Amines are reactive in nature, mainly due to difference in electronegativity between Nitrogen and Hydrogen and, due to lone pair of electrons on Nitrogen. Amines behaves like nucleophiles and as good Lewis bases and there are various reactions to support this. Different degree of amines differ in their reactivity due to steric hindrance and electronic effects. Aromatic amines are less basic due to +R effect, hence have higher \(pK_b\) value than the aliphatic amines. Basic strength of amines also depend on stability of corresponding conjugate acid in aqueous medium.
Which of the following will be having the lowest \(pK_b\) in gaseous medium?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → N,N-Diethylethanamine
The lower the pKb, the stronger the base. In the gaseous phase there is no solvent, so solvation of the protonated amine plays no part; only the +I (electron-releasing) effect of the alkyl groups decides basicity. Each alkyl group pushes electron density onto nitrogen and stabilises the cation formed on protonation, so in the gas phase:
tertiary amine > secondary amine > primary amine > \( \ce{NH3} \)
N,N-Diethylethanamine, \( \ce{(C2H5)3N} \) (triethylamine), is the only tertiary amine in the list, so it is the strongest base in the gas phase and has the lowest pKb.
Why the other options are incorrect
Option (2) sec-Butylamine — Incorrect. \( \ce{CH3CH2CH(NH2)CH3} \) is a primary amine (one alkyl group on N; “sec” refers to the carbon, not the nitrogen), so it is weaker than a tertiary amine in the gas phase.
Option (3) Diethylamine — Incorrect. A secondary amine with two ethyl groups on N; it has one +I group fewer than triethylamine.
Option (4) Ethylamine — Incorrect. A primary amine with only one ethyl group on N; it is the weakest of the three ethyl amines in the gas phase.
NCERT (2026-27 edition):
Alkanamines versus Ammonia – “Due to the electron releasing nature of alkyl group, it pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkylamines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines should increase with increase in the number of alkyl groups. This trend is followed in the gaseous phase. The order of basicity of amines in the gaseous phase follows the expected order: tertiary amine > secondary amine > primary amine > NH3.”