Question

Explain the mechanism of addition of $HCN$ to a carbonyl group in presence of a base.

Answer 1

Base reacts with $HCN$ to form $C{N}^{-}$ ion.
$B+HCN\to B{H}^{+}+C{N}^{-}$
$C{N}^{-}$ ion acts as nucleophile and attacks carbonyl carbon
$C{N}^{-}+R-\underset{\underset{O}{||}}{C}-R^{\prime }\to R-\underset{\underset{O_{-}}{|}}{C}(-R^{\prime })-CN$
The anion thus formed accepts a proton from water to form cyanohydrin.
$H_{2}O+R-\underset{\underset{O_{-}}{|}}{C}(-R^{\prime })-CN\to R-\underset{\underset{OH}{|}}{C}(-R^{\prime })-CN+O{H}^{-}$