Textbook Question
Predict the product of the following Friedel–Crafts alkylation reactions. Assume only one alkyl group adds in each case.
(d)
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19. Reactions of Aromatics: EAS and Beyond
EAS:Friedel-Crafts Alkylation Mechanism
5:05 minutesProblem 15a
Textbook Question
Propose products (if any) and mechanisms for the following AlCl3-catalyzed reactions:
a. chlorocyclohexane with benzene
Verified step by step guidance1
Step 1: Recognize that this reaction involves Friedel-Crafts alkylation, a common electrophilic aromatic substitution reaction catalyzed by AlCl₃. The chlorocyclohexane serves as the alkylating agent, and benzene acts as the aromatic substrate.
Step 2: Understand the role of AlCl₃. It acts as a Lewis acid and interacts with the chlorine atom in chlorocyclohexane, forming a complex that generates a cyclohexyl carbocation (C₆H₁₁⁺). This carbocation is the electrophile in the reaction.
Step 3: Benzene, as an aromatic compound, has a high electron density in its π-system. The cyclohexyl carbocation attacks the benzene ring, leading to the formation of a sigma complex (arenium ion). This intermediate temporarily disrupts the aromaticity of benzene.
Step 4: The sigma complex undergoes deprotonation at the site of electrophilic attack, restoring the aromaticity of benzene. The final product is cyclohexylbenzene (C₆H₅-C₆H₁₁).
Step 5: Regenerate the AlCl₃ catalyst. The chloride ion (Cl⁻) released during the deprotonation step combines with the AlCl₃ complex to reform AlCl₄⁻, ensuring the catalyst is available for further reactions.
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Video duration:
5mKey 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 context, the benzene acts as a nucleophile, attacking the electrophile generated from chlorocyclohexane in the presence of AlCl3, which enhances the electrophilicity of the chlorocyclohexane.
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Lewis Acid Catalysis
Lewis acid catalysis involves the use of a Lewis acid, such as AlCl3, to facilitate chemical reactions by accepting an electron pair. In this reaction, AlCl3 coordinates with the chlorine atom of chlorocyclohexane, forming a more reactive carbocation that can then participate in the electrophilic aromatic substitution with benzene.
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Carbocation Stability
Carbocation stability is crucial in determining the outcome of reactions involving carbocations. The stability of the carbocation formed during the reaction influences the likelihood of substitution occurring. In this case, the stability of the carbocation generated from chlorocyclohexane will affect the efficiency of the electrophilic attack on the benzene ring.
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