Which one of the following compounds possesses the most acidic hydrogen?

Solution

Chemistry ma’am · 3 steps

Look at the anion left behind: more groups that can share the negative charge make a more acidic H.

1. Judge the anion. Remove the acidic H as H⁺ and look at the carbanion that is left. The more that negative charge is spread out, the more stable the anion and the stronger the acid.
2. Resonance beats hybridisation. An sp carbon holds its lone pair well because of its 50% s character, but the charge stays on that one carbon. A carbanion next to C=O or C≡N can push its charge onto oxygen or nitrogen by resonance, and that stabilises it far more.

   Compound	Acidic H on	How the anion is stabilised	Charge spread onto
   (a) CH₃-C≡C-H	sp carbon	s character only, no resonance	one carbon
   (b) CH₃COCH₃	α-carbon	resonance with one C=O	one O
   (c) CH₂(CN)₂	carbon between two CN	resonance with two C≡N	two N
   (d) HC(COOCH₃)₃	carbon between three COOCH₃	resonance with three C=O	three O

3. So. In (d) the carbanion shares its charge over three electron-withdrawing ester groups, more than any other option. It is the most stable conjugate base, so (d) has the most acidic hydrogen.

The idea to keep. For C–H acidity, count the -M groups on the carbon that loses H⁺. Each extra C=O or C≡N spreads the charge further, and resonance outweighs sp hybridisation.

Answer from the NTA final answer key. Solution by PrepWiser.