Three gaseous alkylamines A, B and C are primary, secondary and tertiary respectively. What is the correct order of their basicity in the gaseous phase?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C > B > A
An alkyl group pushes electrons towards nitrogen (+I effect). This makes the lone pair more available to a proton and spreads the positive charge of the ammonium ion formed. In the gaseous phase there is no solvation by water, so basicity rises with the number of alkyl groups.
C (tertiary amine, R3N) — first: three alkyl groups push electrons to nitrogen.
B (secondary amine, R2NH) — second: two alkyl groups.
A (primary amine, RNH2) — third: only one alkyl group.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “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. … 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.”