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


\(+\) I effect increases \(\longrightarrow\)
Steric hindrance increases \(\longrightarrow\)
Reactivity towards \(\mathrm{HCN}\) addition decreases \(\longrightarrow\)
(ii) We know that + I-effect decreases while-I-effect increases the acidic strength of carboxylic acids.
Since + I-effect of isopropyl group is more than that of \(n\)-propyl group, therefore, \(\left(\mathrm{CH}_3\right)_2 \mathrm{CHCOOH}\) is a weaker acid than \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{COOH}\)
Further since - I-effect decreases with distance, therefore \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHBrCOOH}\) is a stronger acid than \(\mathrm{CH}_3 \mathrm{CHBrCH}_2 \mathrm{COOH}\). Thus, the overall acid 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- \(\mathrm{NO}_2\) group at \(m\)-position with respect to- \(\mathrm{COOH}\) 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 \( < \) benzoic acid \( < \) 4-nitrobenzoic acid \( < 3\),4-dinitrobenzoic acid.