Which of the following is the strongest acid?

Aphenol
B4-methoxyphenol
C4-nitrophenol
D4-methylphenol

Answer: C

The nitro group at the para position withdraws electrons by

- M

and

- I

effects and strongly stabilises the phenoxide ion. Methyl and methoxy donate electrons and make the phenols weaker than phenol itself, so 4-nitrophenol is strongest.

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