Technology Type

The Friedel-Crafts alkylation

The Friedel-Crafts alkylation of aromatic compounds involves an acid-catalyzed electrophilic substitution, wherein an alkyl group replaces an aromatic hydrogen. A diverse range of alkylating agents, such as olefins, alkyl halides, and alcohols, is commonly employed for this purpose.

This reaction can be applied to various aromatic substrates, including heteroaromatic compounds and even compounds like ferrocene. These reactions are typically rapid and exothermic, and they are often conducted under mild conditions in the liquid phase.

However, in some cases, vapor-phase processes with more stringent conditions are utilized for certain substrates. When olefins, alkyl halides, and alcohols are used as alkylating agents^.

Catalysts used in the Friedel-Crafts alkylation process are acidic, and their effectiveness depends on various factors, including reaction conditions, alkylating agents, and aromatic substrates. Both Lewis acids and Brønsted acids can act as active catalysts, and frequently used catalysts are classified based on their chemical structures.

Alkylation of Benzene

The alkylation of benzene finds three major commercial applications:

1. Alkylation with ethylene to produce ethylbenzene, which serves as an intermediate for styrene production.
2. Alkylation with propene to produce cumene, which is an intermediate for phenol production, with acetone as a byproduct.
3. Alkylation with long-chain olefins containing 10 – 18 carbon atoms to produce alkylbenzenes, which are then sulfonated to produce surface-active compounds named linear alkylbenzenesulfonates (LAB).

Alkylation of Benzene with ethylene

The alkylation of benzene with ethylene results in the formation of ethylbenzene, a reaction that is fast and exothermic.

The reaction can be carried out in both the liquid phase and the vapor phase, with either pure or diluted ethylene used as the feedstock after purification. The liquid-phase alkylation reaction is typically conducted at atmospheric pressure and temperatures ranging from 85 to 95 °C with AlCl₃ as the most commonly used catalyst. Vapor-phase alkylation of benzene is usually carried out at approximately 300 °C and 40 – 65 bar using acidic heterogeneous catalysts such as Zeolite catalysts.

Alkylation of Benzene with Propene

The alkylation of benzene with propene leads to the production of cumene, which is an exothermic reaction.

Both liquid-phase and vapor-phase processes are employed commercially for the synthesis of cumene. Cumee has historically been produced in a liquid-phase process utilizing a solid phosphoric acid catalyst (SPA) supported on diatomaceous earth, the reaction being conducted at 15 – 35 bar and temperatures of 200 – 250 °C.  More recent cumene production technologies utilize zeolite catalysts.

References

1. Chemcess, 5^th Sep 2024, Alkylation of Aromatic Compounds.