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  4. A compound 'X' neither reacts with sodium, displacing hydrogen nor with phosphorus pentachloride to give hydrogen chloride. X reduces an alkaline solution of Cu (II) salt on gentle warming. The structure of X is

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Q. A compound $ 'X' $ neither reacts with sodium, displacing hydrogen nor with phosphorus pentachloride to give hydrogen chloride. $ X $ reduces an alkaline solution of $ Cu \,(II) $ salt on gentle warming. The structure of $ X $ is

AMUAMU 2011Aldehydes Ketones and Carboxylic Acids

Solution:

Compound $X$ cannot be primary or secondary alcohol because both react with sodium displacing hydrogen and $PCl_5$ to give hydrogen chloride.
$2ROH + 2Na \rightarrow 2RO^- Na^+ + H_2 \uparrow$
$2ROH + PCl_5 \rightarrow \underset{\text{hydrogen chloride}}{RCl + POCl_3 + HCl}$
$X$ is an aldehyde and not a ketone because it reduces Fehling’s solution, ie. alkaline solution of $Cu (II)$ salt.
$\underset{\text{aldehyde}}{RCHO} + \underset{\text{Fehling’s solution }}{2Cu^{2+} + OH^-} \rightarrow \underset{\overset{\text{carboxylate}}{\text{ion}}}{RCOO^-}$
$ + \underset{\overset{\text{cuprous oxide}}{\text{(red ppt.)}}}{Cu_2O \downarrow } + 3H_2O$