Which of the following has highly acidic hydrogen?

Solution

Chemistry ma’am · 3 steps

Look for a CH₂ sitting between two C=O groups. That hydrogen is the one that leaves easily.

1. Judge the anion. An acidic C–H loses H⁺ to leave a carbanion. The more that negative charge can spread onto electronegative oxygen, the more stable the anion and the more acidic the hydrogen.

   Option	Compound	Most acidic C–H	C=O groups that take the negative charge
   (a)	hexane-2,3-dione (1,2-dione)	CH₂ next to C-3	1, the second C=O only pulls by induction
   (b)	hexane-2,5-dione (1,4-dione)	CH₂ next to C-2 or C-5	1, the other C=O is too far away
   (c)	hexan-3-one	CH₂ next to C-3	1
   (d)	hexane-2,4-dione (1,3-dione)	CH₂ at C-3, between both C=O	2

2. Active methylene. In (d) the carbanion at C-3 is conjugated with both carbonyls, so resonance puts the negative charge on either oxygen. This doubly stabilised anion makes a 1,3-diketone far more acidic (pKₐ ≈ 9) than a simple ketone (pKₐ ≈ 19 to 20).
3. So. The most acidic hydrogen is on the CH₂ flanked by two carbonyls in hexane-2,4-dione, option (d).

The idea to keep. For C–H acidity, ask where the negative charge goes. A carbon between two carbonyls (an active methylene) spreads it onto two oxygens, so 1,3-dicarbonyls beat every other dione.

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