(a) How many you account for the following :
(i) Aldehydes are more reactive than ketones towards nucleophiles.
(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) How will you distinguish between these compounds:
(i) Acetaldehyde and benzaldehyde
(ii) Propanone and propanol

Explanation

(a) (i) Due to smaller + I -effect of one alkyl group in aldehydes as compared to larger +I-effect of two alkyl groups, the magnitude of positive charge on the carbonyl carbon is more in aldehydes than in ketones. As a result nucleophilic addition reaction occur more readily in aldehyde than in Ketones.
(ii) The boiling points of aldhydes and ketones are lower than corresponding acids and alcohols due to absence of intermolecular hydrogen bonding.
(iii) Aldehydes and Ketones undergo a number of addition reactions as both possess the carbonyl functional group which reacts a number of nucleophiles such as HCN, `NaHSO_(3)`, alcohols, ammonia derivatives and Grignard reagents.
(b) (i) Distinction between acetaldehyde and benzaldehyde : Acetaldehyde and benzaldehyde can be distinguish by Fehling solution.
Acetaldehyde give red coloured precipitate with Fehling solution while benzaldehyde does not.
`CH_(3)CHO+underset("Fehling solution")ubrace(2Cu^(2+)5OH^(-))rarrunderset("red ppt.")(CH_(3)COO^(-))+Cu_(2)O+H_(2)O`
(ii) Distinction between Propanone and Propanol : Propanone `(CH_(3)COCH_(3))` and propanol `(CH_(3)CH_(2)CH_(2)OH)` can be distinguish by iodoform test. Propanone when warmed with sodium hypoiodide (NaOI) i.e. `I_(2)` in NaOH, it gives yellow ppt of idoform while propanol does not respond to iodoform test.
`underset("Propanone")(CH_(3)COCH_(3))+3NaOI rarr underset(" Yellow ppt ")(CH_(3)Idarr+)CH_(3)COONa + 2NaOH`