When are stereoisomers diastereomers?
1 Answer
Stereoisomers are diastereomers when they (a) have at least two chiral centres, (b) differ in the configuration of one or more (but not all) stereocentres, and (c) are not mirror images of each other.
Consider the four structures in the image below.
Let's number the structures clockwise from 1 to 4 starting at the upper left corner.
(1,2), (2,3), (3,4), and (1,4) are pairs of diastereomers. They all differ in the configuration at one stereocentre, and they are not mirror images of each other.
(1,3) and (2,4) are not diastereomeric pairs. They differ in the configuration at all stereocentres, and they are nonsuperimposable mirror images of each other. They are pairs of enantiomers.
