Question

In which of the following reactions proceeds by $S_{N}1$ mechanism?
(I)$\left(A\right)+SOC{l}_2⟶$

(II) $\left(A\right)+COC{l}_2⟶$
(III) $\left(A\right)+SOC{l}_2\stackrel{Pyridine}{⟶}$
(IV) $\left(A\right)+COC{l}_2\stackrel{Pyridine}{⟶}$

• A. $\left(I\right),\left(III\right)$
• B. $\left(I\right),\left(II\right)$
• C. $\left(III\right),\left(IV\right)$
• D. $\left(I\right),\left(IV\right)$

Answer 1

The correct option is B $\left(I\right),\left(II\right)$

$SOC{l}_2$ and $COC{l}_2$ both replace $OH$ of alcohols by $S_{N}1$ mechanism. They corrdinate to the alcohol, with loss of $HCl$ and formation of a good leaving group (“chlorosulfite”). The chlorosulfite leaving group can spontaneously depart, forming a carbocation, and when it does so, an “intimate ion pair” is formed, where the carbocation and negatively charged leaving group are held tightly together in space. From here, the chlorine can act as a nucleophile attacking the carbocation on the same face from which it was expelled and after expulsion of $S{O}_2$, we have formation of an alkyl chloride with retention of configuration.