Concept:In aqueous medium, basic strength of aliphatic amines depends on the combined effect of the electron-donating inductive effect of alkyl groups and the solvation of the conjugate acid.
Explanation:The
+I effect of methyl groups increases electron density on the nitrogen atom, making methylamines stronger bases than ammonia.
In water, the basicity order is determined by a balance between this inductive effect and the hydration of the ammonium ion formed.
The conjugate acid having more N–H hydrogens is better stabilized by hydrogen bonding with water.
(CH3)2NH has the best combination of inductive effect and solvation, so it is the strongest base.
(CH3)3N has the greatest
+I effect but very poor solvation because its conjugate acid has no N–H hydrogen, lowering its basicity below dimethylamine and methylamine.
Therefore, the correct increasing order of basic strength is:
NH3<(CH3)3N<CH3NH2<(CH3)2NHAnswer:Option D:
NH3<(CH3)3 N<CH3NH2<(CH3)2NH