Arrange the following aldehydes in the decreasing order of their solubility in water.
(A) Butanal
(B) Ethanal
(C) Pentanal
(D) Propanal
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → B > D > A > C
The carbonyl group of an aldehyde forms hydrogen bonds with water, and this makes the small members dissolve. The alkyl chain does not hydrogen bond, so as the chain becomes longer the solubility decreases rapidly.
(B) Ethanal, CH3CHO — two carbons; a lower member, miscible with water in all proportions. Most soluble.
(D) Propanal, CH3CH2CHO — three carbons; less soluble than ethanal.
(A) Butanal, CH3(CH2)2CHO — four carbons; less soluble again.
(C) Pentanal, CH3(CH2)3CHO — five carbons; the longest chain, so the least soluble.
Decreasing solubility: Ethanal > Propanal > Butanal > Pentanal, that is, B > D > A > C.
NCERT (2026-27 edition):
8.3 Physical Properties – “The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bond with water.”
“However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain.”