Solution:

(i) The attacking species when HCN reacts with a compound, is a nucleophile, CN^–. The reactivity of HCN decreases when the negative charge on the compound increases. The +I effect and steric hindrance also increases in the same.

Therefore, the given compounds can be arranged according to their increasing reactivities toward HCN as:

(ii) After losing a proton, carboxylic acids gain a negative charge as shown:

Now, every group that aids in the stabilisation of the negative charge will boost the acid’s strength. As a result, groups with a -I effect will raise the strength of the acids, while groups with a +I effect will decrease it. The Br^– group has a -I effect in the listed compounds, while the CH₃ group has +I effect. As a result, acids that contain Br^– are stronger.

The isopropyl group now has a greater +I impact than the n-propyl group. As a result, CH₃CH₂CH₂COOH is more powerful than (CH₃)₂CHCOOH.

In addition, the +I impact weakens as the distance increases. As a result, CH₃CH₂CH(Br)COOH is a stronger acid than CH₃CH(Br)CH₂COOH.

Hence, the strengths of the given acids increase as:

(CH₃)₂CHCOOH < CH₃CH₂CH₂COOH < CH₃CH(Br)CH₂COOH < CH₃CH₂CH(Br)COOH

(iii) The strength of acids is diminished by electron donating groups, whereas the strength of acids is increased by electron-withdrawing groups, as we saw in the prior situation. Benzoic acid is a stronger acid than 4-methoxybenzoic acid because the methoxy group is an electron-donating group. The nitro group is an electron-withdrawing component that increases the acid’s potency. As 3,4-dinitrobenzoic acid contains two nitro groups, it is a slightly stronger acid than 4-nitrobenzoic acid. Hence, the strengths of the given acids increase as:

4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3, 4-Dinitrobenzoic acid