Question

propanol is less reactive then ethanol toward addition of HCN

Answer 1

Ethanal is an aldehyde containing two carbon atoms.In case of ehanal (aldehyde), one methyl group (alkyl group)is attached to the carbonyl group.Methyl group is +I group,so it tend to decrease the magnitude of charge on carbonyl carbon of aldehyde.As just one such alkyl group is present in case of aldehydes, nucleophilic attack is more facillitated comparatively.
In case of propanone and other ketones, two alkyl groups (+I) are attached to the carbonyl group which decreases magnitude of charge on carbonyl carbon more effectively.Hence attack of nucleophile is difficult,comparatively, in case of propanone and other ketones

Answer 2

Aldehydes and ketones contain carbonyl group,which is highly polar due to difference in electronegativity of carbon and oxygen.Hence aldehydes and ketone both give nucleophilic addition reaction.

Ethanal is an aldehyde containing two carbon atoms.In case of ehanal (aldehyde), one methyl group (alkyl group)is attached to the carbonyl group.Methyl group is +I group,so it tend to decrease the magnitude of charge on carbonyl carbon of aldehyde.As just one such alkyl group is present in case of aldehydes, nucleophilic attack is more facillitated comparatively.