(a) Give reasons :
(i) `CH_(3)-CHO` is more reactive than `CH_(3)COCH_(3)` towards HCN.
(ii) 4-nitrobenzoic acid is more acidic than benzoic acid.
(b) Describe the following :
(i) Acetylation
(ii) Cannizzaro reaction.
(iii) Cross aldol condensation.

Explanation

(i) Ethanal is more reactive than propanone towards HCN because of steric hinderance and electronic reason.
Propanone is sterically hindered by two bulky alkyl group where as, ethanal has one alkyl groups.
2 alkyl group in propanone reduces the electrophilicity of carbonyl group to greater extend and makes it less reactive towards HCN as compare to ethanal which has only one alkyl group.
(ii) is more acid than benzoic acid
because electron withdrawing group increases the acid strength. Hence, `NO_(2)` group (withdrawing group) increases the acidic strength. Hence, 4 nitrobenzoic acid is more acidic than benxoic acid.
(i) Acetylation : When alcohols, phenols are treated with acid chloride or anhydride i.e., acetyl chloride or acetic anhydride, the reaction is termed as acetylation.
`underset("ethanol")(C_(2)H_(5)OH)+underset("(acetyl chloride)")(CH_(3)COCl)rarrunderset("(Ethyl ethanoate)")(CH_(3)COOC_(2)H_(5))+HCl`
(ii) Cannizaro reaction : Aldehyde which do not contain `alpha`-hydrogen, when treates with alkali solution undergo disproportionation i.e., self oxidation-reduction. As a result, one molecule is reduced to corresponding alcohol and other corresponding carboxylic acid.
`underset("formaldehyde")(2H-overset(O)overset(||)C-H)+underset("(Conc.)")(NaOH)rarrunderset("Methanol")(CH_(3)OH)+underset("(Sodium formate)")(H-overset(O)overset(||)C-ONa)`
(iii) Cross - aldol condensation : When condensation of aldehyde or ketone having `alpha`-hydrogen inpresence of dil NaOH is carried out with 2 different aldehydes or two ketones or between one aldehyde and one ketone is called cross aldol condensation. Four products are obtained.
`CH_(3)CHO+CH_(3)CH_(2)CHOoverset("dil")underset("NaOH")rarrCH_(3)overset(OH)overset("| ")"CH"CH_(2)CHO+CH_(3)CH_(2)overset(OH)overset("| ")"CH"-overset(CH_(3))overset("| ")"CH"CHO+CH_(3)overset(OH)overset("| ")"CH"-overset(CH_(3))overset("| ")"CH"-CHO+CH_(3)CH_(2)overset(OH)overset("| ")"CH"-cH_(2)CHO`