Question

What is the major product from the Friedel-Crafts reaction of benzene with 1-chloro-2-methylpropane in the presence of \(\mathrm{AlCl}_{3}\) ?

Short answer

The major product is tert-butylbenzene.

Step-by-step solution

Identify the Electrophile

In the Friedel-Crafts alkylation reaction, an alkyl halide reacts with a Lewis acid to form a carbocation. Here, the 1-chloro-2-methylpropane reacts with the Lewis acid \( \mathrm{AlCl}_3 \) to generate a carbocation. This involves the chlorine atom forming a complex with \( \mathrm{AlCl}_3 \), which results in the generation of a secondary carbocation: \( \text{(CH}_3\text{)}_2\text{CHCH}_2^+ \).

Consider Carbocation Rearrangement

Secondary carbocations can undergo rearrangement to form more stable carbocations. In this case, the secondary carbocation \( \text{(CH}_3\text{)}_2\text{CHCH}_2^+ \) can rearrange to a more stable tertiary carbocation: \( \text{C} \text{(CH}_3\text{)}_3^+ \). This is because tertiary carbocations are more stable than secondary ones.

Electrophilic Attack by Benzene

The benzene ring acts as a nucleophile and attacks the more stable tertiary carbocation \( \text{C(CH}_3\text{)}_3^+ \). The \( \pi \)-electrons from the benzene ring form a bond with the carbocation, resulting in the formation of an arenium ion intermediate.

Restoration of Aromaticity

The arenium ion intermediate loses a hydrogen ion (\( \text{H}^+ \)) to restore the aromaticity of the benzene ring. This results in the formation of the alkylated benzene product, which in this case is tert-butylbenzene.

Key concepts

Carbocation Rearrangement

Carbocation rearrangement is an essential concept in organic chemistry, specifically in reactions like the Friedel-Crafts Alkylation. Carbocations are positively charged ions that play a critical role in the formation of products in these reactions. When a carbocation is formed, it can rearrange if a more stable structure is possible. Stability of carbocations generally increases in the order: primary < secondary < tertiary.

In the context of the Friedel-Crafts reaction in the exercise, the initial carbocation formed was a secondary carbocation from 1-chloro-2-methylpropane. However, this secondary carbocation can rearrange to form a tertiary carbocation by shifting a methyl group. This rearrangement is favorable because tertiary carbocations are more stable due to greater electron donation from adjacent carbon atoms, stabilizing the positive charge. This stability is a key reason that carbocation rearrangements occur, leading to different, often major, reaction products.

Lewis Acids in Organic Chemistry

Lewis acids are compounds that can accept a pair of electrons. In organic chemistry, they are often used to generate electrophiles, which are crucial for many reaction mechanisms such as the Friedel-Crafts Alkylation. In this reaction, a Lewis acid like aluminum chloride (\(\mathrm{AlCl}_3\)) is used.

Why is AlCl3 so effective? It's because AlCl3 can accept electrons from chloride ions, leading to the formation of an electrophilic carbocation. This step is important because the carbocation can then react with benzene, a core step in the electrophilic aromatic substitution process. This interaction between a Lewis acid and an alkyl halide is essential, as it facilitates the removal of halide ions and the generation of carbocations, setting the stage for the subsequent reaction steps.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a fundamental reaction mechanism in organic chemistry where an atom on an aromatic ring is replaced by an electrophile. In the Friedel-Crafts Alkylation reaction, benzene is the aromatic ring participating in this process.

The benzene ring serves as a nucleophile due to its electron-rich nature, allowing it to react with an electrophile, such as the tertiary carbocation formed from the precursor stages of the reaction. When benzene attacks the carbocation, it forms an intermediate called an arenium ion or sigma complex. This intermediate temporarily sacrifices its aromaticity, which is soon restored when the arenium ion loses a proton (\(\text{H}^+\)), resulting in the formation of the final alkylated product.The entire process is driven by the stability of the aromatic ring, and often dictates not only the type of product formed but also the orientation and reactivity patterns observed in electrophilic aromatic substitution reactions across varying conditions and starting materials.