Question: What hydrogen atoms in each compound have a pk_a≤ 25?

A compound is considered to be highly acidic when the intermediate structure formed after the proton abstraction is stabilized by resonance or -I effect of the adjacent group (generally an electronegative element).

The negative charge formed after the proton abstraction undergoes extensive resonance along with the exertion of -I effect of the adjacent group. More the stability of the conjugate base formed, more will be its acidity.

Representation of the resonating structures

The -I effect is shown by the carbonyl group.

Representation of the resonating structures

The -I effect is shown by the carbonyl group.

Representation of the resonating structures

The -I effect is shown by the phenyl group.

Based on the explanations, the hydrogen atoms in each compound have a pK_a ≤ 25, as shown below:

The acidic hydrogen a.

The acidic hydrogen b.

The acidic hydrogen c.