Question
(NEET style.) Arrange the following amines in order of decreasing basicity in aqueous solution: aniline (PhNH), p-nitroaniline (p-ON-CH-NH), methylamine (CHNH), dimethylamine ((CH)NH), trimethylamine ((CH)N).
Solution — Step by Step
Basicity = ability to donate the lone pair on N to a proton. More electron density on N ⇒ more basic. Two competing effects:
- Inductive (electron donation by alkyl groups) — increases basicity in the gas phase: 3°>2°>1°>NH.
- Solvation of conjugate acid — H-bonding by water stabilizes RNH more than RNH because the latter has fewer N–H bonds. So in water, 2° > 1° > 3° > NH for aliphatic amines.
For aromatic amines: lone pair on N delocalizes into the ring, reducing electron density on N. So aniline is much weaker than methylamine.
Standard order in aqueous solution (pKb values shown):
- Dimethylamine (pKb ≈ 3.27) — most basic.
- Methylamine (pKb ≈ 3.36).
- Trimethylamine (pKb ≈ 4.20) — weaker than the other two due to limited solvation.
- Aniline (pKb ≈ 9.4) — weakly basic due to lone-pair delocalization into ring.
- p-Nitroaniline (pKb ≈ 13) — even weaker; the nitro group withdraws electron density from N by resonance and induction.
Why This Works
The order for amines in water is the famous “anomaly” of aliphatic amine basicity. In gas phase or non-polar solvents, (pure inductive ordering). Water disrupts this by hydrogen-bonding to the protonated amine — and 1° amines have three N–H bonds for H-bonding, while 3° amines have only one.
For aromatic amines, the lone pair is partially in the ring (like a quasi-double-bond character), unavailable for protonation. EWGs (–NO) make this worse; EDGs (–OCH) make it better.
Three reflexes for amine basicity in water:
- Aliphatic order: 2° > 1° > 3° > NH.
- Aliphatic vs aromatic: any alkyl amine ≫ any aryl amine.
- Aryl substituents: EDG (–OMe, –NH) increase basicity; EWG (–NO, –CN) decrease basicity.
Alternative Method
Look up pKb values directly:
| Amine | pKb | Relative basicity |
|---|---|---|
| (CH)NH | 3.27 | strongest |
| CHNH | 3.36 | |
| (CH)N | 4.20 | |
| PhNH | 9.40 | |
| p-ON-CH-NH | 13.0 | weakest |
Lower pKb = stronger base. Same order as the analytical reasoning.
Students apply the gas-phase order (3° > 2° > 1°) to aqueous solutions, getting the wrong rank for trimethylamine. Always use the aqueous order for NEET/CBSE problems unless the question specifies gas phase.
Final answer: .