Explain
a) Clemmensen's reduction of aldehydes and ketones.
b) Tollen's test of aldehydes.

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Solution

a) The reaction of aldehydes and ketones with zinc amalgam ( $Z n / H g$ alloy) in concentrated hydrochloric acid, which reduces the aldehyde or ketone to a hydrocarbon, is called Clemmensen reduction.
ex. $R C O R^{'} + Z n - H g + H C l \to R C H_{2} R^{'}$

b) Tollens’ test: Also known as silver-mirror test, is a qualitative laboratory test used to distinguish between an aldehyde and a ketone. It exploits the fact that aldehydes are readily oxidized whereas ketones are not. Tollens’ test uses a reagent known as Tollens’ reagent, which is a colorless, basic, aqueous solution containing silver ions coordinated to ammonia $[A g (N H_{3})_{2}]^{+}$ . It is prepared using a two-step procedure.
Step 1: Aqueous silver nitrate is mixed with aqueous sodium hydroxide.
$A g N O_{3} + N a O H \to A g O H + H N O_{3}$
$2 A g O H \to A g_{2} O + H_{2} O$
Step 2: Aqueous ammonia is added drop-wise until the precipitated silver oxide completely dissolves.
$A g_{2} O + 4 N H_{3} + H_{2} O \to 2 A g (N H_{3})_{2}^{+} 2 O H^{-}$
Tollens’ reagent oxidizes an aldehyde into the corresponding carboxylic acid.
Ex. $C H_{3} C H O + 2 A g^{+} + 2 O H^{-} \to C H_{3} C O O H + 2 A g + H_{2} O$
The reaction is accompanied by the reduction of silver ions in Tollens’ reagent into metallic silver, which, if the test is carried out in a clean glass test tube, forms a mirror on the test tube.

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