compounds (e.g., at 1.2-dichloro. Cyclopentane ) occurs on diastereoisomerism; these two cases are often viewed as cis - trans isomerism , cf. above the section diastereomerism due to double bonds .
Chiral molecules differentiate between the possible enantiomers compared to other chiral molecules. This is because two different chiral molecules are in a diastereomeric relationship to one another. Diastereoselectivity occurs in a chemical reaction both between two chiral starting materials (or reagents ) and between a chiral and a prochiral or achiral starting material, so that one enantiomer is preferred on the product side. Once a chirality is present, it continues in subsequent systems.
In diastereoselective reactions, the different transition states of enantiomers also have different physical properties, which ultimately results in selectivity . Ultimately, enantioselective syntheses are always based on diastereoselective mechanisms, which is why the term diastereoselective synthesis is to be preferred.
- Entry on diastereoisomerism . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.D01679 Version: 2.3 ..
- Hans Beyer , Wolfgang Walter : Textbook of organic chemistry. 19th completely revised edition. S. Hirzel Verlag, Stuttgart 1981, ISBN 3-7776-0356-2 .
- Hans Rudolf Christen , Fritz Vögtle : Organic chemistry. From the basics to research. 2nd volume. 2nd Edition. Otto Salle Verlag et al., Frankfurt am Main 1996, ISBN 3-7935-5398-1 .
- Karl Schwister (Ed.): Pocket book of chemistry. 4th updated edition. Fachbuchverlag Leipzig in Carl-Hanser-Verlag, Munich 2010, ISBN 978-3-446-42211-7 .