Final answer:
In the Friedel-Crafts acylation, the Lewis acid such as AlCl3 coordinates with the acyl chloride to form a complex, enhancing the electrophilicity of the acyl group, which facilitates its addition to the benzene ring.
Step-by-step explanation:
In Friedel-Crafts acylation, the Lewis acid, such as aluminum chloride (AlCl3), plays a crucial role in the reaction mechanism. The Lewis acid acts by accepting a lone pair of electrons from the acyl halide, typically an acyl chloride (RCOCl), to generate a more reactive electrophile. This electrophile is the acylium ion (RCO+), which is highly electron-deficient and thus capable of reacting with the electron-rich benzene ring. Here is the basic sequence of steps in the Friedel-Crafts acylation:
- The Lewis acid (AlCl3) coordinates with the acyl chloride to form a complex.
- Formation of this complex facilitates the loss of the chloride ion (Cl-), resulting in the formation of the acylium ion.
- The benzene ring acts as a nucleophile and attacks the acylium ion, leading to an intermediate carbocation.
- This carbocation undergoes rearrangement to restore aromaticity, resulting in the substitution of an acyl group for a hydrogen atom on the benzene ring.
The primary function of the Lewis acid in this reaction is to increase the electrophilicity of the acyl halide, enabling it to react with the relatively unreactive benzene ring.