Question

(a) How would you account for the following:
(i) Aldehydes are more reactive than ketones towards nucelophiles.
(ii) The boiling points of aldehydes and ketones are lower than of the corresponding acids.
(iii) The aldehydes and ketones undergo a number of addition reactions.
(b) Give chemical tests to distinguish between:
(i) Acetaldehyde and benzaldehyde]
(ii) Propanone and propanol.

Answer 1

(a) (i) This is due to 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) This is due to intermolecular hydrogen bonding in carboxylic acids.

(iii) Due to greater electronegativity of oxygen than carbon the C atom of the C = O bond acquires a partial positive charge in aldehydes and ketones and hence readily undergo nucleophilic addition reactions

(b) (i) Acetaldehyde reacts with NaOI (I₂ / NaOH) to form yellow ppt of iodoform while benzaldehyde does not give this test.

(ii) Propanone give orange-red ppt with 2, 4-DNP reagent and yellow ppt of iodoform with sodium hypoiodite, whereas 1-propanol does not give these tests.