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.

Strongest Bronsted base out of the following will be:=

Toluidine

The correct answer is option 3. Toluidine.

Let us break down why toluidine is the strongest Brønsted base among the given options:

1. Pyrrole

Pyrrole is an aromatic compound where the nitrogen atom is part of the five-membered ring. The lone pair of electrons on the nitrogen atom in pyrrole is involved in maintaining the aromaticity of the ring (contributing to the π-system). Because these electrons are less available for protonation, pyrrole is a very weak base.

2. Pyridine

Pyridine is an aromatic compound with a six-membered ring structure similar to benzene, but with a nitrogen atom replacing one of the carbon atoms. The nitrogen atom's lone pair in pyridine is not involved in the aromatic π-system and is readily available for protonation. This makes pyridine a relatively stronger base than pyrrole, but other factors can affect its basicity in comparison to toluidine.

3. Toluidine

Toluidine is an aniline derivative where the amino group \((-NH_2)\) is directly attached to a benzene ring, and there's a methyl group attached to the ring (the position of the methyl group can vary—ortho, meta, or para). The methyl group is an electron-donating group, which increases the electron density on the nitrogen atom in the amino group. This increased electron density makes the nitrogen's lone pair more available for protonation, enhancing its basicity. Therefore, toluidine is a stronger base than pyridine because of the electron-donating effect of the methyl group.

4. 4-Nitrobenzamine

4-Nitrobenzamine is an aniline derivative with a nitro group \((-NO_2)\) attached to the benzene ring at the para position relative to the amino group. The nitro group is a strong electron-withdrawing group, which significantly reduces the electron density on the nitrogen atom. This makes the lone pair on the nitrogen less available for protonation, greatly reducing its basicity. As a result, 4-nitrobenzamine is the weakest base among the given options.

Conclusion
Toluidine is the strongest base among the options due to the electron-donating effect of the methyl group, which increases the availability of the nitrogen’s lone pair for protonation. This makes toluidine a stronger Brønsted base compared to pyridine, pyrrole, and 4-nitrobenzamine.