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Given are cyclohexanol (I), acetic acid (II), 2,4,6-trinitrophenol (III) and phenol (IV). In these the order of decreasing acidic character will be
A
III > IV > II > I
B
III > II > IV > I
C
II > III > I > IV
D
II > III > IV > I
Detailed Solution
2,4,6-Trinitrophenol (picric acid): three $-NO_2$ groups at the ortho and para positions withdraw electrons strongly by −I and −R effects and delocalise the negative charge of the phenoxide ion; $pK_a \approx 0.4$, a very strong acid.
Acetic acid: the carboxylate ion is stabilised by two equivalent resonance structures with the charge on two oxygen atoms; $pK_a = 4.76$.
Phenol: the phenoxide ion is resonance stabilised, but the charge is delocalised onto less electronegative carbon atoms; $pK_a \approx 10$.
Cyclohexanol: an ordinary alcohol with no resonance stabilisation of its alkoxide ion; $pK_a \approx 18$.
Decreasing order of acidic character: III > II > IV > I
Acetic acid: the carboxylate ion is stabilised by two equivalent resonance structures with the charge on two oxygen atoms; $pK_a = 4.76$.
Phenol: the phenoxide ion is resonance stabilised, but the charge is delocalised onto less electronegative carbon atoms; $pK_a \approx 10$.
Cyclohexanol: an ordinary alcohol with no resonance stabilisation of its alkoxide ion; $pK_a \approx 18$.
Decreasing order of acidic character: III > II > IV > I