What is Diastereomers?


– We have defined stereoisomers as isomers whose atoms are bonded together in the same order but differ in how the atoms are directed in space.

– We have also considered enantiomers (mirror-image isomers) in detail.

– All other stereoisomers are classified as diastereomers, which are defined as stereoisomers that are not mirror images.

– Most diastereomers are either geometric isomers or compounds containing two or more chirality centers.

Cis-trans Isomerism on Double Bonds

– We have already seen one class of diastereomers, the cis-trans isomers, or geometric isomers.

– For example, there are two isomers of but-2-ene:

– These stereoisomers are not mirror images of each other, so they are not enantiomers. They are diastereomers.

 Cis-trans Isomerism on Rings

– Cis-trans isomerism is also possible when there is a ring present.

– Cis- and trans-1,2-dimethylcyclopentane are geometric isomers, and they are also diastereomers.

– The trans diastereomer has an enantiomer, but the cis diastereomer has an internal mirror plane of symmetry, so it is achiral.

What is Diastereomers?

Diastereomers of Molecules with Two or More Chirality Centers

– Apart from geometric isomers, most other compounds that show diastereomerism have two or more chirality centers, usually asymmetric carbon atoms.

– For example, 2-bromo-3-chlorobutane has two asymmetric carbon atoms, and it exists in two diastereomeric forms (shown next).

– Make molecular models of these two stereoisomers.

What is Diastereomers?

– These two structures are not the same; they are stereoisomers because they differ in the orientation of their atoms in space.

– They are not enantiomers, however, because they are not mirror images of each other: C2 has the (S) configuration in both structures, while C3 is (R) in the structure on the left and (S) in the structure on the right.

– The C3 carbon atoms are mirror images of each other, but the C2 carbon atoms are not.

– If these two compounds were mirror images of each other, both asymmetric carbons would have to be mirror images of each other.

– Since these compounds are stereoisomers but not enantiomers, they must be diastereomers.

– In fact, both of these diastereomers are chiral and each has an enantiomer.

– Thus, there is a total of four stereoisomeric 2-bromo-3 chlorobutanes: two pairs of enantiomers. Either member of one pair of enantiomers is a diastereomer of either member of the other pair.

What is Diastereomers?

– We have now seen all the types of isomers we need to study, and we can diagram their relationships and summarize their definitions

SUMMARY: Types of Isomers

What is Diastereomers?

Isomers are different compounds with the same molecular formula.

Constitutional isomers are isomers that differ in the order in which atoms are bonded together. Constitutional isomers are sometimes called structural isomers because they have different connections among their atoms.

Stereoisomers are isomers that differ only in the orientation of the atoms in space.

Enantiomers are mirror-image isomers.

Diastereomers are stereoisomers that are not mirror images of each other.

Cis-trans isomers (geometric isomers) are diastereomers that differ in their cis-trans arrangement on a ring or double bond.

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