Show why p-nitroaniline is a much weaker base (3 pK_b units weaker) than aniline.

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Understand the basicity of aniline: Aniline (C₆H₅NH₂) is a primary aromatic amine where the lone pair of electrons on the nitrogen atom can participate in accepting a proton, making it a base. The basicity of aniline is influenced by the electron density on the nitrogen atom.

Introduce the effect of the nitro group: In p-nitroaniline, a nitro group (-NO₂) is attached to the para position of the benzene ring. The nitro group is an electron-withdrawing group due to its strong -I (inductive) and -M (mesomeric) effects, which reduce the electron density on the benzene ring and, consequently, on the nitrogen atom.

Explain resonance effects: The nitro group at the para position can delocalize the lone pair of electrons on the nitrogen atom through resonance. This delocalization reduces the availability of the lone pair for protonation, thereby decreasing the basicity of p-nitroaniline compared to aniline.

Discuss the inductive effect: The -NO₂ group also exerts a strong inductive electron-withdrawing effect through the sigma bonds, further reducing the electron density on the nitrogen atom. This makes the nitrogen less nucleophilic and less likely to accept a proton.

Relate to pKb values: The pKb value is inversely related to the basicity of a compound. Since p-nitroaniline has a much lower electron density on the nitrogen atom due to the combined inductive and resonance effects of the nitro group, it is a weaker base than aniline, resulting in a higher pKb value (3 units higher).

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주요 개념

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Basicity of Amines

Basicity in amines refers to their ability to accept protons (H+). The strength of a base is often measured by its pKb value, where a lower pKb indicates a stronger base. Aniline, with its amino group attached to a phenyl ring, is a relatively strong base due to the availability of the lone pair of electrons on the nitrogen atom for protonation.

Electron-Withdrawing Groups

Electron-withdrawing groups, such as the nitro group (-NO2) in p-nitroaniline, decrease the electron density on the nitrogen atom. This reduction in electron density makes it less likely for the nitrogen to donate its lone pair to accept a proton, thus weakening the basicity of the compound compared to aniline, which lacks such a group.

Resonance Effects

Resonance effects occur when electron pairs are delocalized over multiple atoms, affecting the stability and reactivity of a molecule. In p-nitroaniline, the resonance between the nitro group and the aromatic ring stabilizes the lone pair on the nitrogen, making it less available for protonation. This contributes to the observed decrease in basicity compared to aniline, which does not have this stabilizing interaction.

Show why p-nitroaniline is a much weaker base (3 pKb units weaker) than aniline.