Question

A compound ${}^{\prime }{X}^{\prime }$ 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

• A. primary alcohol
• B. secondary alcohol
• C. a ketone
• D. an aldehyde

Answer 1

Answer: (D)

Compound $X$ cannot be primary or secondary alcohol because both react with sodium displacing hydrogen and $PC{l}_5$ to give hydrogen chloride.
$2ROH+2Na\to 2R{O}^{-}N{a}^{+}+H_2\uparrow$
$2ROH+PC{l}_5\to \underset{\text{hydrogen chloride}}{RCl+POC{l}_3+HCl}$
$X$ is an aldehyde and not a ketone because it reduces Fehling’s solution, ie. alkaline solution of $Cu(II)$ salt.
$\underset{\text{aldehyde}}{\mathrm{RCHO}}+\underset{\text{Fehling’s solution }}{2C{u}^{2+}+O{H}^{-}}\to \underset{\stackrel{\text{carboxylate}}{\mathrm{ion}}}{RCO{O}^{-}}$
$+\underset{\stackrel{\text{cuprous oxide}}{\text{(red ppt.)}}}{C{u}_{2}O\downarrow }+3H_{2}O$