Give reasons for the following:
(a) Protonation of phenols is difficult whereas ethanol easily undergoes protonation.
(b) The boiling point of ethanol is higher than that of dimethyl ether.
(c) Anisole on reaction with HI gives phenol and CHI as main products and not iodobenzene and CHOH.
(a) Protonation of phenols is difficult whereas ethanol easily undergoes protonation.
(b) The boiling point of ethanol is higher than that of dimethyl ether.
(c) Anisole on reaction with HI gives phenol and CHI as main products and not iodobenzene and CHOH.
Solution
(a) Resonance drains electron density from phenol's oxygen, so H is accepted reluctantly; ethanol's oxygen keeps both lone pairs fully available and protonates easily.
(b) Ethanol self-associates by O–H···O hydrogen bonds; dimethyl ether cannot — hence ethanol's much higher boiling point.
(c) The protonated anisole can only break at O–CH: the O–CH bond has partial double-bond character (resonance) and an S attack on the sp aryl carbon (or a phenyl cation) is impossible. I therefore attacks the methyl carbon:
(b) Ethanol self-associates by O–H···O hydrogen bonds; dimethyl ether cannot — hence ethanol's much higher boiling point.
(c) The protonated anisole can only break at O–CH: the O–CH bond has partial double-bond character (resonance) and an S attack on the sp aryl carbon (or a phenyl cation) is impossible. I therefore attacks the methyl carbon:
CBSE Class 12 Board · 2016 · All India · Subjective