Everything about Diastereoisomer totally explained
» Erythro redirects here. For the fictional planet, see Erythro (Asimov).Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (nonsuperimposable mirror images of each other). Diastereomers can have different physical properties and different reactivity. In another definition diastereomers are pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other .
In simpler terms two stereoisomers are said to be diastereoisomers if they're not mirror images of each other and
one or more stereogenic centres differ between the two stereoisomers. According to this same definition,
cis-trans isomerism is a form of diastereomerism.
If a
molecule contains a single asymmetric
carbon atom or
stereocenter, it'll have two mirror image forms. If a molecule contains two asymmetric carbons, there are up to 4 possible configurations, and they can't all be mirror images of each other. The possibilities continue to multiply as there are more asymmetric centers in a molecule. In general, the number of diastereomers of a molecule can be determined by calculating 2
n, where
n=the number of
chiral centers in the molecule. This holds true except in cases where the molecule has
meso forms.
Tartaric acid contains two asymmetric centers, but two of the "isomers" are equivalent and together are called a
meso compound. This configuration isn't
optically active, while the remaining two isomers are
D- and
L- mirror images,
for example, enantiomers. The meso form is a diastereomer of the other forms.
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(natural) tartaric acid
L-(+)-tartaric acid
dextrotartaric acid
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D-(-)-tartaric acid
levotartaric acid
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mesotartaric acid
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(1:1)
DL-tartaric acid
"racemic acid"
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The families of 4, 5 and 6 carbon
carbohydrates contain many diastereomers because of the large numbers of asymmetric centres in these molecules.
Two common prefixes used to distinguish diastereomers are
threo and
erythro. When drawn in the
Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sites.
Cis-trans isomerism and
conformational isomerism are also forms of diastereomerism.
Diastereoselectivity is the preference for the formation of one or more than one diastereomer over the other in an
organic reaction.
Applications
As stated, two enantiomers will have identical physical properties, while diastereomers will not. This knowledge is harnessed in
chiral synthesis to separate a mixture of enantiomers. This is the principle behind
chiral resolution. After preparing the diastereomers, they're separated by
chromatography or
recrystallization.
Further Information
Get more info on 'Diastereoisomer'.
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