Question
Arrange the following in decreasing order of basicity in aqueous solution: methylamine (), dimethylamine (), trimethylamine (), and ammonia (). Then explain the JEE/NEET trap: why the gas-phase order differs from the aqueous-phase order.
Solution — Step by Step
For amines in aqueous solution, basicity depends on:
- (inductive donation) of alkyl groups — increases basicity by pushing electrons onto N, stabilising the protonated form.
- Steric hindrance — bulky alkyl groups make the lone pair harder to access, decreasing basicity.
- Solvation of the protonated cation — with more bonds is better hydrogen-bonded by water; more H atoms = more solvation = more stable cation = higher basicity.
- : no alkyl groups (no ), small (no steric hindrance), best solvated ( N-H bonds in ).
- : one alkyl ( helps), small (low steric), N-H bonds (good solvation).
- : two alkyl ( stronger), moderate steric, N-H bonds (decent solvation).
- : three alkyl ( strongest), high steric, N-H bond (poor solvation).
The three effects compete. In aqueous solution, the dominant interplay produces this order:
Approximate values in water at :
- :
- :
- :
- :
Smaller = stronger base. The order matches.
In the gas phase (no solvent), there’s no solvation effect — only and steric (which is small for these). So basicity simply tracks :
JEE Main has tested this distinction (gas vs aqueous) repeatedly. The question always specifies the medium — read carefully.
Why This Works
Basicity isn’t a single property — it’s a measure of equilibrium . The position of this equilibrium depends on the medium because solvent stabilisation of the products matters.
In water, has only one N-H bond available for H-bonding, so it’s poorly solvated. In gas phase, no solvation — so wins and tertiary amine becomes the strongest base.
Alternative Method
Use of conjugate acid: a stronger base has a higher for its conjugate acid. So compare of across the four amines. Same ordering.
For exam purposes, memorise the aqueous-phase order: secondary > primary > tertiary > NH₃ for methyl-substituted amines. For ethyl-substituted, the order shifts slightly but the same principle applies. Always assume aqueous unless the problem says otherwise.
Common Mistake
Three traps:
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Ignoring solvation. Students who only consider get the gas-phase order in an aqueous problem. NEET specifically tests the aqueous order — different from inductive prediction.
-
Confusing amines with anilines. Anilines (aryl amines) are less basic than ammonia because the lone pair delocalises into the ring. Don’t apply the methylamine reasoning to aniline.
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Wrong sign of comparison. Smaller = stronger base. Many students reverse this and rank larger as more basic.
Final answer: Aqueous: . Gas phase reverses solvation effect.