The table gives the pKb values of some amines in aqueous phase.
Which of the following amines is the strongest base in aqueous phase?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → N-Ethylethanamine
The smaller the pKb, the larger the Kb and the stronger the base. The table gives 3.25 for N,N-diethylethanamine, 9.38 for benzenamine, 3.00 for N-ethylethanamine and 3.38 for methanamine. The smallest value belongs to N-ethylethanamine, so it is the strongest base of the four.
Why the other options are incorrect
Option (1) N,N-Diethylethanamine — Incorrect. Its pKb, 3.25, is higher than 3.00. It has three ethyl groups, but in water the tertiary amine is less basic than the secondary amine because of steric hindrance and less solvation of its ion.
Option (2) Benzenamine — Incorrect. Its pKb, 9.38, is the largest of the four, so it is the weakest base; the unshared pair on nitrogen is in conjugation with the benzene ring.
Option (4) Methanamine — Incorrect. Its pKb, 3.38, is higher than 3.00 for N-ethylethanamine, so it is a weaker base.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “Larger the value of Kb or smaller the value of pKb, stronger is the base. The pKb values of few amines are given in Table 9.3.”
“…The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3”
Table 9.3: pKb Values of Amines in Aqueous Phase