Question

How will you bring about the following conversations?
(i) Propanone to propane
(ii) Benzoyl chloride to benzaldehyde
(iii) Ethanal to but-2-enal.

Answer 1

Hint: Following changes are occurring: (i) The carbonyl group is reduced to methylene group.
(ii) Acid chloride group is converted to an aldehyde group.
(iii) Two molecules of aldehyde condense in this conversion.

Complete step by step answer:
The conversion of propanone to propane can be carried out with either Clemmensen reduction or Wolff-Kishner reduction.
In Clemmensen reduction, propanone is treated with zinc amalgam and concentrated hydrochloric acid. Carbonyl group is reduced to methylene group.

In Wolff-Kishner reduction, propanone is treated with hydrazine, followed by potassium hydroxide in a high boiling solvent such as ethylene glycol. Carbonyl group is reduced to methylene group.

The conversion of benzoyl chloride to benzaldehyde is carried out by Rosenmund reduction. Benzoyl chloride is treated with hydrogen and palladium- barium sulphate catalyst partially poisoned with quinoline or sulphur. Acid chloride group is converted into aldehyde group.

The conversion of ethanal to but-2-enal is carried out by aldol condensation reaction. Ethanol is acetaldehyde and contains an alpha hydrogen atom. In presence of dilute alkali, two molecules of ethanal condense to form 3-hydroxybutanal.
Upon heating, this 3-hydroxybutanal loses a water molecule to form but-2-enal.

Note:
(i) Do not use the reagents that reduce carbonyl groups to alcohols.
(ii) Do not use the reagents that convert an acid chloride into an ester or a carboxylic acid.
(iii) Do not forget to add the step for water elimination.