(a) Write a note on Rosenmund reduction.
(b) Why is fluoroacetic acid more acidic than chloroacetic acid?
(c) Draw the resonating structures of carboxylate ion.

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Solution

(a) Rosenmund reduction:
Acid chloride is reduced to aldehyde by hydrogenation over palladium catalyst supported on barium sulphate. For example, benzoyl chloride is reduced to benzaldehyde.
$C_{6} H_{5} - C O C l H_{2} - ------ \to P d - B a S O_{4} C_{6} H_{5} - C H O + H C l$
(b) Fluoroacetic acid more acidic than chloroacetic acid.
Electron withdrawing substituent increases the acidity of carboxylic acid by dispersing negative charge by inductive effect and stabilizing the carboxylate anion. This effect is stronger in fluoroacetic acid than in chloroacetic acid because $- I$ effect of $F_{2}$ is greater than that of $C l_{2}$ .
(c) The resonating structures of carboxylate ion are as shown.

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