.:- 1,2-diols, known as pinacols, also undergo a similar type of dehydration reaction when treated with acid to furnish a ketone or aldehyde which have rearranged structure
compared to the initial diol. This transformation is commonly referred to as Pinacol
Rearrangement. Let’s take an example – when pinacol reacts with an acid, it leads to the
formation of pincolone. It is believed that this reaction also involves the formation of a
tertiary carbocation in the first step which is followed by 1,2-shift of the methyl group.
Now, the question will be if we already have a stable carbocation, why should it undergo
rearrangement? The answer is - the oxygen lone pair - which can stabilize the newly
formed carbocation by donating its lone pair to carbocation, thereby completing octet for
all the atoms involved. In fact, one could view this reaction as a ‘push-pull’ reaction! The
carbocation ‘pulls’ the alkyl group along with its bonding electrons towards itself while
the oxygen lone pair ‘pushes’ the alkyl group away.