Question

Which of the phenol derivatives is the weakest acid?

• A. A
• B. B
• C. C
• D. D

Answer 1

The correct option is B B

Lesser the stability of conjugate base, lesser is the acidic strength.
Conjugate bases formed after removal of $H^{+}$ are :


In A and C :$-N{O}_2$ and $-COC{H}_3$ at para positions show $-M$ effect predominantly, thus stabilizing the conjugate base formed.
In D : $-N{O}_2$ at meta position shows $-I$ effect, since resonance/mesomeric effect doesn't operate at meta position.
The conjugate base formed is also stabilized by $-I$ effect.
In B :All the three $-N{H}_2$ groups show $+M$ effect, and it highly destabilizes the conjugate base formed.
So, B forms the weakest conjugate base, and is the least acidic phenol derivative.