Submitted by R. H. Bullard and J. B. Dickey.
Checked by W. H. Carothers and W. L. McEwen.
1. Procedure
In a
12-l. round-bottomed flask fitted with a
mechanical stirrer is placed 1 l. of water. The water is heated to boiling, and
500 g. (4.7 moles) of anhydrous sodium carbonate is added. As soon as the carbonate has dissolved,
250 g. (1.26 moles) of arsenious oxide and
11 g. of crystalline copper sulfate are added with stirring. When all the solids have dissolved the solution is cooled with stirring under a stream of tap water until the temperature falls to 15°.
Concurrently with the preparation of the
sodium arsenite solution, a solution of
benzenediazonium chloride is prepared. To a well-stirred mixture of
186 g. (2 moles) of technical aniline,
400 cc. (4.8 moles) of concentrated hydrochloric acid (sp. gr. 1.19), 1 l. of water, and enough crushed ice to make a volume of about 3 l., is added slowly a solution of
145 g. (2 moles) of 95 per cent sodium nitrite in 500 cc. of water. This requires about thirty to forty minutes.
The
benezenediazonium chloride solution is then added with stirring during a period of one hour to the suspension of
sodium arsenite, cooled in an
ice and salt bath to 0°. The temperature during the reaction is held below 5° (Note
1). Frothing takes place owing to the escape of
nitrogen, but this is easily controlled by the occasional addition of a small quantity of
benzene. Stirring is continued for one hour after the addition of the diazonium chloride solution, and the mixture is filtered to remove the solid material which separates. This is washed with 500 cc. of cold water, and the combined liquors are concentrated over a free flame to a volume of about 1.5 l. (Note
2).
To the hot concentrated solution, which is deep brown in color, concentrated
hydrochloric acid is added until no more tarry material separates (Note
3). The tar is filtered and more
hydrochloric acid is added until, after filtering, a clear pale yellow solution results. It is important to remove all the tar at this time; otherwise, subsequent recrystallizations will not free the product from color. The
phenylarsonic acid is then precipitated by the addition of
250 cc. of concentrated hydrochloric acid (sp. gr. 1.19) (Note
4). When the mixture has cooled (preferably by standing overnight) the
phenylarsonic acid is filtered on a
Büchner funnel and washed with 200 cc. of cold water. The light yellow crystals are dissolved in 500 cc. of boiling water, 20 g. of
Norite is added, the solution filtered hot, and the filtrate allowed to cool. After filtering and drying, the white crystals melt with decomposition at
154–158°, passing into the anhydride, C
6H
5AsO
2. The yield is
160–182 g. (
39–45 per cent of the theoretical amount) (Note
5).
2. Notes
1. This temperature is advised because it appears to be near the optimum with regard to yield and ease of purifying the product. However, a reaction temperature as high as 15° may be used with good results.
2. The solution is concentrated at atmospheric pressure since frothing occurs if reduced pressure is used.
4. Too large an excess of
hydrochloric acid will dissolve some of the product and lower the yield.
5. In occasional runs by this procedure yields as high as
57 per cent have been obtained, but the reasons for this are not known.
3. Discussion
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