Amines are reactive in nature, mainly due to difference in electronegativity between Nitrogen and Hydrogen and, due to lone pair of electrons on Nitrogen. Amines behaves like nucleophiles and as good Lewis bases and there are various reactions to support this. Different degree of amines differ in their reactivity due to steric hinderance and electronic effects. Aromatic amines are less basic due to +R effect, hence have higher \(pK_b\) value than the aliphatic amines. Basic strength of amines also depend on stability of corresponding conjugate acid in aqueous medium.

Conjugate acid of which of the following amine is most stable?

Aniline

The correct answer is option 3. Aniline.

Let us look into why the conjugate acid of aniline is the most stable among the given options.

A conjugate acid is formed when a base (in this case, an amine) accepts a proton \((\text{H}^+)\). The stability of the conjugate acid depends on how well the positive charge on the nitrogen atom (which is formed after protonation) is stabilized.

Analyzing Each Amine:

1. Ethylamine \((\text{C}_2\text{H}_5\text{NH}_2)\):

Conjugate Acid: \(\text{C}_2\text{H}_5\text{NH}_3^+\)

Effect: The ethyl group (\(\text{C}_2\text{H}_5\)) is a weak electron-donating group through an inductive effect. This donation slightly stabilizes the positive charge on the nitrogen, but the effect is minimal

2. Triethylamine \((\text{(C}_2\text{H}_5\text{)}_3\text{N})\):

Conjugate Acid: \(\text{(C}_2\text{H}_5\text{)}_3\text{NH}^+\)

Effect: Triethylamine has three ethyl groups attached to nitrogen. While these groups are electron-donating, their cumulative effect can actually lead to steric hindrance and overcrowding around the nitrogen atom, which reduces the stability of the conjugate acid. Additionally, too much electron density around the nitrogen can destabilize the positive charge.

3. Aniline \((\text{C}_6\text{H}_5\text{NH}_2)\):

Conjugate Acid: \(\text{C}_6\text{H}_5\text{NH}_3^+\)

Effect: The phenyl group (\(\text{C}_6\text{H}_5\)) attached to nitrogen has a unique ability to stabilize the positive charge through **resonance**. When aniline is protonated to form its conjugate acid, the positive charge on the nitrogen can be delocalized over the aromatic ring. This delocalization reduces the electron density on the nitrogen and distributes the positive charge over a larger area, significantly stabilizing the conjugate acid.

4. Diethylamine \((\text{(C}_2\text{H}_5\text{)}_2\text{NH})\):

Conjugate Acid: \(\text{(C}_2\text{H}_5\text{)}_2\text{NH}_2^+\)

Effect: Diethylamine has two ethyl groups, which donate electrons and somewhat stabilize the positive charge on the nitrogen. However, the stabilization is still weaker than the resonance effect seen in aniline.

Why Aniline’s Conjugate Acid is the Most Stable:

Resonance Stabilization: The key factor that makes the conjugate acid of aniline most stable is resonance. The lone pair of electrons on nitrogen in aniline can overlap with the \(\pi\)-system of the benzene ring. When aniline is protonated, the positive charge on nitrogen can be delocalized into the benzene ring, spreading the charge over a larger area and stabilizing the molecule.

Resonance Structures:

Inductive Effect: While alkyl groups like ethyl provide some stabilization through the inductive effect, they cannot compete with the significant stabilization provided by resonance in the case of aniline.

The resonance in aniline's conjugate acid provides superior stabilization compared to the inductive effects in the other amines, making the conjugate acid of aniline the most stable. The correct answer is option 3: Aniline.