Account for the following : (1) The alpha (α)-hydrogens of aldehydes and ketones are acidic in nature. ​

Question

(i) Account for the following :

(1) The alpha (\(\alpha\))-hydrogens of aldehydes and ketones are acidic in nature.

(2) Oxidation of aldehydes is easier than ketones.

(ii) Arrange the following in :

(1) Decreasing reactivity towards nucleophilic addition reaction propanal, acetone, benzaldehyde.

(2) Increasing order of boiling point : Propane, Ethanol, Dimethylether, Propanal

(iii) Give simple chemical test to distinguish between Benzoic acid and Benzaldehyde.

Answer 1

(i) (1) The acidity of \(\alpha\)-hydrogen atoms of carbonyl compounds is due to strong electron withdrawing effect of the carbonyl group and resonance stabilization of the conjugate base.

(2) Aldehydes have H attached to carbonyl C, so aldehydes can be oxidised by mild oxidising agent. Oxidation of ketones involves C – C bond cleavage. 

(ii) (1) Propanal > Benzaldehyde > Acetone

Aldehydes are generally more reactive towards nucleophilic addition reaction.

(2) Propane < Dimethylether < Propanal < Ethanol

(iii) Tollen’s test Benzaldehyde gives Tollen’s test

Benzoic acid does not give Tollens’ test.