Animated Solution for Chemistry - Organic Chemistry: Which one of the following is the strongest base in aqueous solution ?

When we talk about the basicity of amines, we are fundamentally asking one question: how willing is the nitrogen atom to share its lone pair of electrons?

A strong base is generous; it readily donates its lone pair to accept a proton ($H^{+}$). A weak base, on the other hand, holds onto its electrons tightly or has them distracted elsewhere.

In this problem, we are tasked with finding the strongest base among four candidates in an aqueous solution. This environment is crucial because water changes the rules of the game entirely!

In the gas phase, basicity is simple: more electron-donating groups mean a stronger base. But in an aqueous solution, basicity is a delicate tug-of-war between three competing factors:

1. The $+I$ (Inductive) Effect: Alkyl groups push electron density towards the nitrogen, making it more basic. 2. Solvation Effect: Once the amine accepts a proton, it becomes a positively charged cation. Water molecules stabilize this cation through hydrogen bonding. More $N-H$ bonds mean better stabilization. 3. Steric Hindrance: Bulky groups around the nitrogen physically block the incoming proton and the surrounding water molecules.

A Resonance Trap

Let's first look at aniline ($C_6{H}_{5}N{H}_2$). You might think it's a decent base, but it has a fatal flaw.

The lone pair on the nitrogen atom is in conjugation with the $\pi$-electron system of the benzene ring. Through resonance, this lone pair is delocalized over the entire ring.

Because the electrons are "busy" dancing around the ring, they are simply not available to accept a proton. This makes aniline the weakest base among our options.

Inductive vs. Solvation

Now, let's compare the aliphatic amines: methylamine ($1^{\circ }$), dimethylamine ($2^{\circ }$), and trimethylamine ($3^{\circ }$).

If we only considered the $+I$ effect, the order would be $3^{\circ }>2^{\circ }>1^{\circ }$. Trimethylamine has three methyl groups pumping electron density into the nitrogen!

However, we must consider solvation. When these amines are protonated, the $1^{\circ }$ amine forms an ion with three $N-H$ bonds, allowing for excellent hydrogen bonding with water. The $3^{\circ }$ amine ion only has one $N-H$ bond, leading to poor solvation. Furthermore, the three bulky methyl groups in the $3^{\circ }$ amine create severe steric hindrance.

Dimethylamine

So, who wins this tug-of-war? The answer lies in finding the perfect balance.

Dimethylamine ($2^{\circ }$) has two methyl groups providing a strong $+I$ effect, and it still has enough room and $N-H$ bonds to be well-stabilized by water. It hits the "Goldilocks zone" of basicity.

Therefore, for methyl-substituted amines in aqueous solution, the $2^{\circ }$ amine is the strongest. The final order is $2^{\circ }>1^{\circ }>3^{\circ }>\text{Aniline}$. Dimethylamine takes the crown!

A Word of Caution

Before we wrap up, here is a crucial trap to avoid. What if the alkyl groups were ethyls instead of methyls?

Ethyl groups are larger and provide a stronger $+I$ effect. In this case, the inductive effect overpowers the solvation effect. The order shifts to $2^{\circ }>3^{\circ }>1^{\circ }$.

Always pay close attention to the specific alkyl group and the solvent when solving basicity questions in organic chemistry!