See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The basicity of aliphatic amines depends on two competing factors: (1) inductive effect of alkyl groups (electron-donating, increases electron density on N, increases basicity) and (2) solvation effect in protic solvents (more N-H bonds allow better hydrogen bonding stabilization of the conjugate acid, the ammonium ion). Step 1 - Evaluate option (a): In protic solvents for methylamines, the order of basicities (pKb) is experimentally well established as: Me2NH (2°) > MeNH2 (1°) > Me3N (3°) > NH3. This arises because the 2° amine benefits from two methyl groups (inductive effect) while still having one N-H for solvation. The 3° amine (Me3N) has three methyl groups but no N-H for solvation, making its conjugate acid poorly solvated, so it falls below the 1° amine. This order is correct. Step 2 - Evaluate option (b): In protic solvents for ethylamines, the order is: Et2NH (2°) > Et3N (3°) > EtNH2 (1°) > NH3. For ethyl groups, the larger inductive effect of ethyl compared to methyl means the 3° triethylamine is sufficiently electron-rich to outweigh the solvation disadvantage relative to the 1° amine, placing Et3N above EtNH2. This order is also experimentally known to be correct. Step 3 - Evaluate option (c): In the gas phase, there is no solvation effect. Basicity is determined purely by the inductive/electron-donating effect of alkyl groups. More alkyl groups = greater electron density on N = stronger base. Therefore the order is: Me3N (3°) > Me2NH (2°) > MeNH2 (1°) > NH3. This is correct. Step 4 - Since options (a), (b), and (c) are all individually correct statements about amine basicities under the specified conditions, option (d) 'All are correct' is the right answer. Why other options fail: (a), (b), and (c) are not wrong individually; they are all correct, so choosing any single one of them would be incomplete. Therefore, the correct answer is D.