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Which one of the following compounds is most acidic :-
A
o-Nitrophenol
B
o-Cresol (o-methylphenol)
C
Phenol
D
$Cl-CH_2-CH_2-OH$
Detailed Solution
Phenols are more acidic than alcohols, because the phenoxide ion is stabilised by resonance with the benzene ring, while an alkoxide ion is not.
So $Cl-CH_2-CH_2-OH$, an alcohol (even with the $-I$ effect of Cl), is much less acidic than the phenols.
An electron withdrawing group on the ring increases the acidity of phenol by further stabilising the phenoxide ion.
The $-NO_2$ group at the ortho position withdraws electrons by both $-M$ and $-I$ effects, so o-nitrophenol ($pK_a$ about 7.2) is much more acidic than phenol ($pK_a$ 10).
The $-CH_3$ group is electron releasing ($+I$ and hyperconjugation); it destabilises the phenoxide ion, so o-cresol ($pK_a$ about 10.3) is less acidic than phenol.
So the most acidic compound is o-nitrophenol.
So $Cl-CH_2-CH_2-OH$, an alcohol (even with the $-I$ effect of Cl), is much less acidic than the phenols.
An electron withdrawing group on the ring increases the acidity of phenol by further stabilising the phenoxide ion.
The $-NO_2$ group at the ortho position withdraws electrons by both $-M$ and $-I$ effects, so o-nitrophenol ($pK_a$ about 7.2) is much more acidic than phenol ($pK_a$ 10).
The $-CH_3$ group is electron releasing ($+I$ and hyperconjugation); it destabilises the phenoxide ion, so o-cresol ($pK_a$ about 10.3) is less acidic than phenol.
So the most acidic compound is o-nitrophenol.