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JEE Advanced levelSingle correct optionDifficulty 3 of 3si031

Ethanal is treated with dilute NaOH at room temperature to give the aldol P, which on heating gives Q. Which single reagent distinguishes P from Q?

ATollens reagent
BFehling's solution
C2,4-DNP
D $Br X_{2} / CCl X_{4}$

Answer: D

P is 3-hydroxybutanal,

CH X_{3} CH (OH) CH X_{2} CHO

, and Q is but-2-enal,

CH X_{3} CH = CHCHO

. Both are aldehydes, so Tollens, Fehling and 2,4-DNP are positive for both. Only Q has a

C = C

and decolourises bromine in

CCl X_{4}

.

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