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Ch. 19 - Amines
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 19 - AminesProblem 60b
Chapter 19, Problem 60b

Show why p-nitroaniline is a much weaker base (3 pKb 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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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.
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Reductive Amination

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.
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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.
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Related Practice
Textbook Question

Pyrrole undergoes electrophilic aromatic substitution more readily than benzene, and mild reagents and conditions are sufficient. These reactions normally occur at the 2-position rather than the 3-position, as shown in the following example.

b. Explain why pyrrole reacts more readily than benzene, and also why substitution occurs primarily at the 2-position rather than the 3-position.

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Textbook Question

Basicity depends on availability of an electron pair to bond a proton. Correlate structural effects in these amines with their basicities.

(a) Explain this order:

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Textbook Question

Basicity depends on availability of an electron pair to bond a proton. Correlate structural effects in these amines with their basicities.

(b) Explain:

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Textbook Question

The following spectra for A and B correspond to two structural isomers. The NMR singlet at δ1.16 in spectrum A disappears when the sample is shaken with D2O. The singlet at δ0.6 ppm in the spectrum of B disappears on shaking with D2O. Propose structures for these isomers, and show how your structures correspond to the spectra. Show what cleavage is responsible for the base peak at m/z 44 in the mass spectrum of A and the prominent peak at m/z 58 in the mass spectrum of B.

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Textbook Question

Guanidine (shown) is about as strong a base as hydroxide ion. Explain why guanidine is a much stronger base than most other amines.

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Textbook Question

Explain why N,N,2,6-tetramethylaniline (shown) is a much stronger base than N,N-dimethylaniline.

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