Describe the following:
A) Cannizzaro reaction
B) Decarboxylation

Answer

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Hint: Cannizzaro reaction involves the formation of alcohol and salt of an acid from aldehyde in presence of a strong base.
Decarboxylation involves the release of

C O_{2}

from an acid to give alkane.

Complete answer:
A) Cannizzaro reaction: -
It is the formation of alcohol from aldehyde. It was first discovered by Stanislao Cannizzaro in 1853. When two molecules of the same aldehyde which have no

α

-hydrogen atom undergo disproportionation or redox reaction in the presence of strong base

(N a O H / K O H)

to give a molecule of alcohol and a molecule of the salt of an acid.
Here, one aldehyde is oxidised to give acid and another aldehyde is reduced to give alcohol simultaneously.
When two molecules of formic acid are reacted in presence of NaOH, methanol and methanoic acid is produced.

2 H C H O + N a O H \to H C H_{2} O H + H C O O H

When two molecules of benzaldehyde (

C_{6} H_{5} C H O

) react with KOH, it will give benzyl alcohol (

C_{6} H_{5} C H_{2} O H

) and benzoic acid (

C_{6} H_{5} C O O H

2 C_{6} H_{5} C H O + K O H \to C_{6} H_{5} C H_{2} O H + C_{6} H_{5} C O O H

B) Decarboxylation: -
Generally, carboxylation refers to the addition of carbon dioxide in a compound and decarboxylation is the removal of carbon dioxide from a compound. In this reaction carboxylic acid is treated with soda lime (a mixture of NaOH and CaO) to form alkane with the release of carbon dioxide.

R C O O H \overset{N a O H + C a O}{\to} R - H + C O_{2}

For example, when benzoic acid is reacted with soda lime it will give benzene with the release of carbon dioxide gas.

C_{6} H_{5} O H \overset{N a O H + C a O}{\to} C_{6} H_{6} + C O_{2}

Note: The carbon atom that is directly bonded to a carbonyl group is called an alpha carbon. Any hydrogen that is attached to an alpha carbon is called alpha hydrogen. These hydrogens are generally acidic in nature and can react with bases.
Decarboxylation is used in many named reactions like Barton decarboxylation, Kolbe electrolysis, Kochi reaction,Hunsdiecker reaction etc. In ketonic decarboxylation, a carboxylic acid is converted to a ketone.

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