Question

The correct order of decreasing basic strength of $CH_3NH_2$, $(CH_3)_2NH$ and $(CH_3)_3N$ in aqueous solution is:

• A. $(CH_3)_3N \gt (CH_3)_2NH \gt CH_3NH_2$
• B. $CH_3NH_2 \gt (CH_3)_2NH \gt (CH_3)_3N$
• C. $(CH_3)_2NH \gt CH_3NH_2 \gt (CH_3)_3N$
• D. $(CH_3)_2NH \gt (CH_3)_3N \gt CH_3NH_2$

Hint:

In water: $2^\circ \gt 1^\circ \gt 3^\circ$ for methyl amines. Solvation of conjugate acid matters as much as inductive effect.

Answer 1

The Correct Option is C

Step 1: Factors governing basicity of amines in water.
Aqueous basicity of aliphatic amines depends on three factors: (i) inductive $+I$ effect of alkyl groups increasing electron density on N, (ii) steric hindrance reducing solvation of the conjugate acid, and (iii) number of N-H bonds in the conjugate acid available for hydrogen bonding with water.
Step 2: Applying to each amine.
Dimethylamine $(CH_3)_2NH$: two methyl groups give strong $+I$ effect; conjugate acid $(CH_3)_2NH_2^+$ has two N-H bonds for good solvation. Methylamine $CH_3NH_2$: one methyl group, weaker $+I$ effect; conjugate acid has three N-H bonds but less inductive stabilisation. Trimethylamine $(CH_3)_3N$: maximum $+I$ effect, but severe steric hindrance and conjugate acid has only one N-H bond (very poor solvation).
Step 3: Ranking.
The balance of $+I$ effect and solvation gives: $(CH_3)_2NH \gt CH_3NH_2 \gt (CH_3)_3N$ in aqueous solution.
\[ \boxed{(CH_3)_2NH \gt CH_3NH_2 \gt (CH_3)_3N} \]