Submitted by H. T. Clarke and E. E. Dreger.
Checked by Henry Gilman and J. E. Kirby.
1. Procedure
In a
2-l. flask fitted with a
mechanical stirrer, a
separatory funnel, and an
efficient reflux condenser are placed
219 g. (1 mole) of finely powdered acid ammonium o-sulfobenzoate (p. 14) and
200 cc. of dry benzene (Note
1). To this is added with stirring
145 g. (86.3 cc., 1.22 moles) of thionyl chloride (b.p.
76–78°). A tube is connected to the upper end of the reflux condenser which passes into an
ice-cooled flask; from this another tube is provided to carry off the
hydrogen chloride and
sulfur dioxide generated during the reaction
(Fig. 7, p. 97). The mixture is gently warmed on a
steam bath in such a way that gases are evolved fairly briskly (Note
2); continuous stirring is essential. The bulk of the entrained
benzene and
thionyl chloride is condensed in the
ice-cooled receiver; the condensate is returned periodically to the reaction mixture through the separatory funnel. After about fifteen hours' heating, the evolution of gases slackens; an additional
400 cc. of dry benzene is then added and the heating continued until the evolution of gases ceases; this requires about five hours longer.
The hot mixture is then filtered (Note
3) with suction into a
2-l. flask, and the solid material, consisting of
ammonium chloride, is washed with
100 cc. of hot benzene. This solid is then returned to the 2-l. flask and boiled with stirring for one hour with
300 cc. of dry benzene; the mixture while hot is then filtered with suction into the same 2-l. flask. The combined filtrates and washings are then distilled until about
300 cc. of benzene has been removed, after which the solution is cooled in an
ice bath; this causes the greater part of the
sulfobenzoic anhydride to crystallize. The
benzene mother liquor is decanted from these crystals and the bulk of the solvent removed by distillation.
The flask containing the crystals is attached to a condenser and heated until the solid has completely melted; the adhering solvent is then removed from the liquid by slightly reducing the pressure. The clear residue is poured into a dish and allowed to cool. The product thus obtained melts at
121–123°. The residue from the mother liquor, being as a rule somewhat dark in color, is preferably distilled under reduced pressure (Note
4), the flask being heated in a
metal bath. The anhydride distils at
184–186°/18 mm. The second crop may have a slightly lower melting point. The total yield of
o-sulfobenzoic anhydride, which is of sufficient purity for the preparation of
sulfonephthaleins, amounts to
118–121 g. (
64–66 per cent of the theoretical amount) (Note
5). In order to convert this into a product melting at
126–127° (Note
4), it is recrystallized from three times its weight of dry
benzene.
2. Notes
1. The
benzene is most satisfactorily dried by distillation, the moist forerun being rejected.
2. The reaction may become so vigorous that it is necessary to interrupt the heating for a short time.
3. The filtration should be carried out under a
hood, on account of the presence of some unchanged
thionyl chloride.
4. It is stated in the literature that the pure compound melts at
128°. It is very sensitive to moist air, which converts it into the free acid.
5. With larger quantities the yields are significantly increased; the submitters of these directions obtained
74–81 per cent of the theoretical amount with runs of 7.5 moles.
3. Discussion
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