JEE Main · 2024 · 29 Jan 2024 · Shift 2 · General Organic Chemistry

Acidity of a C–H bond tracks the stability of the carbanion left behind on deprotonation, which in turn tracks the s-character of the hybrid orbital holding the negative charge — greater s-character holds the lone pair closer to the nucleus and is more stable.

A is a terminal alkyne C–H: the conjugate base is sp-hybridised (50% s-character), the most stable and most acidic of the four.

B is a vinylic C–H: the conjugate base is sp${}^2$-hybridised (33% s-character).

D is an allylic C–H: the conjugate base is sp${}^3$-hybridised, but the negative charge is delocalised by resonance into the adjacent double bond, giving it more stability than an isolated sp${}^3$ carbanion.

C is a simple alkane C–H: the conjugate base is sp${}^3$-hybridised with no resonance stabilisation available, making it the least stable and least acidic.

Ranking by increasing acidity: C < D < B < A.