In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp
Microscale Synthesis of Tributyl Arsenite
SUNY-Cortland Cortland, NY 13045
Francisco J. Arnáiz* and Mariano J. Miranda Laboratorio de Química Inorgánica, Universidad de Burgos, 09001 Burgos, Spain; *
[email protected] The chemistry of phosphines and arsines has been thoroughly investigated and many analogies are encountered, especially with regard to the formation of complexes with transition metal centers (1). However, while triorganyl phosphites, P(OR)3, form a large number of well-characterized complexes, there is a dearth of derivatives of the analogous arsenites, As(OR)3. This is at least part because arsenites are very sensitive to hydrolysis and are rarely commercially available (2). The most common ways to prepare arsenite esters make use of the reaction of arsenic trichloride with either sodium alkoxides (3, 4 ), or alcohols in the presence of tertiary amines (5). AsCl3 is a moisture-sensitive product for which the recently reported synthesis, direct reaction of As4O6 with excess SOCl2 (6 ), is not as simple as it appears, because the excess SOCl2 is difficult to remove. Here we describe a synthesis for tributyl arsenite, devised for the integrated lab, that makes use of the simplest materials and products with considerable savings in effort, time and cost. Procedure CAUTION: Arsenic compounds are toxic. Butanol is a somewhat toxic, flammable liquid. Use gloves during the process and conduct the experiment in a hood far from open flames. In an ordinary test tube (e.g., 15 × 150 mm) are placed a spin bar, 250 mg of arsenic oxide powder (0.63 mmol of As4O6), and 1 g (13.5 mmol) of n-butanol (weighed by difference). The bottom of the tube is immersed in a silicone oil bath at 140–150 °C, which is standing on a magnetic stirrer so that the butanol boils immediately. The boiling is continued with stirring until all arsenic oxide dissolves. (This takes about 1 hour and most of the butanol vapor condenses in the cold upper part of the tube. Rarely, another 0.5–1-g portion of butanol must be added to replace evaporation losses.) Then a folded piece of filter paper ~15 × 15 cm is introduced into the tube (Fig. 1) and the heating is continued until boiling
Figure 1. The experimental setup, showing placement of the filter paper in the test tube.
ceases. Twenty to 30 min is usually sufficient. Most of the water produced and the excess butanol are absorbed by the paper. At this point the wet part of the paper inside the tube is replaced by the dry external paper and the liquid in the bottom of the tube, consisting mainly of tributyl arsenite, is heated for an additional 15 min. Under these conditions the residual butanol and some tributyl arsenite are absorbed by the paper. Finally the paper is removed, and the tube is capped with a stopper, allowed to cool, and weighed. (The white color usually observed at the edge of the paper after it is removed is due to the As4O6 formed through hydrolysis of the arsenite by ambient moisture.) In a typical run, 0.71 g of As(OBu)3 (95% yield) is obtained. Further purification can by achieved by vacuum distillation (bp 103–104 °C at 4 mmHg). Characterization of the Product A drop of the crude liquid arsenite is picked up with a dry Pasteur pipet and rapidly transferred to an NMR tube containing about 0.5 mL of dry CCl4 (a clear solution must result), and the 1H NMR spectrum is recorded. The spectrum can be compared with that of n-butanol. The most significant difference is that the triplet centered at 3.58 ppm, characteristic of the CH2 vicinal to oxygen in butanol, presents as a multiplet (note that 75As has I = 3/2) centered at 3.90 ppm in the arsenite. The absence of both the CH2 triplet and the OH singlet at 2.30 ppm indicates that butanol was completely removed from the liquid arsenite. (If the sample is prepared without sufficient care, some butanol, from partial hydrolysis, can be detected.) IR spectroscopy is less appropriate for characterizing the compound, owing to difficulties in differentiating pure and partially hydrolyzed As(OBu)3 and in running the spectrum under the usual conditions (plates will probably require polishing to remove the As4O6 deposited). We recommend placing a drop of the liquid between two homemade KBr discs immediately before running the spectrum. Under these conditions the main differences between the spectra of BuOH and As(OBu)3 are (i) a medium-intensity shouldered band with a maximum at 657 cm{1, assignable to ν (As–O), absent in the spectrum of butanol; (ii) a strong wide band at 3345 cm{1, corresponding to ν (OH) in butanol, missing in the spectrum of the arsenite. The arsenic content of the sample can be determined by usual procedures, such as iodometry (7). However, we propose the conversion to As4O6, because acceptable results are obtained and the recycling of wastes is facilitated. Thus, a sample consisting of most of the remaining arsenite is picked up with a dry Pasteur pipet and added to a 25-mL vessel standing on an electronic balance (to weigh by difference). Then about 10 g of acetone and 0.5 g of water are added and the mixture is stirred for 5 min. The resulting As4O6 is
JChemEd.chem.wisc.edu • Vol. 76 No. 9 September 1999 • Journal of Chemical Education
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In the Laboratory
filtered (Hirsch funnel), washed with acetone (2 × 2 mL), and dried in an oven at 60–70 °C for 30 min. CAUTION: It is very important not to dry the product as usual because As4O6 has a significant vapor pressure above 100 °C. In a typical run, 450 mg of As(OBu)3 yielded 140 mg of As4O6 (calcd 145 mg). The IR spectrum of the resulting product is identical to that of a pure sample of As4O6. Discussion
Thermodynamic Considerations As expected for acid anhydrides, the As–O–As bonds in the adamantane-like As4O6 are cleaved by alcohols (ROH) to initially produce As–O–H and As–O–R functionalities. Then, the acidic As–O–H group condenses with ROH to produce H2O and As–O–R. Thus, the whole reaction can be written As4O6 + 12ROH
4As(OR)3 + 6H2O
A rough evaluation of the nature and number of bonds cleaved and formed and the entropy change of the species involved suggests no great variation in free energy for this reaction. Under normal conditions the equilibrium is displaced to the left, so that arsenic oxide does not react significantly with common alcohols. Consistently, triorganyl arsenites hydrolyze readily. However, the low free energy for the reaction, the excess of butanol, the existence of the minimum-boilingpoint azeotrope butanol-water, and the lower volatility of As(OBu)3 compared with that of butanol work together to displace the reaction to the right and to facilitate the isolation of the arsenite after heating. Triorganyl arsenites are considerably more resistant to aerial oxidation than the analogous phosphites, according to the observed trend in availability of high oxidation states for elements of post-transition groups (8). Consequently, to remove excess butanol, tributyl arsenite can be heated in air at 150 °C without oxidation to tributyl arsenate or isomerization to butyl arsonate (in fact, arsonates can decompose to arsenites in an “anti-Arbusov” rearrangement [2]).
Kinetic Considerations The fact that the lower alcohols do not react with As4O6 at the reflux temperature is surely due to their boiling points, which prevent the activation barrier from being reached and thus prevent the reaction from proceeding at a chemically significant rate. The dehydrating capability of pure ethanol and the azeotrope ethanol–water, are well known. However, we have observed that no significant amount of As4O6 reacts even after a mixture of As4O6 is boiled with excess anhydrous ethanol for 6 hours, so that most of the ethanol is volatilized.
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Arsenites, contrary to phosphites, hydrolyze quite readily. This is probably because d-orbitals in arsenic are more available than those in phosphorus, so that nucleophilic attack to arsenites provides a convenient low-energy pathway for reactions. We also have found that transesterification with ethanol proceeds easily, and some solid adducts of fluoroalkyl arsenites with pyridine have been characterized (4 ).
A Final Consideration The piece of filter paper (Fig. 1) has two main functions: to facilitate the removal of excess butanol and water condensed in the cold upper part of the tube (since both solvents wet cellulose), and to work as a block to avoid ejection of the reacting mixture by bumping (this event has never been observed under the above conditions, but stirring is nevertheless strongly recommended). The folded paper multiplies the capacity of a single strip to absorb liquids and at the same time increases the resistance of the paper to tearing. Folding the paper in the middle insures that it remains in place during the heating and facilitates the replacement of the internal wet by the external dry part of the paper, thereby providing more efficient removal of liquids. Recycling All residues containing tributyl arsenite are hydrolyzed with acetone containing some water (As4O6 is significantly soluble in water but insoluble in acetone), as described above. The resulting As4O6 is washed with acetone, dried, and stored for re-use. Acknowledgments To the editor for helpful comments, and to Dirección General de Enseñanza Superior (PB95-0832) and Junta de Castilla y León (Bu04/95) for financial support. Literature Cited 1. McAuliffe, C. A.; Levason, W. Phosphine, Arsine and Stibine Complexes of the Transition Elements; Elsevier: New York, 1979. 2. See, e.g., Smith, J. D. In Comprehensive Inorganic Chemistry; Bailard, J. C.; Emeleus, H. J.; Nyholm, R.; Trotman-Dickerson, A. F., Eds.; Pergamon: New York, 1975; Vol. 2, Chapter 21, p 609. 3. Crafts, J. M. Bull. Soc. Chim. Fr. 1870, 14, 102. 4. Chadha, S. L.; Parkash, R. J. Fluorine Chem. 1995, 74, 293. 5. Moedritzer, K. Inorg. Synth. 1968, 11, 181. 6. Pandey, S. K.; Steiner, A; Roesky, H. B. Inorg. Synth. 1997, 31, 148. 7. See, e.g., McAlpine, R. K. J. Chem. Educ. 1949, 26, 362. 8. See, e.g., Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley: New York, 1995; Chapter 8, pp 241–270.
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
