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Which one of the following compounds has the most acidic nature?
A
Dicyclohexylmethanol, $(C_6H_{11})_2CHOH$
B
Benzyl alcohol, $C_6H_5CH_2OH$
C
Phenol, $C_6H_5OH$
D
Cyclohexanol, $C_6H_{11}OH$
Detailed Solution
Acidity depends on the stability of the anion formed after loss of $H^+$.
Phenol gives the phenoxide ion, in which the negative charge on oxygen is delocalised over the benzene ring by resonance; this stabilises the ion and makes phenol acidic ($pK_a \approx 10$).
In benzyl alcohol the –OH is on an $sp^3$ carbon, so the alkoxide ion formed cannot be stabilised by resonance with the ring.
Cyclohexanol and dicyclohexylmethanol are ordinary alcohols; their alkoxide ions are destabilised by the +I effect of the alkyl groups ($pK_a \approx 16$–18).
Hence phenol is the most acidic.
Phenol gives the phenoxide ion, in which the negative charge on oxygen is delocalised over the benzene ring by resonance; this stabilises the ion and makes phenol acidic ($pK_a \approx 10$).
In benzyl alcohol the –OH is on an $sp^3$ carbon, so the alkoxide ion formed cannot be stabilised by resonance with the ring.
Cyclohexanol and dicyclohexylmethanol are ordinary alcohols; their alkoxide ions are destabilised by the +I effect of the alkyl groups ($pK_a \approx 16$–18).
Hence phenol is the most acidic.