How does acetaldehyde react with the following reagents ?

Dilute aqueous caustic soda

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Hydrazine

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Ammoniacal silver nitrate

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Phosphorus pentachloride.

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Solution

The correct option is A Dilute aqueous caustic soda
(a) Aldehydes and ketones having at least one $α$ -hydrogen undergo a reaction in the presence of dilute aqueous caustic soda as catalyst to form $α$ -hydroxy aldehydes (aldol). This is known as aldol reaction.
$2 C H_{3} - C H O A c e t a l d e h y d e D i l . N a O H \to C H_{3} - C | O H H - C H_{2} - C H O 3 - h y d r o x y b u t a n a l^{+} C H_{3} - C H = C H - C H O B u t - 2 - e n a l$
(b) The carbonyl group of acetaldehyde reacts with hydrazine to produce a hydrazone in which the carbon is double bonded to nitrogen.
(c) When acetaldehyde is warmed with freshly prepared ammoniacal silver nitrate solution, a bright silver mirror is produced. The aldehyde gets oxidized to acetate ion and silver ion is reduced to metallic silver.
$C H_{3} C H O + 2 [A g (N H_{3})_{2}]^{-} + 3 O H^{-} \to C H_{3} C O O^{-} A c e t a t e i o n + 2 A g (s i l v e r m i r r o r) ↓ + 2 H_{2} O + 4 N H_{3}$
(d) Acetaldehyde on reaction with Phosphorus pentachloride, gives gem-dichloride.
$C H_{3} C H O A c e t a l d e h y d e + P C l_{5} P h o s p h o r u s p e n t a c h l o r i d e \to C H_{3} C H C l_{2} g e m - d i c h l o r i d e + P O C l_{3}$ .

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