Question

Which of the following reaction of `(S)-CH_(3)CHD(OH)` proceeds with retention of configuration ?
A. Reaction with `HCl`
B. Reaction with `SOCl_(2)` in the presence of pyridine
C. Reaction with `SOCl_(2)` in a nonpolar solvent with no added base
D. both (1) and (2)

Answer 1

Correct Answer - C
The given alcohol is a `1^(@)` alcohol. With `HCl` , it undergoes `S_(N)2` mechanism . Thus , the product is `R` with inversion of configuration.
With `SOCl_(2)` , reaction proceeds via chlorosulphite. In the absence of base as pyridine, the reaction proceeds with retention of configuration , i.e.,`S_(N)i` type (subsitution , nucleopilic , internal):

In the presence of pyridine, the chloride has the inverted configuration as `S_(N)2` mechanism is operating through the involvement of a pyridine complex :
`ROSOCl + underset("Pyridine")(C_(5)H_(5)N)overset("fast")rarrCl^(-)+ROSOoverset(+)(N)C_(5)H_(5)underset(S_(N)2)overset("slow")rarr`
`[overset(delta-)(Cl)* * *R* * *overset(delta-)OSOoverset(+)NC_(5)H_(5)]overset("fast")rarrCl - R+SO_(2)+C_(5)H_(5)N`