(i) WhenHCN reacts with a compound, the attacking species is a nucleophile, CN−.Therefore, as the negative charge on the compound increases, its reactivitywith HCN decreases. In the given compounds, the +I effect increases as shownbelow. It can be observed that steric hindrance also increases in the same
Hence, the given compounds can be arranged according totheir increasing reactivities toward HCN as:
Di-tert-butyl ketone tert-butyl ketone < Acetone < Acetaldehyde
(ii) Afterlosing a proton, carboxylic acids gain a negative charge as shown:
Now, any group that will help stabilise thenegative charge will increase the stability of the carboxyl ion and as aresult, will increase the strength of the acid. Thus, groups having +I effectwill decrease the strength of the acids and groups having −I effect willincrease the strength of the acids. In the given compounds, −CH3 grouphas +I effect and Br− group has −I effect. Thus, acids containing Br− arestronger.
Now, the +I effect of isopropyl group ismore than that of n-propyl group. Hence, (CH3)2CHCOOHis a weaker acid than CH3CH2CH2COOH.
Also, the −I effect grows weaker as distanceincreases. Hence, CH3CH(Br)CH2COOH is a weaker acidthan CH3CH2CH(Br)COOH.
Hence, the strengths of the given acids increase as:
(CH3)2CHCOOH< CH3CH2CH2COOH < CH3CH(Br)CH2COOH< CH3CH2CH(Br)COOH
(iii) As wehave seen in the previous case, electron-donating groups decrease the strengthsof acids, while electron-withdrawing groups increase the strengths of acids. Asmethoxy group is an electron-donating group, 4-methoxybenzoic acid is a weakeracid than benzoic acid. Nitro group is an electron-withdrawing group and willincrease the strengths of acids. As 3,4-dinitrobenzoic acid contains two nitrogroups, it is a slightly stronger acid than 4-nitrobenzoic acid. Hence, thestrengths of the given acids increase as:
4-Methoxybenzoic acid < Benzoic acid <4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid