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Question
Give reasons for the following:
(i) o-nitrophenol is more acidic than o-methoxyphenol.
(ii) Butan-1-ol has a higher boiling point than diethyl ether.
(iii) (CH3)3C−O−CH3 on reaction with HI gives (CH3)C−I and CH3−OH as the main products and not (CH3)3C−OH and CH3−I.
(i) o-nitrophenol is more acidic than o-methoxyphenol.
(ii) Butan-1-ol has a higher boiling point than diethyl ether.
(iii) (CH3)3C−O−CH3 on reaction with HI gives (CH3)C−I and CH3−OH as the main products and not (CH3)3C−OH and CH3−I.
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Solution
(i) Due to strong -R and - I -effect of the - NO2 group, electron density in the O - H bond decreases and hence the loss of a proton becomes easy.
Further, after the loss of a proton, the o-nitrophenoxide ion left behind is stabilized by reasonance, thereby making o-nitrophenol a stronger acid.
In contrast, to + R effect of the OCH3 group, the electron density in the O - H bond increases, thereby making the loss of a proton difficult.
Furthermore, the o-methoxyphenoxide ion left after the loss of a proton is denstabilized by resonance because the two negative charges repel each other, thereby making o-methoxyphenol a weaker acid. Thus, o-nitrophenol is more acidic than o-methoxyphenol.
(ii) Butan-1-ol molecules undergo association due to intermolecular hydrogen bonding but diethyl ether does not undergo hydrogen bonding. Thus, intermolecular interactions in diethyl ether (dipole-dipole interactions) are weaker than in butan-1-ol. As a result, butan-1-ol has higher boiling point than diethyl ether.
(iii) (CH3)3C−O−CH3 on reaction with HI gives (CH3)3C−I and CH3−OH as the main products.
CH3−CH3|C|CH3−O−CH3+HI→CH3OH+CH3−CH3|C|CH3−I
It is because in ether if one of the alkyl groups is tertiary, the reaction occurs by SN I mechanism and the formation of the product is controlled by the stability of carbocation resulting from cleavage of C-O bond of protonated ether. Since, tert-butylcarbocation is more stable than methyl carbocation, therefore, cleavage of C-O bond gives methyl alcohol and more stable tertbutylcarbocation which then reacts with I− to form tert-butyl iodide.
Further, after the loss of a proton, the o-nitrophenoxide ion left behind is stabilized by reasonance, thereby making o-nitrophenol a stronger acid.
In contrast, to + R effect of the OCH3 group, the electron density in the O - H bond increases, thereby making the loss of a proton difficult.
Furthermore, the o-methoxyphenoxide ion left after the loss of a proton is denstabilized by resonance because the two negative charges repel each other, thereby making o-methoxyphenol a weaker acid. Thus, o-nitrophenol is more acidic than o-methoxyphenol.
(ii) Butan-1-ol molecules undergo association due to intermolecular hydrogen bonding but diethyl ether does not undergo hydrogen bonding. Thus, intermolecular interactions in diethyl ether (dipole-dipole interactions) are weaker than in butan-1-ol. As a result, butan-1-ol has higher boiling point than diethyl ether.
(iii) (CH3)3C−O−CH3 on reaction with HI gives (CH3)3C−I and CH3−OH as the main products.
CH3−CH3|C|CH3−O−CH3+HI→CH3OH+CH3−CH3|C|CH3−I
It is because in ether if one of the alkyl groups is tertiary, the reaction occurs by SN I mechanism and the formation of the product is controlled by the stability of carbocation resulting from cleavage of C-O bond of protonated ether. Since, tert-butylcarbocation is more stable than methyl carbocation, therefore, cleavage of C-O bond gives methyl alcohol and more stable tertbutylcarbocation which then reacts with I− to form tert-butyl iodide.
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