Base catalyzed reactions of carbonyl compounds II

  • Cannizzaro’s Reaction:

            Those carbonyl compounds with no α-hydrogen undergo  disproportionation reaction in the  presence of a strong base to form 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.

i.          The applicability of Cannizzaro reaction in organic synthesis is limited as the yield is not more than 50% for either acid or alcohol formed.

ii.         In case of aldehydes that do have α-hydrogens, the aldol condensation reaction takes place preferentially.

iii.        The α, α, α-Trihalo aldehydes undergo haloform reaction in strongly alkaline medium. e.g. Choral will give chloroform in presence of an alkali.


            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 H2O 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 Cannizzaro’s reaction takes place very slowly when electron-donating groups are present. But the reaction occurs at faster rates when electron withdrawing groups are present.