Question

Question: Rank the labeled protons in each compound in order of increasing acidity.

Short answer

Answer

The order of acidity of the labeled protons in each compound is shown below:

Step-by-step solution

Criteria for acidity

A highly acidic proton when abstracted is stabilized by resonance or -I effect of the adjacent group (generally an electronegative element).

Stability of the negative charge

The negative charge formed after the proton abstraction undergoes extensive resonance along with the exertion of -I effect of the adjacent group. Resonance helps in predicting the acidity of the given compound and is much higher than the -I effect. The acidity is high in those cases where the conjugate base is stabilized by resonance and -I effect.

If H_a is abstracted, the negative charge formed is resonance stabilized by the adjacent carbonyl group. If H_b is abstracted, the negative charge formed is resonance stabilized by both the adjacent carbonyl group and the benzene ring. But when H_c is abstracted, the negative charge formed is a phenylic anion that is not stable.

So, H_a is the most acidic proton, and is the least acidic proton.

If H_a is abstracted, the negative charge formed is not resonance stabilized; rather, it is highly acidic due to the -I effect of the oxygen atom. Generally, alcoholic protons are more acidic the most acidic among the three protons.

If H_b is abstracted, the negative charge formed is resonance stabilized by both the double-bonded carbons. But when H_c is abstracted, the negative charge formed is resonance stabilized by the adjacent carbonyl group. Resonance stabilization is greater for the carbonyl group than the alkene; hence, H_c is more acidic than H_b .

So, H_a is the most acidic proton, and H_b is the least acidic proton.

Hence, the order of acidity of the labeled protons in each compound is shown below