MHTCET 26 Apr 2025 Shift 1 Paper

Concept:Basicity of amines depends on the electron-releasing inductive effect of alkyl groups and on solvation effects in the medium.Explanation:Alkyl groups are electron-donating groups. They push electron density towards the nitrogen atom, making its lone pair more easily available for donation.Therefore, on the basis of the inductive effect alone, the predicted order is: $\mathrm{NH}_3 < \mathrm{RNH}_2 < \mathrm{R}_2\mathrm{NH} < \mathrm{R}_3\mathrm{N}$.This order is actually observed in the gaseous phase, where solvation effects are absent.Thus, in the gaseous phase: $\mathrm{NH}_3 < 1^\circ$ amine $< 2^\circ$ amine $< 3^\circ$ amine.In the aqueous phase, hydration of the protonated amine (ammonium ion) also plays a role. A greater number of N–H bonds allows more hydrogen bonding with water, giving extra stability.Hence, in aqueous medium the order becomes: $\mathrm{NH}_3 < \mathrm{RNH}_2 < \mathrm{R}_3\mathrm{N} < \mathrm{R}_2\mathrm{NH}$.Since the question does not specify any particular medium, the expected general order is the gaseous phase order.Among the given options, only option B matches this order.Answer:Option B: $\mathrm{NH}_3 < \mathrm{R}-\mathrm{NH}_2 < \mathrm{R}_2\mathrm{NH} < \mathrm{R}_3\mathrm{N}$