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 2

Chapter 17, Problem 2

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

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The reaction is an example of an electrophilic aromatic substitution (EAS) reaction. Begin by recognizing that aluminum chloride (AlCl₃) acts as a Lewis acid catalyst, which helps to generate the electrophile (Cl⁺) from molecular chlorine (Cl₂).

In the first step, AlCl₃ interacts with Cl₂ to form a complex: \( \text{Cl}_2 + \text{AlCl}_3 \rightarrow \text{Cl}^+ + \text{AlCl}_4^- \). This generates the highly reactive electrophile, \( \text{Cl}^+ \).

Next, the benzene ring, which is electron-rich due to its delocalized π-electrons, attacks the \( \text{Cl}^+ \) electrophile. This forms a sigma complex (also called an arenium ion), where one of the benzene carbons is bonded to the chlorine atom, and the aromaticity of the ring is temporarily lost.

In the subsequent step, the sigma complex loses a proton (H⁺) from the carbon bonded to the chlorine atom. This proton is abstracted by \( \text{AlCl}_4^- \), regenerating \( \text{AlCl}_3 \) and forming HCl as a byproduct. The aromaticity of the benzene ring is restored in this step.

Finally, the overall reaction results in the formation of chlorobenzene (C₆H₅Cl) and HCl, with \( \text{AlCl}_3 \) acting as a catalyst and being regenerated at the end of the reaction.

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Key Concepts

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

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In this mechanism, the aromatic system donates electrons to the electrophile, forming a sigma complex. This process is crucial for understanding how benzene reacts with various electrophiles, including chlorine in the presence of a catalyst like aluminum chloride.

Role of Aluminum Chloride as a Catalyst

Aluminum chloride (AlCl3) acts as a Lewis acid catalyst in the reaction between benzene and chlorine. It enhances the electrophilicity of chlorine by forming a complex with it, which generates a more reactive electrophile, Cl+. This activation is essential for facilitating the EAS mechanism, allowing the chlorine to effectively attack the benzene ring.

Formation of the Sigma Complex

The sigma complex, or arenium ion, is an intermediate formed during the electrophilic aromatic substitution process. When the electrophile attacks the benzene ring, it disrupts the aromaticity, leading to a positively charged intermediate. This step is critical as it determines the stability of the reaction pathway and influences the final product formation after deprotonation.

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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.

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.

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

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