Textbook Question
Design a synthesis for each of the following, using an intramolecular reaction:
d.
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19. Reactions of Aromatics: EAS and Beyond
EAS:Friedel-Crafts Alkylation Mechanism
8:22 minutesProblem 15b
Textbook Question
Propose products (if any) and mechanisms for the following AlCl3-catalyzed reactions:
b. methyl chloride with anisole
Verified step by step guidance1
Step 1: Recognize that the reaction involves Friedel-Crafts alkylation, a common electrophilic aromatic substitution reaction. Anisole (methoxybenzene) acts as the aromatic compound, and methyl chloride (CH₃Cl) is the alkylating agent. AlCl₃ serves as the Lewis acid catalyst.
Step 2: Understand the role of AlCl₃ in generating the electrophile. AlCl₃ interacts with methyl chloride to form a carbocation intermediate. The reaction can be represented as: CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻.
Step 3: Identify the reactive site on anisole. The methoxy group (-OCH₃) is an electron-donating group, which activates the aromatic ring and directs substitution to the ortho and para positions relative to the methoxy group.
Step 4: Propose the mechanism for the electrophilic aromatic substitution. The carbocation (CH₃⁺) generated in Step 2 attacks the activated aromatic ring of anisole at either the ortho or para position. This forms a sigma complex (arenium ion), which is stabilized by resonance.
Step 5: Complete the reaction by restoring aromaticity. The sigma complex loses a proton (H⁺), regenerating the aromatic ring and yielding the alkylated product. The final products are ortho-methyl anisole and para-methyl anisole, with para-methyl anisole typically being the major product due to steric considerations.
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Video duration:
8mKey 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, anisole, which contains a methoxy group that activates the aromatic ring, can undergo EAS with methyl chloride acting as the electrophile. Understanding EAS is crucial for predicting the products of the reaction.
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Lewis Acid Catalysis
Lewis acid catalysis involves the use of a Lewis acid, such as AlCl3, to enhance the reactivity of electrophiles. In this reaction, AlCl3 coordinates with methyl chloride, increasing its electrophilic character and facilitating its interaction with the aromatic ring of anisole. Recognizing the role of Lewis acids is essential for understanding the mechanism and product formation.
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Nucleophilic Attack and Rearrangement
In the proposed reaction, the nucleophilic aromatic ring of anisole attacks the activated methyl chloride, leading to the formation of a sigma complex. This intermediate can undergo rearrangement to restore aromaticity, resulting in the final product. Grasping the concepts of nucleophilic attack and the stabilization of intermediates is vital for predicting the outcome of the reaction.
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