(a) Account for the following :
(i) Propanal is more reactive than propanone towards nucleophilic reagents.
(ii) Electrophilic substitution in benzoic acid taken place at meta position.
(iii) Carboxylic acids do not distinguish between the following pairs of compounds :
(b) Give simple chemical test to distinguish between the following pairs of compounds :
(i) Acetophenone and benzaldehyde (ii) Benzoic acid and ethylbenzoate.

Explanation

(iii) Due to resonance, the carboxyl carbon of carboxylic acid is less positive and hence less electrophillic than carbonyl carbon of adethydes and ketons. Therefore, many nucleophilic addition reactions of aldehydes and ketones do not take place with carboxylic acids although it has also carbonyl group.
(b) Acetophenone and Benzaldehyde
Tollen's reagent test
`{:(" "C_(6)H_(5)CHO+5[Ag(NH_(3))_(2)]^(+)+3OH^(-)),("Benezaldehyde" " "darr),(" "C_(6)H_(5)COO^(-)+2H_(2)O+2Ag darr +4NH_(3)),(" ""Silver mirror"),(C_(6)H_(5)overset(O)overset(||)(C-CH_(3))overset("Tollen's reagent")to"no siliver mirror formed"),("Acetophenone"):}`
(ii) Benzoic acid and ethylbnzoate
`NAHCO_(3)` Test
`{:(C_(6)H_(5)COOH+NaHCO_(3)toC_(6)H_(5)COONa+CO_(2)uarr+H_(2)O),(" ""Benzoic acid" " " "Sod. benzoate"):}`
Benzoic acid produces brisk efferescence.
`underset("Ethyl benzoate")(C_(6)H_(5)COOH)+NaHCO_(3)underset("Solution")overset(NaHCO_(3))to"No effervesence due to evolutin of "CO_(2) gas.`