Friedel-Crafts Alkylation

Friedel-Crafts reactions form a new carbon-carbon bond to a benzene ring. This is important in synthesis because it attaches a carbon chain to an aromatic ring, which most other reactions of benzene cannot do.In Friedel-Crafts alkylation, benzene reacts with a haloalkane in the presence of a halogen carrier such as AlCl₃, forming an alkylbenzene. Conditions must be anhydrous, because water reacts with aluminium chloride and destroys the catalyst.Mechanism (electrophilic substitution)
1. The halogen carrier generates the electrophile, a carbocation:
CH₃CH₂Cl + AlCl₃ → CH₃CH₂⁺ + AlCl₄⁻2. The delocalised electrons of benzene attack the carbocation, forming a positively charged intermediate.
3. The intermediate loses H⁺, restoring the stable delocalised ring.
4. The catalyst is regenerated: H⁺ + AlCl₄⁻ → AlCl₃ + HClOverall:C₆H₆ + CH₃CH₂Cl → C₆H₅CH₂CH₃ + HClThe product is ethylbenzene. With chloromethane the product would be methylbenzene.The alkylbenzene formed is more reactive than benzene, so it can be alkylated again, and a mixture of products may form.Worked example: what mass of ethylbenzene can be made from 7.80 g of benzene?
n(benzene) = 7.80 ÷ 78.0 = 0.100 mol. The ratio is 1 : 1, so at most 0.100 mol of ethylbenzene, C₈H₁₀ (M_r 106), forms.mass = 0.100 × 106 = 10.6 g