An Acid catalyzed rearrangement in which 1,2-diol or pinacole is converted to carbonyl compound is called pinacole- pinacolone rearrangement.

Pinacole : 2,3- dimethyl butane-2,3-diol 

Pinacolone :3,3-dimethyl-2- butanone

Reaction of Pinacole-pinacolone rearrangement:

Mechanism of pinacole-pinacolone rearrangement:

Mechanism of pinacole-pinacolone rearrangement takes place in four steps

Step(01):

In this step hydroxyl group of pinacole is protonated by acid because reaction takes place in acidic medium.

Step(o2):

In this step water is removed and forming a tertiary carbocation, there this carbocation is stable .

Step(03):

In this step, methyl shift takes place to positively charged carbon ion.

Step(04):

In this step oxygen atom is deprotonated giving our product i.e. pinacolone.

Unsymmetrical Pinacolone:

In unsymmetrical pinacolone reaction we form a stable carbocation and we show methyl or either phenyl shift. Thus it show migratory aptitude. Migratory aptitude order is

Here we form

• Stable carbocation
• Migration of suitable group
• Regioselectivity

Reaction:

Mechanism Of Unsymmetrical pinacolone:

In Unsymmetrical pinacolone two possibilities occurs

First Possibilty:

Second Possibility:

• More stable carbocation give more major product
• Less stable carbocation give minor product

Cyclic Pinacole-pinacolone rearrangement:

Cyclic pinacole-pinacolone rearrangement forms spirocylic ring systems. This is also called ring expansion rearrangement.

Mechanism:

Step(01)Protonation:

In this step protonation of hydroxyl group takes place in acidic medium as follows;

Step(02)Deprotonation:

In this step removal of water takes place for the formation of carbocation.

Step(03)Methyl shift:

In this step methyl shift takes place for the formation of stable carbocation.

Step(04):

In this step deprotonation of oxygen takes place forming C=O group.