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Why p-Nitroaniline is Less Basic than Aniline

Understanding why p-nitroaniline is less basic than aniline requires an exploration of the chemical structures of both compounds, as well as their electronic properties. The key difference between these two compounds is the presence of a nitro group (-NO₂) in p-nitroaniline, which has significant implications on their basicity. Let’s break down the chemistry step by step to understand this phenomenon.

Basicity in Organic Compounds

Basicity in organic compounds, particularly amines, depends on the availability of the lone pair of electrons on the nitrogen atom. In aniline (C₆H₅NH₂), the nitrogen atom in the amino group has a lone pair of electrons. The basicity of aniline is influenced by how readily this lone pair can accept protons (H⁺). When comparing p-nitroaniline and aniline, we need to consider how the nitro group affects the availability of this lone pair.

Influence of Electron-Withdrawing Groups

The nitro group (-NO₂) is a strong electron-withdrawing group due to its high electronegativity and resonance effects. In p-nitroaniline, the nitro group is attached to the para position relative to the amino group on the benzene ring. This positioning allows the nitro group to exert its electron-withdrawing influence through the ring.

Because the nitro group pulls electron density away from the ring and towards itself, it reduces the electron density available on the nitrogen atom's lone pair in the amino group. Consequently, the nitrogen in p-nitroaniline becomes less able to donate its lone pair to accept a proton, leading to decreased basicity.

Resonance Effect in p-Nitroaniline

A critical factor in why p-nitroaniline is less basic than aniline is the resonance effect. In p-nitroaniline, resonance structures can be drawn where the lone pair of electrons on the nitrogen in the amino group participates in conjugation with the π-electrons of the benzene ring. This delocalization of electrons means that the lone pair is not entirely localized on the nitrogen atom, making it less available for protonation.

Moreover, the nitro group at the para position further stabilizes this resonance by withdrawing electron density from the ring, enhancing the delocalization. As a result, the nitrogen's lone pair is less nucleophilic, and thus, the basicity of p-nitroaniline is lower compared to aniline, where no such electron-withdrawing group is present.

Inductive Effects and Their Role

In addition to the resonance effect, the inductive effect plays a significant role in determining why p-nitroaniline is less basic than aniline. The nitro group exerts a strong -I effect (inductive electron-withdrawing effect), pulling electron density through the σ-bonds of the molecule. This decreases the electron density around the nitrogen atom, weakening its ability to donate electrons and decreasing its basicity.

Aniline, lacking an electron-withdrawing group, does not experience such a strong inductive effect, allowing its nitrogen atom to maintain a higher electron density and thus a stronger ability to accept protons.

Conclusion

In summary, the reason why p-nitroaniline is less basic than aniline can be attributed to the electron-withdrawing effects of the nitro group. Both resonance and inductive effects reduce the electron density around the nitrogen atom in p-nitroaniline, making it less capable of accepting protons. In contrast, aniline, with no such electron-withdrawing groups, maintains higher basicity due to the availability of the nitrogen's lone pair for protonation.