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Give a plausible explanation for each one of the following: (a) Although phenoxide ion has more number of resonating structures than carboxylate ion, carborylic acid is a stronger acid than phenol. (b) There are two -NH2 groups in semicarbazide. However, only one is involved in the formation of semicarbazones. |
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Answer» (a) The delocalisation of benzene electrons contributes little towards the stability of phenoxide ion. The carboxylate ion is much more resonance stabilized than phenoxide ion. So, it is easier to lose a proton than phenol. Hence, carboxylic acid is a stronger acid than phenol. (b) Semicarbazide has two -NH2 groups. One of them, which is directly attached to C=O is involved in resonance. Thus electron density on this group decreases and it does not act as a nucleophile. In contrast, the lone pair of electrons on the other-NH2 group is available for uncleophilic attack. |
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