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CANNIZZARO REACTION: DISPROPORTIONATION:


(1) Introduction:
(1) INTRODUCTION:
The aldehydes having absence of α-hydrogens when treated with concentrate strong base undergoes to disproportionate reaction and furnish an alcohol and a carboxylic acid is called Cannizzaro reaction. In this reaction one molecule of aldehyde is reduced to the corresponding alcohol, while a second one is oxidized to the carboxylic acid.

 (2) 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. 
(3) 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):
(4) ILLUSTRATIONS & EXAMPLES OF CANNIZZARO REACTION
(1) Formaldehyde is disproportionated to formic acid and methyl alcohol in strong alkali.
(3) Furfural gives furoic acid and furfuryl alcohol in presence of strong alkali.
(2) Benzaldehyde can be converted to benzoic acid and benzyl alcohol.

When a mixture of two different aldehyde (alpha hydrogen less)  like formaldehyde and a non enolizable aldehyde (benzaldehyde) 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.
Illustrative example: Benzyl alcohol and formic acid are obtained when a mixture of benzaldehyde and formaldehyde is treated with alkali.
Important note:
(1) In cross cannizaro reaction if one of the reactant is formaldehyde, then oxidation of formaldehyde take place , and reduction of another aldehyde take place .The reason may be: the initial nucleophillic addition of hydroxide anion is faster on formaldehyde as there are no electron donating groups on it.
(2) The preferential oxidation of formaldehyde in crossed Cannizzaro reactions may be utilized in the quantitative reduction of some aldehydes. 
(6) INTRAMOLECULAR CANNIZARO REACTION:
 α-keto aldehydes can be converted to α-hydroxy carboxylic acids by an intermolecular Cannizzaro reaction.
Illustrative Example: Phenylglyoxal undergoes intramolecular cannizzaro reaction by giving Mandelic acid (α-hydroxyphenylacetic acid or 2-Hydroxy-2-phenylethanoic acid)
Illustrative Example: Phthalaldehyde can undergo intramolecular Cannizzaro reaction by giving (o-hydroxymethyl) benzoic acid.

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