A simple method for making p-arsanilic acid

Abstract

The preparation of what is now known as primary arsanilic acid, the starting point and the basis of Ehrlich's synthesis of salvarsan, was first mentioned by Béchamp 1 in 1863. In spite of Béchamp's published work, and that of many others, the fact remains that the literature does not contain definite directions for making the primary arsanilic acid. Many laboratories have spent much effort, money and time in trying to evolve the synthesis from the general directions given in the literature, but without much success. The process in its highest form is generally understood to be a trade secret.

Without going into details, it can be shown that the directions either call for too much aniline or too high a temperature, or both. By controlling one or both of these factors one can obtain the primary free from secondary in large yields, practically pure, with ease.

One thousand c.c. of crude arsenic acid (75 per cent.) are heated in an open beaker or vessel, at 120° to 140° C. for 12 to 15 hours by means of an oil or “Crisco” bath. This should concentrate the acid to practically 100 per cent.

Fourteen hundred c.c. of dry aniline oil are cooled with an ice mixture to 0° C. or lower, and the cooled arsenic acid is added slowly with vigorous stirring. The mixture soon becomes thick and then granular, after which it is finely ground and thoroughly mixed. This powder has roughly the composition of (C₆H₅NH₂)₃-(H₃A_sO₄)₂.

Two hundred grams of this powder are heated in an Erlenmeyer flask, by means of a “Crisco” bath to 160° C., when the powder begins to melt. The substance is stirred continually and when it has all melted, a reflux condenser is then attached; for one and a half hour it is heated from 160°-170°, and then one hour from 180° to 183° C.