(R)-Butan-2-ol is treated with $SOCl X_{2}$ in the presence of pyridine. Pyridine ensures that the reaction proceeds by an $S_{N} 2$ pathway with inversion at the stereocentre. The product is

Answer: D

Priorities in butan-2-ol are

OH > CH X_{2} CH X_{3} > CH X_{3} > H

; in 2-chlorobutane they are

Cl > CH X_{2} CH X_{3} > CH X_{3} > H

, the same order. Since

Cl

takes the place of

OH

with inversion of the spatial arrangement, the descriptor changes from R to S: (S)-2-chlorobutane.