The present invention relates to a method of preparing 2-sulfochloride benzoates, and, more especially, relates to a method of preparing such compounds which comprises diazotizing an anthranilic acid ester to form a 2-diazonium chloride benzoate and then reacting the 2-diazonium chloride benzoate with sulfur dioxide to form the 2-sulfochloride benzoate. This invention also relates to a method of synthesizing saccharin which comprises pyrolyzing a 2-sulfochloride benzoate at elevated temperatures and then ammonolyzing the resulting o-sulfobenzoic anhydride to saccharin.
Saccharin is variously known as o-benzosulfimide; gluside; benzoylsulfonic imide; and is the anhydride of o-sulfimide benzoic acid having the formula: ##STR1## Saccharin is employed in the manufacture of syrups, medicine (substitute for sugar), soft drinks, foods, and the like, and is a nonnutritive sweetener which can readily be converted to sodium or soluble saccharin, and is a white, crystalline powder. It has an exceedingly sweet taste (500 times that of cane sugar), a melting point of about 226.degree. C. to 230.degree. C., and is soluble in amyl acetate, ethyl acetate, benzene and alcohol; slightly soluble in water, chloroform and ether.
Saccharin has been made from toluene by the following series of reactions: ##STR2## The imide is converted to the sodium salt to increase the solubility in water. Saccharin can also be prepared by converting a mixture of toluene sulfonic acids into the sodium salt, then by distillation with phosphorus trichloride and chlorine to obtain the ortho-toluene sulfonyl chloride, which by means of ammonia is converted into ortho-toluenesulfamide. This is oxidized with permanganate, treated with acid and saccharin crystallized out. It is reported that the slight bitter taste associated with the saccharin prepared by either of the above methods is caused by the presence of o-toluamide. Moreover, the disposal of the p-toluenesulfonyl chloride obtain by-product in the above processes has also been a problem.
Both of these objections have been attempted to be overcome by two more recent processes, the first [A] commencing with thianaphthene (prepared from styrene and sulfur) and the other [B] with anthranilic acid, as follows [see Noller, Chemistry of Organic Compounds, 2nd Edition, pp. 556-7 (1957)]: ##STR3## Nevertheless, it too is reported that even when utilizing these alternate routes there still is a slight bitter taste associated with the saccharin, here probably caused by the presence of certain trace contaminants.