* 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.
* The Cannizzaro reaction is initiated by the nucleophilic attack of a hydroxide ion on the carbonyl carbon of an aldehyde molecule by giving a hydrate anion, which can be further deprotonated to give a dianion in a strongly alkaline medium. Note that, in the 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.
1) Formaldehyde is disproportionated to formic acid and methyl alcohol in strong alkali.
2) Benzaldehyde can be converted to benzoic acid and benzyl alcohol.
3) Furfural gives furoic acid and furfuryl alcohol in presence of strong alkali.
4) Crossed Cannizzaro reaction: When a mixture of formaldehyde and a non enolizable aldehyde is treated with a strong base, the later is preferentially reduced to alcohol while formaldehyde is oxidized to formic acid. This variant is known as crossed Cannizzaro reaction.
E.g. Benzyl alcohol and formic acid are obtained when a mixture of benzaldehyde and formaldehyde is treated with alkali.
The reason may be: the initial nucleophilic addition of hydroxide anion is faster on formaldehyde as there are no electron donating groups on it.
The preferential oxidation of formaldehyde in crossed Cannizzaro reactions may be utilized in the quantitative reduction of some aldehydes.
5) α-keto aldehydes can be converted to α-hydroxy carboxylic acids by an intermolecular Cannizzaro reaction.
E.g. Phenylglyoxal undergoes intermolecular cannizzaro reaction by giving Mandelic acid (α-hydroxyphenylacetic acid or 2-Hydroxy-2-phenylethanoic acid)
6) Phthalaldehyde can undergo intramolecular Cannizzaro reaction by giving (o-hydroxymethyl) benzoic acid.
Chloral, CCl3CHO undergoes haloform reaction instead of Cannizzaro reaction to give chloroform and formate anion since the CCl3- group is a good leaving group rather than the hydride ion, H-.