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Among the following four compounds
a. Phenol
b. Methyl phenol
c. Metanitrophenol
d. Paranitrophenol
the acidity order is
a. Phenol
b. Methyl phenol
c. Metanitrophenol
d. Paranitrophenol
the acidity order is
A
d > c > a > b
B
c > d > a > b
C
a > d > c > b
D
b > a > c > d
Detailed Solution
The acidity of a phenol depends on the stability of its phenoxide ion.
An electron-withdrawing group ($-NO_2$) disperses the negative charge of the phenoxide ion and increases acidity; an electron-donating group ($-CH_3$) intensifies the charge and decreases acidity.
p-Nitrophenol: the nitro group at the para position withdraws electrons by both −I and −R effects, so the charge is delocalised onto the nitro group; most acidic ($pK_a$ = 7.1).
m-Nitrophenol: at the meta position only the −I effect operates; less acidic than the para isomer ($pK_a$ = 8.3).
Phenol: no substituent ($pK_a$ = 10.0).
Methyl phenol (cresol): the +I and hyperconjugation effects of $-CH_3$ make it the least acidic ($pK_a \approx$ 10.2).
Acidity order: d > c > a > b
An electron-withdrawing group ($-NO_2$) disperses the negative charge of the phenoxide ion and increases acidity; an electron-donating group ($-CH_3$) intensifies the charge and decreases acidity.
p-Nitrophenol: the nitro group at the para position withdraws electrons by both −I and −R effects, so the charge is delocalised onto the nitro group; most acidic ($pK_a$ = 7.1).
m-Nitrophenol: at the meta position only the −I effect operates; less acidic than the para isomer ($pK_a$ = 8.3).
Phenol: no substituent ($pK_a$ = 10.0).
Methyl phenol (cresol): the +I and hyperconjugation effects of $-CH_3$ make it the least acidic ($pK_a \approx$ 10.2).
Acidity order: d > c > a > b