Question
A primary amine of formula has possible structural isomers. Identify them and determine which one would be most basic in aqueous solution.
Solution — Step by Step
has degree of unsaturation . Saturated, no rings — so it’s an open-chain amine.
For a primary amine (), has formula (butyl group). The four butyl isomers give four structural isomers.
- n-butylamine:
- sec-butylamine:
- iso-butylamine:
- tert-butylamine:
Basicity in aqueous solution depends on:
(a) Inductive effect (+I groups donate, increasing electron density on N → more basic). (b) Solvation of the conjugate acid (). Smaller, less hindered cations solvate better, stabilising them, increasing . (c) Steric hindrance around N (larger groups can shield the lone pair).
For primary aliphatic amines, the inductive and steric effects largely cancel. In aqueous solution, n-butylamine (least branched, most easily solvated conjugate acid) is typically slightly more basic than the more branched isomers.
But the differences are small ( to for all four). Among them:
- n-butylamine: (most basic)
- sec-butylamine:
- iso-butylamine:
- tert-butylamine: (least basic of these)
Final answer: n-butylamine is the most basic in aqueous solution among the four primary isomers.
Why This Works
In gas phase, basicity follows pure inductive donation: 3° > 2° > 1° amines. But in aqueous solution, the order shuffles because:
- ions need to be solvated (stabilised) by water via H-bonds.
- More groups around = fewer atoms on available for H-bonding to water = less solvation = less stabilisation of the cation = lower .
Net result for aqueous solution: 2° ≥ 1° > 3° for aliphatic amines. Among 1° amines specifically, less branched is slightly more basic.
Gas phase (intrinsic): 3° > 2° > 1° > NH
Aqueous solution: 2° > 1° ≈ 3° > NH (anomalous order due to solvation)
Alternative Method
For exam shortcuts, just memorise that for primary amines, n-butylamine is the most basic in water. JEE rarely asks about the four C4 amines specifically; usually it tests the more broadly applicable rule (1° vs 2° vs 3° comparison).
When comparing 1° vs 2° vs 3° aliphatic amines in aqueous solution, remember the anomalous order: 2° is the most basic, not 3°. JEE Main 2024 had this directly.
Common Mistake
Students apply the “more alkyl = more basic” rule blindly and conclude tert-butylamine is most basic. Wrong — that’s the gas-phase order. In water, the trend reverses for tertiary amines because of poor solvation of bulky .
Another common trap: confusing basicity with nucleophilicity. They often correlate, but not always — for example, in steric situations, a less basic amine can be a better nucleophile if it’s less hindered.
A third trap: forgetting that aniline () is much less basic than aliphatic amines (about times less), because the lone pair on N is delocalised into the ring. Don’t compare aliphatic and aromatic amines using the same rules.