Write a note on Haloform reaction.

Answer:

Aldehydes and ketones having a CH₃–CO– (methyl keto) group, i.e. acetaldehyde and methyl ketones, react with a halogen in the presence of sodium hydroxide (sodium hypohalite, NaOX) to give a haloform (CHX₃) and the sodium salt of a carboxylic acid with one carbon atom less. This is the haloform reaction.

The three H atoms of the methyl group are replaced by halogen, and the trihalomethyl ketone is then cleaved by OH^–:

CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ + CH₃COONa + 3NaI + 3H₂O

With iodine, yellow iodoform (CHI₃) is precipitated; this iodoform test is used to detect the CH₃CO– group (and the CH₃CH(OH)– group of alcohols such as ethanol, which are first oxidised). The reaction is also used to convert methyl ketones into carboxylic acids with one carbon less.