Which of the following groups when present at para position increases the basic strength of aniline?

NH₂

The correct answer is option 2. \(NH_2\).

To understand how substituents affect the basic strength of aniline, it's important to consider the electronic effects they impart on the aniline molecule. The basic strength of aniline depends on the availability of the lone pair of electrons on the nitrogen atom for protonation. Electron-donating groups increase the electron density on the nitrogen, making it more basic, while electron-withdrawing groups decrease the electron density, making it less basic.

Let us analyze each substituent:

1. \(COOH\) (carboxyl group):

The carboxyl group is an electron-withdrawing group through both resonance and inductive effects. At the para position, it withdraws electron density from the benzene ring, reducing the electron density on the nitrogen atom of the aniline. This decreased electron density makes the nitrogen's lone pair less available for protonation, thereby decreasing the basicity of aniline.

2. \(NH_2\) (amino group):

The amino group is an electron-donating group through both resonance and inductive effects. When the \( -NH_2 \) group is at the para position, it can donate electron density via resonance into the benzene ring, which in turn increases the electron density on the nitrogen atom of the aniline. This increased electron density makes the nitrogen's lone pair more available for protonation, thereby increasing the basicity of aniline.

3. Br (bromine):

Bromine is an electron-withdrawing group through induction but can donate electrons via resonance (though this effect is weaker compared to its inductive effect). The overall effect of bromine at the para position is not strongly electron-donating, so it does not significantly increase the basicity of aniline. However, it also does not greatly decrease it.

4. \(NO_2\) (nitro group):

The nitro group is a strong electron-withdrawing group through both resonance and inductive effects. At the para position, it strongly withdraws electron density from the benzene ring, significantly reducing the electron density on the nitrogen atom of the aniline. This strong electron-withdrawing effect makes the nitrogen's lone pair much less available for protonation, greatly decreasing the basicity of aniline.

Given these effects, the \(NH_2\) group is the one that increases the basic strength of aniline when present at the para position, as it donates electron density to the ring and enhances the availability of the nitrogen's lone pair for protonation.