Which of the following has an achiral stereoisomer: 2,3-dichlorobutane; 2,3-dichloropentane; 2,3-dichloro-2,3-dimethylbutane; 1,3-dichlorocyclopentane; 1,3-dibromocyclobutane?

1 Answer
Dec 5, 2015

This really asks you what an achiral stereoisomer is. Once you know that and you draw out the srtuctures, it's really not too difficult.

Simply put, if you have a mirror plane down the middle of the molecule and corresponding chiral carbons on either side of the plane have identical substituents, then you have the possibility for a net cancellation of the rotation of plane-polarized light. In other words, optical inactivity, or an overall achiral molecule.

So, which of these molecules is symmetric? All except the second one. (It doesn't matter whether you choose to draw out the wedges/hashes or not.)