Ch. 17 - Reactions of Aromatic Compounds

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 17 - Reactions of Aromatic Compounds

Problem 3

Chapter 17, Problem 3

p-Xylene undergoes nitration much faster than benzene. Use resonance forms of the sigma complex to explain this accelerated rate

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Identify the structure of p-xylene. It is a benzene ring with two methyl groups attached at the para positions. These methyl groups are electron-donating groups through hyperconjugation and inductive effects, which increase the electron density on the benzene ring.

Understand the nitration reaction. Nitration involves the electrophilic substitution of a nitro group (-NO₂) onto the benzene ring. The rate of this reaction depends on the electron density of the ring, as a higher electron density makes the ring more reactive toward the electrophile (NO₂⁺).

Draw the resonance forms of the sigma complex (arenium ion) formed during the nitration of p-xylene. Focus on the intermediates where the positive charge is delocalized over the ring. The methyl groups stabilize the positive charge through hyperconjugation and inductive effects, making the intermediates lower in energy compared to benzene.

Compare the resonance stabilization of the sigma complex in p-xylene versus benzene. In p-xylene, the electron-donating methyl groups provide additional stabilization to the sigma complex, which lowers the activation energy for the reaction and accelerates the rate of nitration.

Conclude that the increased rate of nitration in p-xylene is due to the electron-donating effects of the methyl groups, which enhance the electron density of the ring and stabilize the sigma complex through resonance and hyperconjugation.

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Key Concepts

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

Nitration of Aromatic Compounds

Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) replaces a hydrogen atom on an aromatic ring. This reaction typically requires a strong electrophile, such as the nitronium ion (NO2+), generated from a mixture of concentrated nitric acid and sulfuric acid. The rate of nitration can vary significantly among different aromatic compounds based on their electronic properties.

Resonance and Stability of Sigma Complex

The sigma complex, or arenium ion, is an intermediate formed during electrophilic aromatic substitution. It features a positive charge that can be delocalized over the aromatic ring through resonance. The more resonance structures that can be drawn for the sigma complex, the more stable it becomes, which enhances the reaction rate. In the case of p-xylene, the presence of electron-donating methyl groups increases the number of resonance forms, stabilizing the sigma complex.

Electron-Donating Effects of Alkyl Groups

Alkyl groups, such as methyl groups in p-xylene, are electron-donating through hyperconjugation and inductive effects. This donation of electron density increases the electron richness of the aromatic ring, making it more reactive towards electrophiles like the nitronium ion. Consequently, p-xylene's increased electron density facilitates the formation of the sigma complex, leading to a faster nitration reaction compared to benzene, which lacks such electron-donating substituents.

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Textbook Question

(b) Explain why m-xylene undergoes nitration 100 times faster than p-xylene.

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Textbook Question

Step 2 of the iodination of benzene shows water acting as a base and removing a proton from the sigma complex. We did not consider the possibility of water acting as a nucleophile and attacking the carbocation, as in an electrophilic addition to an alkene. Draw the reaction that would occur if water reacted as a nucleophile and added to the carbocation. Explain why this type of addition is rarely observed.

Propose a mechanism for the aluminum chloride–catalyzed reaction of benzene with chlorine.

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Textbook Question

Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results.

Propose a mechanism for the bromination of ethoxybenzene to give o- and p-bromoethoxybenzene.

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