What is the Difference Between Friedel Crafts Acylation and Alkylation?

The Friedel-Crafts reactions are a set of chemical reactions that involve the addition of an alkyl or acyl group to an aromatic ring. There are two types of Friedel-Crafts reactions: alkylation and acylation. The main differences between these two reactions are:

1. Type of Reactants: In Friedel-Crafts alkylation, an aromatic compound reacts with an alkyl halide, while in Friedel-Crafts acylation, an aromatic compound reacts with an acyl halide (also called an acid halide).
2. Product Formation: Friedel-Crafts alkylation adds a simple carbon chain to the benzene ring, while Friedel-Crafts acylation adds an acyl group, creating a ketone or aldehyde.
3. Control over Reaction: Friedel-Crafts acylation is significantly easier to control compared to Friedel-Crafts alkylation. This is because multiple alkylations can occur in alkylation reactions, making it difficult to control the reaction and obtain the desired product.
4. Limitation: Both Friedel-Crafts acylation and alkylation reactions fail when attempting to perform them on deactivated aromatic rings.

In summary, Friedel-Crafts alkylation involves the addition of a simple carbon chain to the benzene ring, while Friedel-Crafts acylation adds an acyl group, creating a ketone or aldehyde. The acylation reaction is easier to control compared to the alkylation reaction, but both reactions have limitations when performed on deactivated aromatic rings.

Comparative Table: Friedel Crafts Acylation vs Alkylation

Here is a table comparing the differences between Friedel-Crafts acylation and alkylation:

Both Friedel-Crafts alkylation and acylation reactions involve the electrophilic substitution of an aromatic ring, but they differ in the reactants, products, and mechanism. In the alkylation reaction, an alkyl halide reacts with the aromatic ring, leading to the formation of an alkylated aromatic ring product. In contrast, the acylation reaction involves an acyl halide (acid halide) reacting with the aromatic ring, resulting in a monoacylated aromatic ring product.

The Lewis acid catalyst (typically aluminum chloride) is used in both reactions, but the specific Lewis acid can vary. The mechanisms of these reactions also differ: the alkylation reaction involves carbocation formation, rearrangement, and aromatic bond formation, while the acylation reaction involves acyl cation formation, aromatic bond formation, and hydrogen abstraction.

The major product formation in Friedel-Crafts alkylation reactions results from the more stable carbocation formed during the rearrangement process, whereas the acylation reaction does not have a carbocation rearrangement, and the major product forms directly from the acyl cation.