Pinacol-Pinacolone Rearrangement: Mechanism & Examples | Chemca

Organic Chemistry

Pinacol-Pinacolone Rearrangement

By Chemca Editorial Team • Last Updated: January 2026 • 11 min read

The Pinacol-Pinacolone Rearrangement is an acid-catalyzed reaction that converts 1,2-diols (vicinal diols) into carbonyl compounds (ketones or aldehydes) via a 1,2-migration. The reaction is named after the classic conversion of Pinacol to Pinacolone.

1. General Reaction

Pinacol (2,3-Dimethylbutane-2,3-diol) is treated with a mineral acid like $H_2SO_4$ to yield Pinacolone (3,3-Dimethylbutan-2-one).

$$ \underset{\text{Pinacol}}{(CH_3)_2C(OH)-C(OH)(CH_3)_2} \xrightarrow{H^+} \underset{\text{Pinacolone}}{CH_3-CO-C(CH_3)_3} + H_2O $$

Driving Force: The formation of a stable carbonyl group ($C=O$) from a carbon-oxygen single bond.

2. Detailed Mechanism

The mechanism involves four key steps involving carbocation intermediates.

Step 1: Protonation

One of the hydroxyl groups accepts a proton from the acid to form an oxonium ion.

$$ (CH_3)_2C(OH)-C(OH)(CH_3)_2 + H^+ \rightleftharpoons (CH_3)_2C(OH)-C(\overset{+}{O}H_2)(CH_3)_2 $$

Step 2: Loss of Water (Carbocation Formation)

Water leaves, generating a tertiary carbocation. This is generally the rate-determining step for symmetrical diols.

$$ (CH_3)_2C(OH)-C(\overset{+}{O}H_2)(CH_3)_2 \rightarrow (CH_3)_2C(OH)-\overset{+}{C}(CH_3)_2 + H_2O $$

Step 3: 1,2-Alkyl Shift (Rearrangement)

A methyl group migrates from the adjacent carbon to the cationic center. This migration creates a new carbocation that is stabilized by resonance from the oxygen lone pair.

$$ (CH_3)_2C(OH)-\overset{+}{C}(CH_3)_2 \xrightarrow{\text{1,2-Me Shift}} CH_3-\overset{+}{C}(OH)-C(CH_3)_3 $$

Note: The new carbocation is actually an oxonium ion resonance structure: $CH_3-C(=\overset{+}{O}H)-C(CH_3)_3$.

Step 4: Deprotonation

Loss of a proton from the oxygen yields the final ketone product.

$$ CH_3-C(=\overset{+}{O}H)-C(CH_3)_3 \rightarrow CH_3-CO-C(CH_3)_3 + H^+ $$

3. Migratory Aptitude

General Order

When the diol is unsymmetrical, the group that migrates is determined by its Migratory Aptitude. In general:
Aryl > Alkyl (Tertiary > Secondary > Primary) > Methyl

Note: Electron-donating groups on the aryl ring increase migratory aptitude (e.g., p-Anisyl > Phenyl > p-Nitrophenyl).

Wait! Before migration, decide which -OH protonates first. The -OH is removed from the carbon that produces the most stable initial carbocation (e.g., benzylic or tertiary).

4. Ring Expansion

Cyclic diols undergo ring expansion during this rearrangement.

Example: Cyclohexane-1,2-diol derivative

A 5-membered ring with a $-\overset{+}{C}H_2$ group adjacent to it will expand to a 6-membered ring to relieve ring strain and achieve stability.

5. Solved Example: Unsymmetrical Diol

Substrate: $Ph-C(OH)(Ph)-C(OH)(CH_3)_2$

Protonation Site: Stable carbocation forms at $Ph-C-Ph$. So, remove OH from the $Ph_2$ carbon. Carbocation: $Ph_2C^+ - C(OH)Me_2$.
Migration: A Methyl group migrates from C2 to C1.
Product: $Ph_2C(Me)-CO-Me$ (3,3-Diphenylbutan-2-one).

Pinacol Rearrangement Quiz

Test your concepts on 1,2-shifts and carbocations. 10 MCQs with explanations.

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