Aldehydes and ketones react with vicinal diols (1,2-diols) in the presence of dry HCl gas to form cyclic acetals and ketals, respectively [1].

This reaction is stereoselective:

1. cis-cyclopenta-1,2-diol: The two hydroxyl (-OH) groups are on the same side of the ring. They are spatially close enough to react with Acetone to form a stable five-membered cyclic ketal (isopropylidene derivative).
2. trans-cyclopenta-1,2-diol: The two hydroxyl groups are on opposite sides of the rigid cyclopentane ring. They cannot come close enough to bridge with the acetone molecule to form a cyclic ketal due to ring strain.

Therefore, Acetone acts as the distinguishing reagent; it reacts with the cis isomer but not the trans isomer.