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Question

Which of the phenol derivatives is the weakest acid?


A
A
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B
B
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C
C
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D
D
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Solution

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 :NO2 and COCH3 at para positions show M effect predominantly, thus stabilizing the conjugate base formed.
In D : NO2 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 NH2 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.

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