26 Cannizzaro Reaction The disproportionation reaction of aldehydes without α-hydrogens in presence of a strong base to produce an alcohol and a carboxylic.

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Presentation transcript:

26 Cannizzaro Reaction The disproportionation reaction of aldehydes without α-hydrogens in presence of a strong base to produce 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-hydrogen, the aldol condensation reaction takes place preferentially.

Mechanism of Cannizzaro reaction:

Limitation Cannizzaro reaction is limited to aldehydes such as formaldehyde and benzaldehyde, which has no hydrogen on the carbon next to the –CHO group (no α hydrogen).

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) Crossed Cannizzaro reaction: When a mixture of formaldehyde and a non enolizable aldehyde (aldehyde with no α-hydrogens) 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.

Procedure Dissolve 7.2gm of KOH in 7ml D.W. in a beaker. Cool the solution and pour into a 125ml reagent bottle. Add 7.3ml benzaldehyde cork the bottle securely and shake the mixture vigorously until it has been converted to a thick emulsion. Allow the mixture to stand for 24hr or longer. By which the time the reaction should have been completed. After this period, add about 30ml water to completely dissolve potassium benzoate. Transfer everything to a separation funnel, rinse the bottle with 10ml of ether and again add to the solution in the separating funnel. Shake the separating funnel well, separate the lower aqueous layer.  (Add 10ml ether, then extract the aqueous layer) repeat this addition of ether and extraction of aqueous layer for two times.

Extraction of Benzoic acid from aqueous layer: Prepare a mixture of (20ml of conc. HCl + 20ml water + Crushed ice). Pour the aqueous solution into this the mixture with stirring. Benzoic acid precipitates in this time. Filtrate the precipitated benzoic acid. To remove the remaining ppt., wash the flask with cold water. Dry the filtrate and weigh your product.