Chemistry · Amines · NEET
More CH3 groups push more electron density onto nitrogen (+I effect), which would make (CH3)3N the strongest. But in water two other effects work against the tertiary amine: (1) steric crowding of three CH3 groups blocks the lone pair and blocks the added H+, and (2) once protonated, the cation must be stabilised by water through hydrogen bonds. (CH3)3NH+ has only ONE N-H bond to hydrogen-bond with water, while (CH3)2NH2+ has TWO. So dimethylamine's cation is solvated better and is more stable. The middle amine gets the best balance of electron push plus good solvation, so it wins.
That pure +I (inductive-only) order is the GAS-PHASE order, where there is no water to solvate the cation. In the gas phase only electron push matters, so more methyl groups means more basic. In WATER, solvation and steric effects flip the tertiary amine down to last place. NEET usually specifies 'in aqueous solution', which means you must use the balanced order, not the pure inductive order.
Basicity in water depends on how stable the protonated cation (R-NH+) is. Water stabilises the cation by making hydrogen bonds to its N-H hydrogens. Primary cation CH3NH3+ has three N-H bonds, secondary (CH3)2NH2+ has two, tertiary (CH3)3NH+ has only one. More N-H bonds means better solvation. So solvation alone favours 1 degree over 2 degree over 3 degree, which is the exact opposite of the +I effect. The final order is the tug-of-war between these two trends.
Only in water (aqueous solution). The concept question always says 'in water' or 'aqueous solution' for a reason. In the gas phase the order reverses to (CH3)3N > (CH3)2NH > CH3NH2 because there is no solvent to stabilise the cation. Always check the phrase in the question before choosing an option.
Ammonia has no electron-pushing methyl group, so it is less basic than all three methylamines in water. Full aqueous order with ammonia: (CH3)2NH > CH3NH2 > (CH3)3N > NH3. Adding even one CH3 group raises basicity above NH3, which is why methylamines are all stronger bases than ammonia in water.
The correct order of the basic strength of methyl-substituted amines in aqueous solution is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
(CH3)2NH > CH3NH2 > (CH3)3N. Dimethylamine (secondary) is the strongest and trimethylamine (tertiary) is the weakest in aqueous solution.
Its three methyl groups crowd the nitrogen (steric hindrance), and its protonated cation (CH3)3NH+ has only one N-H bond, so water can barely solvate it. Poor solvation plus crowding make it the weakest in water.
In water the order is (CH3)2NH > CH3NH2 > (CH3)3N (balance of +I, steric, solvation). In the gas phase it is (CH3)3N > (CH3)2NH > CH3NH2 (pure +I effect, no solvent).
Yes, all three are more basic than NH3 in water because the CH3 groups push electron density onto nitrogen. Order with ammonia: (CH3)2NH > CH3NH2 > (CH3)3N > NH3.
Inductive (+I) electron push from methyl groups, steric hindrance around nitrogen, and solvation (hydrogen bonding of the cation by water). The observed order is the net result of all three.
