Among the following marked proton of which compound shows lowest pKₐ value?

Solution

Chemistry ma’am · 3 steps

Remove the marked H and look at the carbanion. The one spread over a C=O and a benzene ring is most stable.

1. Judge the anion. A lower pKₐ means a more acidic proton. A C–H proton is more acidic when the carbanion left behind is more stable, and the best stabiliser is resonance that spreads the negative charge.
2. Ketone beats acid. In -COOH the lone pair on -OH already pushes electrons into the C=O, so that C=O pulls less on an α-carbanion. A ketone C=O has no such competition and forms a better enolate. So α-H of a ketone is more acidic than α-H of the matching acid.

   Option	Compound	Groups stabilising the carbanion	Stability of anion
   (a)	CH₃COOH	acid C=O only, weakened by -OH	lowest
   (b)	CH₃COCH₃	ketone C=O only	moderate
   (c)	C₆H₅CH₂COCH₃	ketone C=O and benzene ring	highest
   (d)	C₆H₅CH₂COOH	benzene ring and a weakened acid C=O	less than (c)

3. So. In (c) the negative charge spreads into the oxygen of the ketone and also around the benzene ring. That makes its marked H the most acidic, with the lowest pKₐ, so the answer is (c).

The idea to keep. For C–H acidity, count how many groups can share the carbanion's charge. A ketone C=O pulls harder than a COOH carbonyl, and an extra phenyl ring adds more delocalisation.

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