Arrange the following compounds in increasing order of their solubility in water:
(A) n-butanol
(B) Butan-1-amine
(C) Methanamine
(D) Aniline
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (D), (A), (B), (C)
An organic compound dissolves in water when it can form hydrogen bonds with water, and its solubility falls as the hydrophobic hydrocarbon part grows. Both alcohols and amines form such hydrogen bonds; the nitrogen of an amine, with its more available lone pair, is the better acceptor of a hydrogen bond from water.
(C) Methanamine, \( \ce{CH3NH2} \): only one carbon atom; it is very highly soluble in water.
(B) Butan-1-amine, \( \ce{CH3(CH2)3NH2} \): four carbon atoms, but the –\( \ce{NH2} \) group bonds strongly with water; it mixes with water in all proportions.
(A) n-Butanol, \( \ce{CH3(CH2)3OH} \): the same four-carbon chain; it is only partly soluble (about 8 g in 100 g of water).
(D) Aniline, \( \ce{C6H5NH2} \): the large phenyl group is strongly hydrophobic, and the lone pair of nitrogen is partly delocalised into the ring; it is the least soluble (about 3.5 g in 100 g of water).
In increasing order of solubility in water:
\( \displaystyle \ce{C6H5NH2} \lt \ce{C4H9OH} \lt \ce{C4H9NH2} \lt \ce{CH3NH2} \)
Shortcut: Aniline, with its big hydrophobic ring, must come first and the one-carbon methanamine last. Only Option (3) starts with (D).