Friedel Crafts Alkylation Acylation — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Friedel-Crafts Alkylation/Acylation Friedel-Crafts reactions are a set of electrophilic aromatic substitution (EAS) reactions that allow for the attachment of alkyl groups (alkylation) or acyl groups (acylation) to aromatic rings using a Lewis acid catalyst. Evolution of hydrogen chloride (HCl) gas (acidic fumes) if alkyl or acyl chlorides are used. The reaction mixture may darken (brown/black) due to the formation of side products or impurities, especially if not run at controlled temperatures. After aqueous workup, the organic product separates from the aqueous layer. Generally thermodynamically favorable and exothermic, driven by the formation of stable aromatic products. 1. Formation of Electrophile: The Lewis acid (e.g., AlCl3) reacts with the alkyl halide to form a carbocation (or a highly polarized complex) or with the acyl halide/anhydride to form a resonance-stabilized acylium ion. 2. Electrophilic Attack: The electron-rich aromatic ring acts as a nucleophile, attacking the electrophile to form a sigma complex (arenium ion), which is a delocalized carbocation and temporarily loses its aromaticity. 3. Deprotonation and Aromaticity Restoration: A base (typically the Lewis acid complex, e.g., AlCl4-) abstracts a proton from the carbon bearing the new substituent, restoring the aromaticity of the ring and regenerating the Lewis acid catalyst. In acylation, the Lewis acid often complexes with the ketone product, requiring aqueous workup to release the product and regenerate the catalyst. Carbocation Rearrangements: Primary alkyl halides often rearrange to more stable secondary or tertiary carbocations in alkylation, leading to different products than expected (e.g., 1-chloropropane yields isopropylbenzene). This does NOT occur in acylation. Polysubstitution (Over-alkylation): Alkyl groups are activating and ortho/para directors. The alkylbenzene product is more reactive than the starting benzene, leading to multiple alkylations if not controlled. Deactivated Aromatic Rings: Rings with strong electron-withdrawing groups (e.g., -NO2, -COOH, -SO3H, -CHO, -COR) or strongly basic groups (e.g., -NH2 which complexes with Lewis acids) do not undergo Friedel-Crafts reactions. Lewis Acid Complexation: Amines (e.g., aniline) or other basic functional groups can react with the Lewis acid, rendering it inactive and preventing the reaction. Reagent Limitations: Vinyl halides (e.g., chloroethene) and aryl halides (e.g., chlorobenzene) generally do not form stable carbocations/acylium ions and thus do not undergo Friedel-Crafts reactions. Stoichiometry in Acylation: A stoichiometric (or slightly greater) amount of Lewis acid is often required for acylation because it complexes with the ketone product, deactivating it and requiring an aqueous workup for product isolation.