Arrange the following compounds in increasing order of their property as indicated :
(a) Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone [Reactivity towards HCN]
(b) `CH_(3)CH_(2)CH(Br)CO OH, CH_(3)CH(Br)CH_(2)CO OH, (CH_(3))_(2)CHCO OH,CH_(3)CH_(2)CH_(2)CO OH` [Acid strength]
(c) Benzoic acid, 4-Nitrobenzoic acid, 3,4–Dinitrobenzoic acid, 4–Methoxybenzoic acid [Acid strength]

Explanation

(i) The reactivity of aldehydes and ketones towards HCN addition decreases as the +1 - effect of the alkyl groups increases. Secondly it decreases with increase in steric hindrance ot the nucleophilic attack by `CN^(-)` at the carbonyl carbon. Thus, the decreasing order of rectivity towards HCN is,

(ii) We know that + l-effect decreases while -l-effect increases the acidic strenght of carboxylic acids. Since + l-l-effect of isopropyl group is more than that of propyl group, therefore, `(CH_(3))_(2)CHCOOH` is a weaker acid than `CH_(3)CH_(2)CH_(2)COOH`. Further since -l- effect decreases with distance, therefore `CH_(3)CH_(2)BrCOOH` is a stronger acid than `CH_(3)CHBrCH_(2)COOH`. Thus, the overall aicd strength increases in the order.

(iii) Since electron-donating groups decreases the acidic strength, therefore, 4-methoxy benzoic acid is a weaker acid than benzoic acid. Further since electron withdrawing groups increase the acidic strength, therefore, both 4-nitrobenzoic acid and 3,4-dinitrobenzoic acid are stronger acids than benzoic acid. Further due to the presence of an additional `-NO_(2)` group at /w-position with respect to -COOH group, 3,4-dinitrobenzoic acid is a stronger acid than 4-nitrobenzoic acid. Thus, the overall acidic strength increases in the order: 4-methoxy benzoic acid lt benzoic acid lt 4-nitrobenzoic acid lt 3,4-dinitrobenzoic acid.