Submitted by E. C. Wagner and Marion F. Fegley.
Checked by R. L. Shriner and R. M. Hedrick.
1. Procedure
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).
2. Notes
1. A thermometer with the graduation marks on the upper half of the stem is convenient.
2. The rate of absorption of
phosgene is dependent upon the speed and efficiency of the stirring. This also determines the amount of product obtained in each treatment with
phosgene.
3. Operation below room temperature is without advantage. At 60° or above the yield of
isatoic anhydride is decreased, or the process may yield precipitated material from which little or no
isatoic anhydride can be obtained. Cooling of the mixture might become advisable during operation on a scale larger than specified.
4. It is advisable to precipitate the
isatoic anhydride in several successive crops as directed, rather than to attempt to complete the reaction in one step, because the accumulation of precipitated
isatoic anhydride slows the rate of absorption of
phosgene to such an extent that prolonged passage of the gas at a decreasing and eventually very low rate would be necessary.
5. It is important to clean the glass gas-dispersing tip with hot
dioxane before reassembling the apparatus.
3. Discussion
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