Submitted by C. S. Palmer
1. Procedure
(
A)
Arsonoacetic Acid.—
One hundred grams (0.51 mole) of powdered arsenious oxide is added to the hot solution obtained by dissolving
160 g. (4 moles) of sodium hydroxide in 300 cc. of water. After the solution has cooled to 20°,
48 g. (0.51 mole) of chloroacetic acid is added. The suspension is well stirred during about five minutes, when a strongly exothermic reaction begins, the temperature rises to 70–75°, and a clear solution results.
The reaction mixture is permitted to stand at room temperature during one hour, or longer if desired (Note
1). The solution is acidified with
160 cc. of glacial acetic acid and, after the temperature has been lowered to 40° by cooling, the precipitated
arsenious oxide is filtered off by suction and washed with 50 cc. of water.
The filtrate is poured into a solution containing
185 g. (0.76 mole) of crystallized barium chloride dissolved in 600 cc. of hot water.
Barium arsonoacetate, Ba (O
2CCH
2AsO
3Ba)
2 (hydrated), forms a thick, finely divided precipitate. The mixture is stirred for several minutes and then allowed to stand until the following day, when it is filtered upon a
15-cm. Büchner funnel and washed thoroughly with water (Note
2). The yield of air-dried product is
220 g. (
96 per cent of the theoretical amount) (Note
3).
Sodium arsonoacetate is prepared by adding the freshly filtered and washed
barium arsonoacetate as obtained above (without drying) to a solution of
108 g. (0.76 mole) of anhydrous sodium sulfate in 500 cc. of hot water. The mixture is mechanically stirred for one hour, the
barium sulfate filtered off, and the filtrate concentrated on the
steam bath until crystallization commences. Upon cooling and stirring (Note
4),
sodium arsonoacetate separates and is filtered by suction, the filtrate being concentrated to obtain a second crop of crystals. The yield of the combined fractions is
100–110 g. (
80–88 per cent of the theoretical amount).
(
B)
Arsenoacetic Acid.—A solution of
12.5 g. of sodium arsonoacetate and
30 g. of sodium hypophosphite (NaH2PO2 · H2O) in
150 cc. of cold 15 per cent sulfuric acid is allowed to stand at room temperature. After two or three days, the yellow precipitate is filtered off, washed with water, and dried in a vacuum over
sulfuric acid or
phosphorus pentoxide. A second crop is obtained by allowing the mother liquid to stand for two days longer.
The arsenoacetic acid consists of minute yellow needles, which do not melt below 260° although they undergo considerable decomposition above 200°. The yield is 5 g. (74 per cent of the theoretical amount).
2. Notes
1. The reaction between
sodium arsenite and
chloroacetic acid is very rapid, as may easily be demonstrated by titrating 1-cc. portions of the solution with 0.1
N iodine, according to the usual volumetric method for
arsenious acid, before and after the reaction with
chloroacetic acid. The excess of
sodium arsenite is necessary, as was learned by means of an iodimetric study of the reaction.
2. If the thick paste is not allowed to stand overnight, filtration is more cumbersome and it is extremely difficult to wash the precipitate free from arsenites. With this precaution, it is found that after five washings with 250-cc. portions of water, the final filtrate, as well as the desired barium salt, is practically free from iodine-reducing compounds.
3. The air-dried sample contains 13 per cent of water of hydration, as determined by actual analysis.
4. Stirring is required during the crystallization process, since otherwise the product tends to form a solid cake. The sodium salt is obtained without water of crystallization. Usually it contains a trace of
sulfate but this does not interfere with its subsequent use. The
sulfate may be removed by recrystallization from water.
3. Discussion
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved