JEE Main · 2010 · 11 Apr 2010 · Offline · General Organic Chemistry

The correct order of increasing basicity of the given conjugate bases (

R = CH_{3}

) is

(A) $RCO \overline{O} < HC = \overline{C} < \overline{R} < \overline{N} H_{2}$
(B) $\overline{R} < HC \equiv \overline{C} < RCO \overline{O} < \overline{N} H_{2}$
(C) $RCO \overline{O} < \overline{N} H_{2} < HC \equiv \overline{C} < \overline{R}$
(D) $RCO \overline{O} < HC \equiv \overline{C} < \overline{N} H_{2} < \overline{R}$ ✓ Correct

Answer: (D)

Solution

A conjugate base is weaker when its parent acid is stronger, so ranking the acids ranks the bases in reverse. The acid strengths run

RCOOH > HC \equiv CH > N H_{3} > RH

, carboxylic acid much the strongest and the alkane the weakest. Reversing this gives the increasing basicity of the conjugate bases

RCO O^{-} < HC \equiv C^{-} < N H_{2}^{-} < R^{-}

. The carboxylate is the weakest base because resonance spreads its charge, while the carbanion, with no such stabilisation, is the strongest.

JEE Main · 2010 · 11 Apr 2010 · Offline · Single correct option