Question
Arrange the following in decreasing order of basicity in (a) gas phase and (b) aqueous solution: , , , . Explain the difference between gas-phase and aqueous trends.
Solution — Step by Step
In the gas phase, basicity is governed solely by the inductive effect of alkyl groups, which donate electron density to nitrogen and stabilise the positive charge of the conjugate acid.
More groups → more electron donation → stronger base.
Order: .
In water, three factors compete:
- Inductive effect (favors more substitution): increases basicity.
- Solvation of conjugate acid (favors more N-H bonds available for H-bonding to water): decreases basicity as you replace H with .
- Steric effect (more methyl groups crowd the nitrogen): slightly decreases basicity.
The observed order in water:
Secondary amine wins because it balances inductive donation (better than primary) with adequate solvation (better than tertiary).
has no N-H bonds; its conjugate acid has only one. So solvation by water is poor, and the energetic gain of protonation is reduced. Trimethylamine ends up below dimethylamine in water.
has no alkyl groups, so no inductive donation. Its solvation is good (three N-H bonds in conjugate acid), but the lack of inductive support keeps it least basic in water.
Final answers:
- Gas phase:
- Aqueous:
Why This Works
In gas phase, no solvent → only intrinsic electronic effects matter. In water, solvation matters because it stabilises ions. The conjugate acid benefits from more N-H bonds for hydrogen-bonding to water.
This is a classic JEE Advanced topic — students who memorise only the gas-phase trend get the aqueous question wrong every time.
Alternative Method
Use the experimental values: , , , . Lower = stronger base. The values confirm the aqueous order.
Students often write the gas-phase order in aqueous problems. Always check whether the question specifies gas phase or aqueous solution. Most exam questions are in water (the default), so use the dimethylamine-on-top order.
JEE Main has asked this question almost every alternate year. Memorise both orders separately and the reasoning behind the aqueous anomaly. The “inductive vs solvation” trade-off is the key insight to write in subjective answers.