One hundred and thirty-seven grams (1 mole) of anthranilic acid is dissolved, with the aid of gentle warming, in a mixture of 1 l. of water and 126 ml. of concentrated hydrochloric acid (sp. gr. 1.19). The solution is filtered into a 2-l. three-necked flask fitted with a gas-tight, mechanically driven Hershberg stirrer. Through one of the side necks extends an inlet tube which ends in a coarse sintered-glass gas-dispersing tip extending well into the liquid into the liquid in the flask. The inlet tube is connected with a cylinder of phosgene through an empty safety flask. In the third neck of the flask is mounted an addition tube, through the vertical arm of which a thermometer (Note 1) is mounted so that the bulb is immersed in the reaction liquid. The outlet is attached to the lateral arm of the addition tube and is connected to an empty safety flask which in turn is connected to a Drechsel bottle charged with ammonium hydroxide.

With the stirrer in rapid motion (Note 2) phosgene is passed into the solution of anthranilic acid at such a rate that bubbles of gas escape slowly into the ammonia scrubber (about two bubbles per second). Isatoic anhydride appears as a precipitate soon after the stream of phosgene is started. The temperature rises but is prevented from exceeding 50° (Note 3) by regulation of the rate at which phosgene is introduced. The stream of phosgene is continued for 2–4 hours, or until the rate of absorption is clearly much decreased (Note 4). The flask is disconnected, and residual phosgene is blown out by passing a current of air through the mixture. The product is collected on a Büchner funnel and is washed with three 500-ml. portions of cold water. The first crop amounts to 54–56 g.

The mother liquor is returned to the reaction flask, the apparatus reassembled, and the passage of phosgene resumed (Note 5). When the rate of absorption has noticeably decreased (1–1.5 hours) the precipitated isatoic anhydride is collected on a filter and washed. The second crop amounts to 34–54 g. A third passage of phosgene at a considerably reduced rate will often yield a small additional crop (10–24 g.) of isatoic anhydride (Note 6).

The product is dried in air and then at 100°. The total yield is 118–123 g. (72–75%) of a white or nearly white product which decomposes at 237–240° cor.; this material is pure enough for most purposes. It may be recrystallized from 95% ethanol (about 30 ml. per gram) or from dioxane (about 10 ml. per gram). The former solvent permits the higher recovery (89–90%) and, except for the large volume required, is to be preferred. The mother liquor may be used for recrystallization of several successive lots of isatoic anhydride. The purified compound decomposes at 243° cor. (Note 7).