Question
Why a compound in which hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group attached to an alkyl group?
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Answer
$\rightarrow$ The ionization of an alcohol and a phenol takes place as follows:
$\rightarrow$ Due to the higher electronegativity of $sp^2$ hybridised carbon of phenol to which $-OH$ is attached, electron density decreases on oxygen. This increases the polarity of $O-H$ bond and results in an increase in ionization of phenols than that of alcohols.
$\rightarrow$ Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.
$\rightarrow$ The delocalisation of negative charge $($structures $I-V)$ makes phenoxide ion more stable and favours the ionization of phenol.
$\rightarrow$ Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion.
$\rightarrow$ Due to the higher electronegativity of $sp^2$ hybridised carbon of phenol to which $-OH$ is attached, electron density decreases on oxygen. This increases the polarity of $O-H$ bond and results in an increase in ionization of phenols than that of alcohols.
$\rightarrow$ Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.
$\rightarrow$ The delocalisation of negative charge $($structures $I-V)$ makes phenoxide ion more stable and favours the ionization of phenol.
$\rightarrow$ Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion.
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