Question

The correct order of rate of reaction towards pinacol pinacolone rearrangement is:

A.1>2>3
B.2>1>3
C.3>2>1
D.1>3>2

Answer 1

In pinacol pinacolone rearrangement intermediate form is carbocation so out of these three compounds which form the most stable intermediate i.e. carbocation which is more reactive towards pinacol pinacolone rearrangement. So rate of reaction depends upon the stability of intermediate forms.

Complete step by step answer:
As we know, in pinacol pinacolone rearrangement intermediate formed is carbocation hence reactivity or rate of this compound towards pinacol pinacolone rearrangement depends upon stability of that carbocation.So first see in which compound stable carbocation forms.
Consider compound (1)

Compound (2)

Compound(3)

Now, carbocation formed in compound (1) is stabilized due to resonance formed by one Ph group,
Carbocation formed in compound (2) is stabilized due to resonance formed by two Ph group
Carbocation formed in compound (3) isn't stabilized due to resonance.
Therefore carbocation formed in compound (2) is more stable while carbocation formed in compound (3) is less stable.
Hence correct order of stability of carbocation is (2)> (1)> (3)
And therefore, on the basis of that stability of carbocation intermediate the correct order of rate towards pinacol pinacolone rearrangement is (2)> (1)> (3).
Hence the correct option is option B.

Note:
Pinacol pinacolone rearrangement mechanism proceed via four steps
Step1: Since the reaction carried out in the acidic medium hydroxide group of pinacol is protonated by acid.
Step2: Water is removed from the compound and stable carbocation is formed.
Step3: Methyl group shifts to positively charged carbon in rearrangement of compound.
Step3: Oxygen atom which is doubly bonded to the carbon is now deprotonated and gives corresponding pinacolone.