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State reasons for the following:(i) pKb value for aniline is more than that for methylamine.(ii) Ethylamine is soluble in water whereas aniline is not soluble in water.(iii) Primary amines have higher boiling points than tertiary amines. |
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Answer» (i) In aniline, the lone pair of electrons on N-atom are delocalised over benzene ring due to resonance. As a result, electron density on the nitrogen atom decreases. In contrast, in methylamine, +I-effect of CH3– group increases electron density on the nitrogen atom. Therefore, aniline is a weaker base than methylamine, hence its pKb value is more than that for methylamine. (ii) Ethylamine is soluble in water due to formation of inter-molecular hydrogen bonds with water molecules. However, in aniline due to large hydrophobic part, i.e., hydrocarbon part, the extent of hydrogen bonding decreases considerably and hence aniline is insoluble in water. (iii) Due to the presence of two H-atoms on N-atom, primary amines undergo extensive intermolecular hydrogen bonding whereas tertiary amines have no H-atoms on the nitrogen atom, do not undergo H-bonding. As a result, primary amines have higher boiling points than tertiary amines. |
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