Give the product of the reaction of excess benzene with each...

Give the product of the reaction of excess benzene with each of the following reagents:
a. isobutyl chloride $+\mathrm{AlCl}_{3}$
b. propene + HF
c. neopentyl chloride $+\mathrm{AlCl}_{3}$
d. dichloromethane $+\mathrm{AlCl}_{3}$

Key Concepts

Formation of Halomethyl Cations

Some reagents, like dichloromethane, can generate halomethyl cations in the presence of a Lewis acid such as AlCl3. These electrophilic species, for example a chloromethyl cation, participate in electrophilic aromatic substitution giving products like benzyl halides. This concept demonstrates that even seemingly simple molecules can act as useful electrophiles under the right conditions.

Limitations in Friedel–Crafts Alkylation

Certain substrates, such as neopentyl halides, pose challenges in Friedel–Crafts alkylations. Steric hindrance and the reluctance of a primary carbocation to form (or its tendency to rearrange) can result in low reactivity or the formation of rearranged products. This is an important consideration when choosing an alkylating agent for aromatic substitution.

Alkene Protonation and Hydroarylation

Alkenes can act as the precursors to carbocations under acidic conditions. In the presence of an acid such as HF, an alkene can be protonated to generate a carbocation. This carbocation then functions as the electrophile in an electrophilic aromatic substitution mechanism, allowing an alkene to be used to alkylate benzene.

Carbocation Stability and Rearrangement

A key aspect of many Friedel–Crafts reactions is the stability of the carbocation that is formed. Primary carbocations are typically unstable and may undergo rearrangements, such as hydride or alkyl shifts, to form more stable secondary or tertiary carbocations. These rearrangements can lead to unexpected products if the initially formed carbocation shifts to a more stable form before reacting with benzene.

Friedel–Crafts Alkylation

Friedel–Crafts alkylation involves the reaction of an alkyl halide with benzene in the presence of a Lewis acid catalyst such as AlCl3. The Lewis acid helps generate a carbocation (or a complex with significant carbocation character) from the alkyl halide, which then serves as the electrophile. This reaction is widely used for the introduction of alkyl groups onto aromatic rings.

Electrophilic Aromatic Substitution

This is the general reaction mechanism where an aromatic ring such as benzene, acting as a nucleophile, reacts with an electrophilic species. The reaction proceeds via the formation of a non-aromatic intermediate (often called the arenium ion) and is restored to aromaticity by the loss of a proton. Friedel–Crafts alkylation is a classic example of this type of reaction, where an electrophile is generated to attack benzene.

Please give Ace some feedback