Question

How can diastereomers be chiral?

Answer 1

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.