How can diastereomers be chiral?

1 Answer
Jun 19, 2016

Diastereomers are geometric isomers that are NOT mirror images.

Explanation:

Given this definition, it is fairly easy to suggest examples. Now take #"cis-butene"# versus #"trans-butene"#. The connectivity is the same for each isomer: #C1# connects to #C2# ........to #C4#, however, these isomers are manifestly geometrically different.

If we go to an organic molecule with 2 stereocentres, which I acknowledge was the subject of your question, there could be #R,R# versus #S,S#, which are clearly optical isomers; but there could also be the other pair, #R,S#, and #S,R#. Sometimes #S,R# is the same as #R,S#, i.e. in a symmetric meso compound. We would say that #R,R# and #R,S# share a diastereomeric relationship; i,e. same connectivity but different geometry.

It is worth trying to get your head around this, and also practise building models, and representing those models on the printed page.