Question

Order of basic strength of amines in aqueous solution

Answer 1

The ammonium cation (protonated form) of aliphatic amine is solvation-stabilised by H2O molecules via Hydrogen-bonding in aqueous solution.

Ammonium cation of primary amine (RNH3+) is maximum solvated of all types (primary, secondary & tertiary) due to greater number of hydrogens directly attached to nitrogen and hence their hydrogen bonding with oxygen of water(solvent) molecules.

In total, three effects are most useful to decide basic character of simple aliphatic amines in aqueous solution. (a) Inductive effect – which favors tertiary amine to be more basic due to greater + I effect of three alkyl groups directly attached to nitrogen (b) Solvation effect- which favor primary amine to be more basic due to greater solvation of its cation. (c) Steric factor – which causes tertiary amine to be less basic due to steric hindrance to approach of proton towards nitrogen.

As a combined result of above three factors, in aq solution, the order of basic character (pKb values in brackets) in isomeric aliphatic amines are different in different cases-

(CH3)2NH [3.27] > CH3NH2 [3.38] > (CH3)3N [4.22] > NH3 [4.75]
(C2H5)2NH [3.0] > (C2H5)3N [3.25] > C2H5NH2 [3.29] > NH3 [4.75]
Me2CH-NH2 > NH3 > (Me2CH)2NH > (Me2CH)3N
NH3 > Me3C-NH2 > (Me3C)2NH > (Me3C)3N.