The correct order of the basic strength of methyl-substituted amines in aqueous solution is:

Solution & Explanation

\textbf{Answer:} (A) In water, the order is governed by the balance of inductive (+I), steric, and solvation (H-bond stabilisation of the cation) effects. \textbf{Solution:} Three opposing factors decide basicity of methylamines in aqueous medium: 1. \textbf{+I effect}: more $-\mathrm{CH}{}_3$ groups push electron density onto $\mathrm{N}$, increasing basicity (favours $(\mathrm{CH}{}_3){}_3\mathrm{N}>(\mathrm{CH}{}_3){}_2\mathrm{NH}>\mathrm{CH}{}_3\mathrm{NH}{}_2$). 2. \textbf{Steric hindrance}: more/bulkier groups hinder protonation, decreasing basicity for $(\mathrm{CH}{}_3){}_3\mathrm{N}$. 3. \textbf{Solvation}: the conjugate acid is stabilised by H-bonding to water. More $\mathrm{N}-\mathrm{H}$ bonds in the cation mean more H-bonds, so $\mathrm{CH}{}_3\mathrm{NH}{}_3{}^{+}$ and $(\mathrm{CH}{}_3){}_2\mathrm{NH}{}_2{}^{+}$ are better solvated than $(\mathrm{CH}{}_3){}_3\mathrm{NH}{}^{+}$. The net result in water places the secondary amine highest: $(\mathrm{CH}{}_3){}_2\mathrm{NH}>\mathrm{CH}{}_3\mathrm{NH}{}_2>(\mathrm{CH}{}_3){}_3\mathrm{N}$ Hence option (A) is correct.