Question

What do you mean by pro-R and pro-S?

Answer 1

A prochiral molecule is a molecule that itself is achiral but can be converted into chiral molecule in one step.

Thus, butane is achiral. It can be converted to 2-chlorobutane which is chiral. The C atom (to which Cl atom is attached) is chiral. Hence, butane is a prochiral molecule.

Two H atoms are attached to second C atom of butane. Either of these can be replaced with chlorine.

Replacement of one of H atom attached to second C atom of butane will give R-2-chlorobutane. This H atom of second C atom of butane is called "pro-R".

Replacement of another H atom attached to second C atom of butane will give S-2-chlorobutane. This H atom of second C atom of butane is called "pro-S".

Thus, pro-R is that substituent on prochiral molecule which when substituted with some other group will give R configuration.

Thus, pro-S is that substituent on prochiral molecule which when substituted with some other group will give S configuration.