(a) Account for the following:
(i) CH3CHO is more reactive than CH3COCH3 towards reaction with HCN.
(ii) 2-Fluorobutanoic acid is a stronger acid than 3-Fluorobutanoic acid.
(b) Write the chemical equations to illustrate the following name reactions:
(i) Etard reaction.
(ii) Rosenmund’s reaction.
(c) Give the mechanism of cyanohydrin formation when carbonyl compounds react with HCN in the presence of alkali.
(a) (i) CH3CHO (aldehyde) is more reactive than CH3COCH3 (ketone) towards reaction with HCN due to the steric and electronic reasons:
⇒ Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent.
⇒ Electronically two alkyl groups reduce the positivity of the carbonyl carbon more effectively in ketones than in aldehydes.
(ii) 2-Fluorobutanoic acid is a stronger acid than 3-Fluorobutanoic acid due to the following reason:
⇒ Inductive effect decreases with distance and hence the conjugate base of 2-fluorobutanoic acid is more stable and more acidic than 3-Fluorobutanoic acid.
(b) (i) Etard reaction:
Toluene is oxidized to benzaldehyde with a solution of chromyl chloride (CrO2Cl2) in CS2 or CCl4. The brown chromium complex thus precipitated is separated and decomposed with dilute acids to give benzaldehyde.
The above reaction is called Etard reaction.
(ii) Rosenmund reaction
When benzoyl chloride is treated with H2 and Pd-BaSO4, it forms benzaldehyde. This reaction is called Rosenmund reaction.
(c) Addition of hydrogen cyanide: Both aldehydes and ketones add a molecule of hydrogen cyanide to form cyanohydrins:
⇒ With pure HCN, reaction occurs very slowly, Therefore, it is catalyzed by a base. The CN- ion thus generated being a strong nucleophile readily adds to the carbonyl compounds to yield the carbonyl cyanohydrins.
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