(a) Account for the following:
(i) $C l - C H_{2} C O O H$ is a stronger acid than $C H_{3} C O O H$ .
(ii) Carboxylic acids do not give reactions of carbonyl group.
(b) Write the chemical equations to illustrate the following name reactions:
(i) Rosenmund reduction
(ii) Cannizzaro's reaction
(c) Out of $C H_{3} C H_{2} - C O - C H_{3}$ and $C H_{3} C H_{2} - C H_{2} - C O - C H_{3}$ , which give iodoform test?

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Solution

(a)(i) Because of $- I$ effect of Cl atom in $C l C H_{2} C O O H$ and $+ I$ effct of $C H_{3}$ group in $C H_{3} C O O H$ the electron density in the $O - H$ bond in $C l C H_{2} C O O H$ is much lower than $C H_{3} C O O H$ . As a result $O - H$ bond in $C l C H_{2} C O O H$ is much weaker than in $C H_{3} C O O H$ therefore loses a proton more easily than $C H_{3} C O O H$ . Hence, $C l C H_{2} C O O H$ acid is stronger acid than $C H_{3} C O O H$ .
(ii) Carboxylic acids are resonance hybride of the following structure:
Similarly, a carbonyl group of aldehydes and ketones may regarded as resonance hybride of following structures.
Because of contribution of structure (IV). the carbonylmcarbon in aidehydes and ketones is electrophilic. On the other hand electrophilic character of carboxyl carbon is reduced due to contribution of structure (ii). As carbonyl carbon of carboxyl group is less electropositive than carbonyl carbon in aldehydes and ketones therefore carboxylic acids do not give nucleophilic addition reactions of aldehydes and ketones.
(b)(i) Ref. image (ii) Ref. image
(c) $C H_{3} - C H_{2} - C H_{2} - C O H_{3}$

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