Question

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

• A. $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2$
• B. $CH_3NH_2> (CH_3)2NH > (CH_3)_3N$
• C. $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N$
• D. $(CH_3)_3N > CH_3NH_2> (CH_3)_2NH$

Answer 1

The Correct Option is C

To determine the correct order of the basic strength of methyl substituted amines in an aqueous solution, we need to consider the inductive effect as well as the solvation effect.

1. Inductive Effect: Methyl groups are electron-donating, which increases the electron density on nitrogen in amines. This increases the availability of the lone pair of electrons for donation, enhancing basicity.
   • In this sense, the order of basicity should be: CH_3NH_2 < (CH_3)_2NH < (CH_3)_3N
2. Solvation Effect: The basicity in aqueous solution is significantly affected by the solvation of the ion formed after the donation of the lone pair of electrons.
   • Tertiary amines like (CH_3)_3N have more steric hindrance which inhibits proper solvation, making them less basic in water.
   • Primary amines like CH_3NH_2 are better solvated than tertiary amines.
   • Secondary amines like (CH_3)_2NH strike a balance between the inductive effect and solvation, leading to the highest basicity in aqueous solution.
3. Combining these effects, the correct order of basic strength in aqueous solution is found to be: (CH_3)_2NH > CH_3NH_2 > (CH_3)_3N.

This matches the given correct answer: (CH_3)_2NH > CH_3NH_2 > (CH_3)_3N.