Textbook Question
What is the major product(s) of each of the following reactions?
e. nitration of p-methoxybenzaldehyde
f. nitration of p-tert-butylmethylbenzene
19. Reactions of Aromatics: EAS and Beyond
Electron Withdrawing Groups
4:14 minutesProblem 50g
Textbook Question
Predict the major products formed when benzene reacts (just once) with the following reagents.
(g) 1-chloro-2,2-dimethylpropane + AlCl3
Verified step by step guidance1
Step 1: Recognize that the reaction involves benzene and a reagent (1-chloro-2,2-dimethylpropane) in the presence of AlCl₃, which is a Lewis acid catalyst. This indicates that the reaction is an electrophilic aromatic substitution (EAS).
Step 2: Understand the role of AlCl₃. It interacts with the alkyl halide (1-chloro-2,2-dimethylpropane) to generate a carbocation. The chlorine atom is removed as Cl⁻, leaving behind a tertiary carbocation due to the stability of the 2,2-dimethylpropane structure.
Step 3: Write the structure of the carbocation formed. The tertiary carbocation has the formula (CH₃)₃C⁺, which is highly stable due to hyperconjugation and inductive effects from the methyl groups.
Step 4: Benzene acts as a nucleophile and attacks the carbocation, forming a sigma complex (also known as an arenium ion). This intermediate disrupts the aromaticity of benzene temporarily.
Step 5: The aromaticity of benzene is restored by the loss of a proton (H⁺) from the sigma complex, resulting in the major product: tert-butylbenzene ((CH₃)₃C-C₆H₅).
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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 process, the aromatic system retains its stability while allowing for the introduction of new substituents. Understanding EAS is crucial for predicting the products of reactions involving benzene and electrophiles.
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Friedel-Crafts Alkylation
Friedel-Crafts alkylation is a specific type of electrophilic aromatic substitution where an alkyl group is introduced to an aromatic ring using an alkyl halide and a Lewis acid catalyst, such as AlCl3. This reaction can lead to the formation of branched or linear alkyl groups on the aromatic compound, but it may also result in carbocation rearrangements that affect the final product.
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Carbocation Stability and Rearrangement
Carbocation stability is a key factor in determining the outcome of alkylation reactions. Tertiary carbocations are more stable than secondary or primary ones, often leading to rearrangements during the reaction. In the case of 1-chloro-2,2-dimethylpropane, the formation of a stable tertiary carbocation can influence the major product formed when it reacts with benzene.
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