Hydrochloric Acid → 1,3-Dihydro-1-oxoisobenzofuran-5-carboxylic acid

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To 800 ml of fuming sulfuric acid, containing about 27% of SO3, 260 g (1.56 m) of terephthalic acid are added, under stirring, in 15 minutes and without exceeding the temperature of 25° C. To the thick suspension thus obtained, 120 g (1.33 m) of 1,3,5-trioxane are added under stirring without exceeding the temperature of 35° C., then stirring is continued for 20-30 minutes without cooling, whereby the temperature of the mixture rises to 45-50° C. The mixture is heated to 120° C. and it is noted that, already at 90° C., the mass becomes clear whilst at 120° C. a light exothermia is observed which brings the temperature to 135-140° C. The mixture is kept 6 hour under stirring at this temperature, then it is cooled to 20° C. and poured in 3000 g of coushed ice without exceeding the temperature of 25° C. To the mixture thus obtained, a 15% w/w solution of sodium hydroxide is added to a pH≡6 (about 6500-7000 ml thereof are needed), by keeping the temperature at 35-40° C. by water-cooling, then a 5% w/w solution of sodium hydroxide is added thereto at the temperature of 35-40° C. up to pH≡8 (about 300 ml are needed). The solid is removed by filtration on Dicalite® in a buchner and washed with water. To the filtered solution thus obtained, 35% hydrochloric acid is added up to pH≡1 (requiring about 1600-1800 ml of 35% HCl) and the suspension thus obtained is heated to 35° C. The solid is filtered, washed 3 times with 500 ml of deionized water at 40° C. and suspended in 1000 ml of deionized water. The suspension is heated under stirring at 50-55° C. and kept 1 hour under these conditions, then it is hot filtered. The solid is washed with deionized water and dried in vacuo at 50° C. to constant weight. Thus, 180 g of light-brown coloured 5-carboxyphthalide with a purity (HPLC) >95% are obtained.

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