phthalic anhydride (Aldrich)
methanol

Powdered phthalic anhydride (74 g, 0.5 mol) and methanol (100 cm³, 2.47 mol) were heated under reflux with magnetic stirring on an oil bath for 8 h. The clear solution was distilled to remove most of the methanol. Toluene (100 cm³) was added to the residue and distillation was continued until the still-head temperature reached 110 ºC. After cooling to 30 ºC, the resulting thick white precipitate was filtered and the residue washed with toluene (50 cm³). Petroleum ether (30/40, 100 cm³) was added to the combined filtrates to give a dense oil which crystallised as long white needles after standing overnight. Filtration and washing with petroleum ether (60/80, 50 cm³) gave the product, 44.5 g, 49% yield.

A subsequent perusal of the literature reveals a much simpler procedure, 0.5 h reflux and no toluene: "Under a nitrogen atmosphere, a mixture of 75.0 grams (0.506 mole) of phthalic anhydride in 120.0 grams (3.75 moles) of methanol was stirred and heated under reflux for 30 minutes. During this time a complete solution was attained. The reaction mixture was concentrated under reduced pressure to a residual oil. The oil was placed in a refrigerator where it solidified to yield when dried, 93.0 grams of phthalic acid, mono-methyl ester; m.p. 80-81.5 ºC." in quantitative yield.

Chang Jun H, Baum J S, US 4892578 (9 Jan 1990)

δ_H (CDCl₃, 200 MHz) 7.68m (1H), 7.61m (1H), 7.56m (2H), 3.92s (3H)

Chang Jun H, Baum J S, US 4892578 (9 Jan 1990)