Chemistry · Amines · NEET
When an amine acts as a base it grabs a proton and becomes the substituted ammonium cation, R-NH3+. Solvation (hydration in water) means water molecules cluster around this positive ion and hold it by hydrogen bonds. This lowers the energy of the cation. The more stable this cation is, the more the amine wants to form it, so the stronger the base. Solvation stabilises the PRODUCT cation, not the neutral amine.
Three effects fight each other. The +I effect of extra CH3 groups pushes electron density to nitrogen and raises basicity, favouring 3 degree > 2 degree > 1 degree. But solvation works the opposite way: (CH3)3NH+ has only one N-H bond, so it makes fewer hydrogen bonds with water and is poorly hydrated. (CH3)2NH2+ has two N-H bonds and is better hydrated. Steric hindrance of three bulky groups also blocks the proton and water. The net winner in water is the secondary amine: (CH3)2NH > CH3NH2 > (CH3)3N.
In the gas phase there is no water, so there is no solvation. Only the +I (electron-releasing) effect decides basicity. So in gas phase the pure inductive order holds: (CH3)3N > (CH3)2NH > CH3NH2 > NH3. In water, solvation and steric effects join in and reshuffle the order. This is the single most tested point: solvation is the reason the neat gas-phase order breaks down in aqueous solution.
Solvation increases the basicity of whichever cation it can stabilise best. It depends on the number of N-H bonds in the ammonium cation, because each N-H is a hydrogen-bond donor to water. Primary ammonium (3 N-H) is best solvated, tertiary ammonium (1 N-H) is worst solvated. So solvation pulls the order toward 1 degree > 2 degree > 3 degree, opposite to +I. The observed order is the compromise between these two opposite trends.
Read the NCERT hint: 'discrepancies in Kb values arise from solvation, steric hindrance, and inductive effect.' +I favours more alkyl groups (3 degree best). Solvation favours more N-H bonds (1 degree best). Steric hindrance penalises crowding (3 degree worst). All three summed give the real aqueous order for methylamines: (CH3)2NH > CH3NH2 > (CH3)3N > NH3. You do not calculate this in NEET; you remember it and know WHY the tidy +I order fails.
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.
Hydration is solvation when the solvent is water. Since NEET amine basicity is discussed in aqueous solution, the two words mean the same thing here: water molecules surrounding and stabilising the ammonium cation by hydrogen bonds.
The primary ammonium ion, RNH3+, because it has three N-H bonds and can donate the most hydrogen bonds to water. The tertiary ammonium ion R3NH+ has only one N-H bond and is the least solvated.
(CH3)2NH > CH3NH2 > (CH3)3N > NH3. The secondary amine is the strongest base in water. Remember it directly for NEET, because you cannot derive it from +I alone.
Because the simple +I based prediction (3 degree > 2 degree > 1 degree) does not match the measured Kb values in water. NCERT tells you the reason is that solvation and steric hindrance also act, not just the inductive effect.
For aniline, basicity is mainly lowered by resonance delocalisation of the nitrogen lone pair into the ring, which dominates over solvation. Solvation is the key deciding factor mainly among aliphatic amines where +I and hydration compete.
