Basicity Order of Aliphatic Amines vs Ammonia (Gas Phase vs Water)
Chemistry · Amines · NEET
In the gas phase, more alkyl groups always mean a stronger base, so the order is 3° amine > 2° amine > 1° amine > ammonia. In water this order breaks because the cation must be stabilised by water molecules (solvation), and a bulky 3° amine cation gets solvated poorly. For methylamines in water the real order is (CH3)2NH > CH3NH2 > (CH3)3N > NH3. Memory hook: "In gas, count the alkyls; in water, count the N-H bonds that water can hold onto."
Gas phase depends only on the +I effect (3° highest). In water, solvation of the cation flips the order so the secondary amine becomes the strongest base; the bulky tertiary amine falls below the primary amine. Ammonia stays lowest in both.
Your doubts, answered
Why is the basicity order different in the gas phase and in water?
In the gas phase there is no solvent. Only one factor matters: the +I (electron-releasing) effect of alkyl groups. More alkyl groups push more electron density onto nitrogen and stabilise the positive cation better, so the order is simply 3° > 2° > 1° > NH3. In water a second factor appears: the ammonium cation formed must be stabilised by hydrogen bonding to water molecules. This is called solvation. Solvation depends on how many N-H bonds the cation has and how easily water can surround it. So in water two effects fight each other, and the order can change.
Why is a secondary amine more basic than a tertiary amine in water?
When a tertiary amine like (CH3)3N accepts a proton, its cation (CH3)3NH+ has only ONE N-H bond, and the three bulky methyl groups block water from getting close. So it is solvated poorly. A secondary amine cation like (CH3)2NH2+ has TWO N-H bonds and is smaller, so water can hydrogen-bond to it strongly. Better solvation means a more stable cation and a stronger base. This is why the tertiary amine falls below the secondary amine in water.
Why is dimethylamine the strongest base among the methylamines in water?
Dimethylamine (CH3)2NH sits at the balance point. It has two methyl groups giving a good +I push (more electron density than methylamine), and its cation still has two N-H bonds for good solvation (unlike trimethylamine). So it wins on the combined effect. The final aqueous order for methylamines is (CH3)2NH > CH3NH2 > (CH3)3N > NH3. This exact order was asked in NEET 2019.
Does the +I effect stop working in water?
No. The +I effect of alkyl groups still increases electron density on nitrogen in water too. It never disappears. What changes is that a SECOND factor (solvation) now also matters. For the tertiary amine, the loss of solvation is bigger than the gain from the extra +I effect, so its net basicity drops. Basicity in water is the sum of +I effect, steric hindrance, and solvation.
Is ammonia always the weakest base in these comparisons?
Among ammonia and the aliphatic (alkyl) amines, yes. Ammonia has no electron-releasing alkyl group, so its nitrogen has the least extra electron density. In both gas phase and water, ammonia stays at the bottom: below every alkyl amine. Do not confuse this with aniline, which is aromatic and even weaker than ammonia for a different reason (lone-pair delocalisation into the ring).
⚠️ The NEET trap ✗ In water the basicity order is (CH3)3N > (CH3)2NH > CH3NH2 > NH3 because more methyl groups always give a stronger base. ✓ That order is only true in the GAS PHASE. In water, solvation of the cation matters, and the bulky (CH3)3N is solvated poorly. The aqueous order is (CH3)2NH > CH3NH2 > (CH3)3N > NH3. 🧠 If the question says 'aqueous solution' and you pick tertiary as strongest, you fell for the trap. In water, the secondary amine wins.
Real NEET questions
NEET 2019
The correct order of the basic strength of methyl-substituted amines in aqueous solution is:
A · (CH3)2NH > CH3NH2 > (CH3)3N ✓
B · (CH3)3N > CH3NH2 > (CH3)2NH
C · (CH3)3N > (CH3)2NH > CH3NH2
D · CH3NH2 > (CH3)2NH > (CH3)3N
Solution: The key phrase is 'aqueous solution', so solvation matters, not just the +I effect. Three factors compete: (1) +I effect: more CH3 groups raise basicity (favours 3° > 2° > 1°). (2) Steric hindrance: bulky groups block protonation, hurting (CH3)3N. (3) Solvation: the cation is stabilised by H-bonding to water; more N-H bonds and a smaller size give better solvation, so CH3NH3+ and (CH3)2NH2+ are solvated better than (CH3)3NH+. The net result places the secondary amine on top: (CH3)2NH > CH3NH2 > (CH3)3N. Ammonia would sit below all of these. Answer (A).
NEET 2025
The correct order of decreasing basic strength of the given amines is: N-ethylethanamine = (C2H5)2NH; ethanamine = C2H5NH2; N-methylaniline = C6H5NH(CH3); benzenamine/aniline = C6H5NH2.
A · N-ethylethanamine > ethanamine > N-methylaniline > benzenamine ✓
B · Benzenamine > ethanamine > N-methylaniline > N-ethylethanamine
C · N-methylaniline > benzenamine > ethanamine > N-ethylethanamine
D · N-ethylethanamine > ethanamine > benzenamine > N-methylaniline
Solution: First split into aliphatic vs aromatic. Aliphatic amines (lone pair fully on nitrogen, boosted by +I of alkyl groups) are stronger bases than aromatic amines (lone pair delocalised into the ring). So both aliphatic amines beat both aromatic ones. Within the aliphatic pair: (C2H5)2NH has two +I ethyl groups, so it is more basic than C2H5NH2. Within the aromatic pair: N-methylaniline has an extra +I methyl on nitrogen, so it is more basic than plain aniline. Final decreasing order: (C2H5)2NH > C2H5NH2 > C6H5NH(CH3) > C6H5NH2. Answer (A).
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the basicity order of amines in the gas phase?
In the gas phase there is no solvent, so only the +I effect counts. The order is tertiary amine > secondary amine > primary amine > ammonia (for example (CH3)3N > (CH3)2NH > CH3NH2 > NH3). More alkyl groups means more electron density on nitrogen and a more stable cation.
What is the basicity order of methylamines in water?
In water it is (CH3)2NH > CH3NH2 > (CH3)3N > NH3. The tertiary amine drops because its bulky cation is solvated (hydrogen-bonded to water) poorly. This was the answer to NEET 2019.
Why does solvation lower the basicity of tertiary amines?
When a tertiary amine accepts a proton, its cation has only one N-H bond and three bulky alkyl groups that block water. So water cannot hydrogen-bond to it well. Poor solvation means a less stable cation, which lowers basicity.
Are aliphatic amines stronger bases than ammonia?
Yes. Every simple aliphatic (alkyl) amine is a stronger base than ammonia in both the gas phase and water, because alkyl groups donate electron density (+I effect) to nitrogen, which ammonia lacks.
Is this the same reason aniline is a weak base?
No. Aniline is weak for a different reason: its nitrogen lone pair is delocalised into the benzene ring (resonance), so it is less available to grab a proton. That topic is covered in 'Why Is Aniline a Weaker Base than Ammonia?'
