Textbook Question
Show how Friedel–Crafts acylation might be used to synthesize the following compounds.
c. n-butylbenzene
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19. Reactions of Aromatics: EAS and Beyond
EAS:Friedel-Crafts Acylation Mechanism
3:24 minutesProblem 53e
Textbook Question
Predict the major products of the following reactions.
(e) p-methylanisole + acetyl chloride + AlCl3
Verified step by step guidance1
Identify the type of reaction: This is a Friedel-Crafts acylation reaction, where an aromatic compound reacts with an acyl chloride in the presence of a Lewis acid catalyst (AlCl₃). The reaction introduces an acyl group (-COCH₃) onto the aromatic ring.
Analyze the structure of p-methylanisole: p-Methylanisole has two substituents on a benzene ring: a methoxy group (-OCH₃) and a methyl group (-CH₃) in the para position relative to each other. Both groups are electron-donating and activate the ring towards electrophilic substitution, directing incoming groups to the ortho and para positions relative to themselves.
Determine the most reactive positions: The methoxy group is a stronger electron-donating group than the methyl group, so it will dominate the directing effects. The ortho and para positions relative to the methoxy group are the most reactive. However, the para position relative to the methoxy group is already occupied by the methyl group, so the acyl group will primarily add to the ortho positions relative to the methoxy group.
Generate the intermediate: The acetyl chloride (CH₃COCl) reacts with AlCl₃ to form the acylium ion (CH₃C⁺=O), which is the electrophile in this reaction. This electrophile will attack the most reactive positions on the aromatic ring, leading to the formation of a sigma complex (arenium ion).
Complete the reaction: The sigma complex will lose a proton to regenerate the aromaticity of the ring, resulting in the major product. The major product will be p-methylanisole with an acetyl group (-COCH₃) added to the ortho position relative to the methoxy group.
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3mKey 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 case, p-methylanisole, which has a methoxy group that activates the ring, will undergo substitution with acetyl chloride in the presence of a Lewis acid catalyst like AlCl3, facilitating the reaction.
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Activating and Deactivating Groups
In EAS reactions, substituents on the aromatic ring can either activate or deactivate the ring towards further substitution. The methoxy group in p-methylanisole is an activating group, which enhances the electron density of the ring, making it more reactive towards electrophiles like acetyl chloride, while the methyl group also directs substitution to the para position.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the case of p-methylanisole reacting with acetyl chloride, the presence of the activating groups will lead to the major product being the para-substituted acetophenone derivative, as the methoxy and methyl groups direct the electrophile to the para position relative to themselves.
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