Question
Why p-nitrophenol is a stronger acid than phenol ?

Answer

(1) In p-nitrophenol, nitro group $\left( NO _2\right)$ is an electron withdrawing group present at para position which enhances the acidic strength ($ -1$ effect). The $O - H$ bond is under strain and release of proton $\left( H ^{+}\right)$becomes easy. Further pnitrophenoxide ion is more stabilised due to resonance.
(2) Since the absence of electron withdrawing group (like $NO _2$ ) in phenol at ortho and para position, the acidic strength of phenol is less than that of
p-nitrophenol.​​​​​​​

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