Friedel-Crafts Acylation of M-Xylene

Purpose: The purpose of this experiment is to demonstrate the preparation of a tertiary alcohol by the reaction of a Grignard reagent with an ester.

Background: Friedel and Crafts discovered that alkyl groups could be substituted onto the aromatic ring by reaction of arenes with alkyl halides in the presence of aluminum chloride, AlCl3. The role of aluminum chloride, a strong Lewis acid, is to convert the alkyl halide into a reactive electrophilic intermediate, in the form of a carbocation or a highly polarized carbon-halogen bond. This electrophile then undergoes attack by an arene, which functions as a Lewis base, resulting in aromatic substitution. They also explored the reaction of aluminum chloride with acid chlorides, and anhydrides, to produce acylonium ions, which functions as electrophiles, just as alkyl carbocations do. Upon reaction of acylonium ions with an arene, an acyl group is introduced onto the ring to provide an aryl ketone. The overall reaction is named the Friedel-Crafts acylation.

The acylation performed for this experiment involves reaction of phthalic anhydride, a cyclic anhydride, with m-xylene in the presence of aluminum chloride to afford 2-(2', 4' -dimethylbenzoyl)benzoic acid, a ketoacid. The initial product of the acylation shows one mole of aluminum chloride has reacted with the acid function to form the salt RCO2AlCl2, and a second mole of aluminum chloride is complexed to the carbonyl group. Adding ice and hydrochloric acid decomposes the complex to produce 2-(2,4' -dimethylbenzoyl)benzoic acid and water-soluble aluminum salts.

Procedure: 0.7g of anhydrous aluminum chloride was placed in a glass vial which was then sealed with the cap and put to the side. 0.3g of phthalic anhydride and 1.8mL of m-xylene was put into a conical vial and the mixture was cooled to 0˚C in an ice-water bath. The anhydrous aluminum chloride was then quickly added to the mixture and stirred for about two minutes, and then allowed to warm to room temperature. Once the reaction occurred with the appearance of bubbles the reaction was then heated under reflux for about 20 minutes. The reaction mixture was then cooled to 0˚C with stirring. The cold reaction mixture was poured into a 15mL screw-cap centrifuge tube containing 2mL of ice water and was thoroughly stirred. 0.5mL of concentrated hydrochloric acid was slowly poured into the tube along