Q.Why p-nitrophenol is a stronger acid than phenol ?

Step 1. Recall the phenoxide resonance structures (Fig. 11.1, section 11.4.4). Phenoxide itself has five resonance contributing structures, spreading its negative charge over the oxygen and the ring's ortho/para carbons.

Step 2. Add the -NO2 group's own resonance conjugation. In p-nitrophenoxide, the -NO2 group sits exactly at one of those already-negatively-charged para positions, and the nitro group's own strongly electron-withdrawing character lets it accept and further delocalise that negative charge onto ITS OWN oxygens (an additional, sixth resonance structure, as detailed in Problem 11.5), placing negative charge on highly electronegative oxygen atoms -- an especially stable arrangement.

Step 3. Compare stabilities. p-Nitrophenoxide (six resonance structures, with the extra one putting charge on electronegative nitro oxygens) is therefore MORE stabilised than plain phenoxide (five resonance structures, charge only ever on ring carbons or the original oxygen). A more stable conjugate base means the parent acid ionises more readily.

Step 4. Conclude. Because -NO2 is electron-withdrawing (by both induction and resonance) and further stabilises the already-delocalised phenoxide charge, p-nitrophenol ionises more than plain phenol and is therefore the stronger acid.

Comparing the number and quality of resonance structures available to phenoxide versus p-nitrophenoxide, per Problem 11.5's worked acid-strength comparison in section 11.4.4

• Citing only the inductive effect of -NO2 without mentioning its resonance contribution, which is the stronger and more complete part of the explanation here (an extra resonance structure placing charge on the nitro oxygens).
• Confusing this with an electron-DONATING substituent's effect (which would instead WEAKEN the acid, as with cresol in MCQ (vii)).