Question

Predict the configuration of A and B at chiral carbon.

• A. A- R; B-S
• B. A- S; B-R
• C. A- R; B-R
• D. A- S; B-S

Answer 1

The correct option is A A- R; B-S

$S_{N}i$ stands for Substitution Nucleophilic Intermolecular.
In this reaction when a chiral alcohol is reacted with $SOC{l}_2$, chlorination happens internally with retention of configuration.

But, when external agents are added (nucleophilic solvent like pyridine ) there is a inversion in the product because here the $C{l}^{-}$ attacks the chiral carbon from the back externally as a $S_{N}2$ mechanism.

By CIP rule, configuration of substrate is 'R'
Hence, configuration of A and B are R and S respectively.