Question

(a) Besides destabilizing the anilinium ion, how else might a nitro group affect basicity? (b) Why does the nitro group exert a larger base-weakening effect from the para position than from the nearer meta position?

Short answer

A nitro group affects the basicity of an aniline derivative by decreasing electron density on the nitrogen atom through resonance and inductive effects, making the amine less basic. The base-weakening effect is stronger at the para position, where both resonance and inductive effects fully participate, compared to the meta position where the resonance effect is less prominent.

Step-by-step solution

Understand the Basicity of Amines

Amines are basic compounds due to the lone pair of electrons on the nitrogen atom. These electrons can be donated to a proton (H+) to form the conjugate acid of the amine. A more stable conjugate acid of the amine indicates a stronger base.

Role of Nitro Group on Basicity

A nitro group (-NO_2) influences the basicity of amines through two major effects: 1. Resonance/Conjugation effect: The nitro group has a structure that allows it to withdraw electron density from the rest of the molecule by resonance. This decreases the electron density around the nitrogen atom, making it less likely to donate its electrons to a proton. 2. Inductive effect: The nitro group is an electron-withdrawing group (EWG) due to the presence of the highly electronegative nitrogen and oxygen atoms. This leads to electron density being pulled away from the nitrogen through the sigma bonds, which further decreases the electron density on the nitrogen atom and weakens the basicity of the amine.

Basicity Differences between Meta and Para position

When the nitro group is at the meta position, the electron-withdrawing inductive effect is still present, but the resonance effect is not as pronounced since the nitro group is not directly conjugated with the nitrogen atom. This results in a weaker base-weakening effect in the meta position compared to the para position. On the other hand, when the nitro group is at the para position, both the resonance and inductive effects are fully at play. The nitro group directly contributes to the delocalization of the nitrogen's lone pair, which results in a weaker basicity of the amine compared to the meta position.

Summary

In conclusion, a nitro group affects the basicity of an aniline derivative through resonance and inductive effects, making the amine less basic. The base-weakening effect is stronger at the para position due to the full participation of both resonance and inductive effects, compared to the meta position where the resonance effect is less prominent.

Key concepts

Resonance Effect in Amines

The basicity of amines is primarily determined by the availability of their nitrogen's lone pair of electrons for binding with protons (H⁺). In the context of aromatic amines, such as anilines, the resonance effect plays a pivotal role. Resonance refers to the delocalization of electrons within a molecule, allowing the electrons to be distributed over several atoms, thereby stabilizing the system.

Take aniline as an example, the lone pair on the nitrogen can participate in resonance with the aromatic ring, slightly delocalizing the charge and reducing the electron density on the nitrogen. This reduces the basicity since the lone pair is less available to react with protons. When an electron-withdrawing group like a nitro group is present, it further pulls electron density through resonance, significantly diminishing the basicity of the amine. The concept illustrates why the nature and position of substituents on the aromatic ring can deeply impact an amine's basicity through resonance effects.

Inductive Effect on Basicity

Alongside resonance, the inductive effect is another key factor influencing the basicity of amines. The inductive effect is the transmission of charge through a chain of atoms in a molecule, which occurs due to the electronegativity difference between adjacent atoms.

An electron-withdrawing group such as a nitro group will cause a negative inductive effect, pulling electron density away from the amine's nitrogen along the sigma bonds. This decreases the electron density around the nitrogen atom, lowering the availability of the lone pair for bonding with protons and thus reducing the basicity of the amine. The strength of the inductive effect diminishes with increasing distance from the group causing it; hence, its impact is more profound when the electron-withdrawing group is closer to the amine nitrogen.

Meta vs Para Position in Amines

The position of substituents on an aromatic ring can significantly alter an amine's basicity due to variations in resonance and inductive effects. Comparing the meta and para positions, the para position allows for certain groups to interact directly with the amine nitrogen through a continuous system of p-orbitals.

For example, when a nitro group is at the para position of aniline, it exerts both inductive and resonance effects on the nitrogen's lone pair, making the amine much less basic. In contrast, at the meta position, the resonance effect is markedly less because the nitro group is not aligned in the same plane as the nitrogen's lone pair, although the inductive effect persists. This results in the para-substituted amine being less basic than its meta counterpart due to the combined influence of both resonance and inductive effects at the para position.