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In Friedel-Crafts alkylation, the carbocation intermediate plays a crucial role. The reaction with hexamethylbenzene and methyl chloride involves generating a reactive carbocation species. When aluminum chloride (AlCl鈧�) interacts with methyl chloride, it facilitates the departure of the chloride ion ( Cl鈦� ). This leaves behind a methyl cation ( CH鈧冣伜 ), commonly referred to as a carbocation intermediate. This species is highly electrophilic, meaning it seeks out electron-rich areas to react with. Since this carbocation is positive, it's a key player in driving the reaction forward by attacking the aromatic ring of hexamethylbenzene, which is electron rich due to its multiple methyl groups.

At the heart of many aromatic reactions, including Friedel-Crafts alkylation, is electrophilic aromatic substitution (EAS). Here, the electron-rich aromatic ring of hexamethylbenzene acts as a target for the electrophilic methyl cation ( CH鈧冣伜 ). Normally, in EAS, the aromatic ring would directly undergo substitution. However, due to the already high methylation of hexamethylbenzene, the methyl group doesn't attach directly onto the ring. Instead, it goes to one of the methyl substituents, forming a larger alkyl group. This adjustment avoids disrupting the aromatic stability by not altering the 蟺 -electron system of the benzene ring.

The role of the Lewis acid catalyst, aluminum chloride ( AlCl鈧� ), is integral to the Friedel-Crafts alkylation process. It plays two major roles:

• Activation: AlCl鈧� activates methyl chloride by removing the Cl鈦� ion, creating the methyl carbocation ( CH鈧冣伜 ). This activation step is what transforms a stable molecule into a highly reactive intermediate, ready for the next step in the reaction.
• Stabilization: Post electrophilic attack on the benzene ring, the reaction's intermediate needs stabilization. The chloride ion, initially displaced by AlCl鈧� , helps stabilize the newly formed carbocation on the aromatic structure, forming what is described as Salt A in the exercise. This salt is then reactive enough to undergo further reaction with sodium bicarbonate, leading to the formation of product B.

These roles together highlight why the presence of a Lewis acid catalyst is indispensable in facilitating Friedel-Crafts alkylation.