Animated Solution for Chemistry - Organic Chemistry: The correct order of increasing basic nature for the bases ${\text{NH}}_3,{\text{CH}}_3{\text{NH}}_2$ and $({\text{CH}}_3{)}_2\text{NH}$ is

When we talk about the basicity of amines in an aqueous solution, we are essentially looking at a competition. It is a competition to see which molecule is the most eager to grab a proton ($H^{+}$) from water.

According to the Bronsted-Lowry theory, a base is a proton acceptor. Amines have a lone pair of electrons on the nitrogen atom, which acts as the perfect landing pad for a proton.

When an amine accepts a proton, it transforms into a positively charged ammonium cation. The golden rule here is simple: the more stable this resulting conjugate acid is, the stronger the original base was.

In the watery arena of an aqueous solution, the stability of the ammonium cation is governed by a delicate balance of three main factors:

1. The Inductive Effect ($+I$): Alkyl groups are electron-donating. They push electron density towards the nitrogen atom, stabilizing the positive charge. 2. Solvation (Hydrogen Bonding): Water molecules surround the cation and stabilize it through hydrogen bonding. The more hydrogen atoms attached to the nitrogen, the better the solvation. 3. Steric Hindrance: Bulky alkyl groups can physically block water molecules from approaching the cation, hindering solvation.

Let's look at our three contenders: ammonia ($N{H}_3$), methylamine ($C{H}_{3}N{H}_2$), and dimethylamine ($(C{H}_3{)}_{2}NH$).

Ammonia is our baseline. It has no alkyl groups, so it relies entirely on its three hydrogen atoms for solvation. It has no $+I$ effect to boost its electron density.

Methylamine steps up the game. It has one methyl group. This group exerts a $+I$ effect, pushing electron density onto the nitrogen. This makes the lone pair more available and stabilizes the resulting cation better than ammonia.

Dimethylamine takes it even further. With two methyl groups, the $+I$ effect is doubled! The electron density on the nitrogen is significantly higher.

For methyl-substituted amines, the alkyl groups are relatively small. Because they are small, steric hindrance does not become a major issue until we reach the tertiary amine (trimethylamine).

Therefore, up to the secondary amine, the inductive effect dominates over the loss of one hydrogen bond. The two methyl groups in dimethylamine make it the strongest base among the three. Methylamine comes in second, and ammonia, lacking any electron-donating groups, is the weakest.

This leads us to our final, elegant order of increasing basic nature: