Cannizzaro Reaction Mechanism

1.0Introduction

The Cannizzaro reaction is a redox reaction in which two molecules of a non-enolizable aldehyde (i.e., an aldehyde without an α-hydrogen) undergo disproportionation in the presence of a strong base (like NaOH or KOH), producing:

• One molecule of carboxylic acid (as its salt),
• One molecule of primary alcohol.

Disproportionation = One molecule is oxidized, the other is reduced.

General Reaction:

2 RCHO + OH⁻ → RCOO⁻ + RCH₂OH

2.0Conditions for Cannizzaro Reaction

• The aldehyde must not contain any α-hydrogen.
• A strong base such as NaOH or KOH is used.

Common examples of such aldehydes:

• Formaldehyde (HCHO)
• Benzaldehyde (C₆H₅CHO)

Alpha hydrogen (α-H) is the hydrogen atom attached to the α-carbon, i.e., the carbon adjacent to the carbonyl group (C=O).
If an aldehyde has an α-hydrogen, it can form an enolate and typically undergoes aldol condensation instead.

However, if no α-hydrogens are present, the Cannizzaro reaction takes place, as enolate formation is not possible.

3.0Cannizzaro Reaction Mechanism

The mechanism explains how two aldehyde molecules yield one alcohol and one acid. In 1853, Stanislao Cannizzaro obtained benzyl alcohol and potassium benzoate from benzaldehyde.

 A hydroxide ion attacks the aldehyde carbonyl, forming a tetrahedral intermediate. This intermediate collapses, reforming the carbonyl and transferring a hydride ion to another aldehyde molecule. This results in one carboxylate ion and one alcohol ion, which receives a proton from the solvent. The acid product often requires post-treatment with acid to convert carboxylate to carboxylic acid. The reaction is third order overall: second order in aldehyde, first order in base.

Rate = k[RCHO]²[OH⁻]

At high base concentration, a second pathway becomes significant:

Rate = k[RCHO]²[OH⁻] + k’[RCHO]²[OH⁻]²

The second term involves the reaction of a divalent anion (RCHO²⁻) with another aldehyde. Deuterium labeling in D₂O confirms direct hydride transfer, as no deuterium appears at the alcohol’s alpha carbon.

Step 1: Nucleophilic Attack by Hydroxide

Hydroxide ion (OH⁻) attacks the carbonyl carbon of one molecule of the aldehyde, forming a tetrahedral alkoxide intermediate.

Step 2: Hydride Ion Transfer

The alkoxide ion formed donates a hydride ion (H⁻) to a second molecule of the aldehyde.

Step 3: Formation of Products

• The first molecule becomes a carboxylate ion (C₆H₅COO⁻).
• The second molecule gets reduced to benzyl alcohol (C₆H₅CH₂OH).

Overall Reaction

C₆H₅CHO + OH⁻ → C₆H₅CH₂OH + C₆H₅COO⁻

If KOH is used, the salt formed will be potassium benzoate (C₆H₅COOK).

4.0Cross Cannizzaro Reaction

When two different aldehydes without α-hydrogens are used in the reaction, it's called the Cross Cannizzaro Reaction.

In a typical Cannizzaro reaction, only 50% of the aldehyde is converted to alcohol and acid, making it inefficient. In the crossed Cannizzaro variant, a sacrificial aldehyde like formaldehyde is used as the reductant, increasing the yield of the desired alcohol from a more valuable aldehyde. Formaldehyde is oxidized to sodium formate while the target aldehyde is reduced to alcohol. This improves efficiency and product yield when dealing with non-enolizable aldehydes.

A common example:

Mixing formaldehyde and benzaldehyde in NaOH.

• Formaldehyde is preferentially oxidized to formate (HCOO⁻).
• Benzaldehyde is reduced to benzyl alcohol.

5.0Uses of Cannizzaro Reaction

• Preparation of alcohols and acids from aldehydes that cannot undergo aldol condensation.
• Useful in organic synthesis where selective reduction and oxidation of aldehydes are needed.
• Used for studying mechanisms of redox organic reactions in academic chemistry.

Industrially, the Crossed Cannizzaro reaction combined with aldol condensation helps produce valuable polyols.

• Neopentyl glycol: used in polyesters for resins, paint coatings, lubricants, and plasticizers; its structure gives high resistance to light, heat, and hydrolysis.
• Pentaerythritol: used in explosives and paints; its esters serve as emulsifiers, lubricants, and emollients.
• Trimethylolpropane: often replaces glycerol in manufacturing alkyd resins, polyesters, and polyurethanes.