Question

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

• A. \((CH_3)_2NH>CH_3NH_2> (CH_3)_3N\)
• B. \((CH_3)_2N> CH_3NH_2>(CH_3)_2NH\)
• C. \((CH_3)_2N>(CH_3)_2NH>CH_3NH_2 \)
• D. \(CH_3NH_2>(CH_3)_2NH>(CH_3)_3N \)

Answer 1

The Correct Option is A

To determine the correct order of the basic strength of methyl-substituted amines in aqueous solution, we need to understand the factors that affect the basicity of amines.

The basicity of amines depends on the availability of the lone pair of electrons on the nitrogen atom to accept a proton. Several factors influence this availability:

1. Inductive Effect: Alkyl groups such as methyl groups, being electron-releasing in nature, increase the electron density on the nitrogen atom through the +I effect. This increases the basicity.
2. Steric Hindrance: The presence of bulky groups around the nitrogen atom can hinder the solvation by water molecules, which is needed for protonation. This can reduce the basicity.
3. Solvation Effect: In aqueous solution, solvation stabilizes the conjugate acid formed after protonation. This is more pronounced in less hindered amines.

Based on these factors, let's analyze each option:

1. (CH_3)_2NH (Dimethylamine): This has two methyl groups that increase the electron density on nitrogen by the inductive effect, but it does not suffer much from steric hindrance. Thus, it is highly basic.
2. CH_3NH_2 (Methylamine): This has one methyl group, which provides some +I effect, and has low steric hindrance, allowing good solvation, making it moderately basic.
3. (CH_3)_3N (Trimethylamine): It has three methyl groups, providing a strong +I effect, but suffers from significant steric hindrance, which reduces solvation and, consequently, the basicity.

Hence, the correct order of basicity in an aqueous solution is:

(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N

This is because dimethylamine has the best combination of the inductive effect and minimal steric hindrance, followed by methylamine, and finally trimethylamine due to steric limitations.