Definition:
•In the presence of concentrated alkali , aldehydes containing no α - hydrogens undergo self oxidation & reduction (Redox) reaction to yield a mixture of an alcohol & a salt of carboxylic acid.
•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
Mechanism:
A nucleophile such as a hydroxide ion is used to attack the carbonyl group of the given aldehyde, causing a disproportionation reaction and giving rise to an anion carrying 2 negative charges.
This resulting intermediate can now function as a hydride reducing agent. Due to its unstable nature, the intermediate releases a hydride anion. This hydride anion proceeds to attack another aldehyde molecule. Now, the doubly charged anion is converted into a carboxylate anion and the aldehyde is converted into an alkoxide anion
In the final step water offers a proton to the alkoxide anion which gives rise to the final alcohol product. The reaction can proceed since the alkoxide is more basic than water. Now, the carboxylate ion gives rise to the final carboxylic acid product when acid workup is used (the acid workup is required since carboxylate is less basic than water and therefore cannot obtain a proton from water).
Application of Cannizzaro Reaction:
Both alcohols and organic acids are well known for their biological actions.
•Antibacterial properties.
•Preservatives for food
•pharmaceutical local application as antiseptics.
•Benzyl alcohol has some local anesthetic properties, it is useful as antipruritic and is the reason for its inclusion in some dental remedies and injectables in pharmaceutical preparations intended for local application, benzyl alcohol has been used up to 10 % in ointments as antipruritic and to prevent secondary infections.
Mechanism:
A nucleophile such as a hydroxide ion is used to attack the carbonyl group of the given aldehyde, causing a disproportionation reaction and giving rise to an anion carrying 2 negative charges.
This resulting intermediate can now function as a hydride reducing agent. Due to its unstable nature, the intermediate releases a hydride anion. This hydride anion proceeds to attack another aldehyde molecule. Now, the doubly charged anion is converted into a carboxylate anion and the aldehyde is converted into an alkoxide anion
In the final step water offers a proton to the alkoxide anion which gives rise to the final alcohol product. The reaction can proceed since the alkoxide is more basic than water. Now, the carboxylate ion gives rise to the final carboxylic acid product when acid workup is used (the acid workup is required since carboxylate is less basic than water and therefore cannot obtain a proton from water).
Application of Cannizzaro Reaction:
Both alcohols and organic acids are well known for their biological actions.
•Antibacterial properties.
•Preservatives for food
•pharmaceutical local application as antiseptics.
•Benzyl alcohol has some local anesthetic properties, it is useful as antipruritic and is the reason for its inclusion in some dental remedies and injectables in pharmaceutical preparations intended for local application, benzyl alcohol has been used up to 10 % in ointments as antipruritic and to prevent secondary infections.
