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Sunday, November 10, 2019

MECHANISM OF CANNIZARO REACTION:


Step – (1):
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.
Step – (2):
In this second step, the hydroxide behaves as a base. Now a hydride ion, H- is transferred either from the mono anionic 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.

CONDITION FOR CANNIZARO REACTION:


(1) Primary condition of cannizaro reaction is absence of alpha hydrogen in aldehyde but (CH3)2CH-CHO gives cannizaro reaction although it has one alpha hydrogen.
(2) CCl3-CHO does not give cannizaro reaction while it has no alpha hydrogen it give halo form reaction.
(3) The overall order of the reaction is usually 3.
(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.

(5) Transfer of hydride is rate determining step.
(6) In cannizaro reaction kinetic isotopic  effect is observed
Step -(1):
Step-(2): 



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.
(3) Furfural gives furoic acid and furfuryl alcohol in presence of strong alkali.