Question

The correct order of relative acidic strength of the following compounds is:

• A. phenol > o-nitrophenol > m-nitrophenol > p-nitrophenol
• B. p-nitrophenol > m-nitrophenol > o-nitrophenol > phenol
• C. p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol
• D. o-nitrophenol > m-nitrophenol > p-nitrophenol > phenol

Answer 1

Answer: (C)

Nitrophenol is more acidic than phenol because $-N{O}_2$ group is electron withdrawing group and it stabilises the phenoxide ion. The effect is more significant when the substituent is present in $o$ or $p$- position than m-position to the $-OH$ group. Further due to intramolecular hydrogen bonding the acidic strength of $o$-isomer is weaker than para-isomer. Thus, acidic strength decreases as:
$p$-nitrophenol $>o$-nitrophenol $>m$-nitrophenol $>$ phenol