Cannizzaro Reaction:

• The disproportionation reaction of aldehydes without α-hydrogens in presence of a strong base to furnish an alcohol and a carboxylic acid is called Cannizzaro reaction. One molecule of aldehyde is reduced to the corresponding alcohol, while a second one is oxidized to the carboxylic acid.

• The applicability of Cannizzaro reaction in organic synthesis is limited as the yield is not more than 50% for either acid or alcohol formed.
• In case of aldehydes that do have α-hydrogens, the aldol condensation reaction takes place preferentially.
• The α,α,α-Trihalo aldehydes undergo haloform reaction in strongly alkaline medium. E.g. Choral will give chloroform in presence of an alkali.

MECHANISM OF CANNIZZARO REACTION:

• The cannizzaro reaction is initiated by the nucleophilic attack of a hydroxide ion to the carbonyl carbon of an aldehyde molecule by giving a hydrate anion. This hydrate anion can be deprotonated to give an anion in a strongly alkaline medium. In this second step, the hydroxide behaves as a base.

• Now a hydride ion, H^- is transferred either from the monoanionic species or dianionic species onto the carbonyl carbon of another aldehyde molecule. The strong electron donating effect of O^- groups facilitates the hydride transfer and drives the reaction further. This is the rate determining step of the reaction.

• Thus one molecule is oxidized to carboxylic acid and the other one is reduced to an alcohol.
• When the reaction is carried out with D₂O as solvent, the resulting alcohol does not show carbon bonded deuterium. It indicates the hydrogen is transferred from the second aldehyde molecule, and not from the solvent. 
• The overall order of the reaction is usually 3 or 4.
• The Cannizzaro reaction takes place very slowly when electron-donating groups are present. But the reaction occurs at faster rates when electron withdrawing groups are present.

ILLUSTRATIONS & EXAMPLES OF CANNIZZARO REACTION:

1) Formaldehyde is disproportionated to formic acid and methyl alcohol in strong alkali.

2) Benzaldehyde can be converted to benzoic acid and benzyl alcohol.

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