The isotopic exchange of arsenic trichloride and chlorine

J. lnoru. NucL Chem., 1956, Vol. 2, pp. 260 to 262. Pergamon Press Ltd., London

THE ISOTOPIC E X C H A N G E OF A R S E N I C T R I C H L O R I D E AND CHLORINE J. HARDING OWEN* and ROWLAND E. JOHNSON Department of Chemistry, Florida State University, Tallahassee, Florida (Received 5 D e c e m b e r

1955)

Abstract--Wc have measured the rate of exchange of isotopic chlorine between arsenic trichk)ridc and chlorine in carbon tetrachloride solution. We find no simple mechanism indicated, the reaction is catalysed by hydrogen chloride, and the formation of arsenic pentachloride is shown to be ver~ tmlikelv. I~ m a n y o f its c o m p o u n d s , arsenic s h o w s a r e l u c t a n c e to a s s u m e a c o - o r d i n a t i o n n u m b e r o f 5, w h e r e a s o t h e r m e m b e r s o f this family f o r m such c o m p o u n d s readil), T h e use o f 4 f orbitals in b o n d i n g by member's o f the s e c o n d l o n g p e r i o d t~ might explain the p e n t a - c o - o r d i n a t i o n o f a n t i m o n y , but we c a n n o t fully e x p l a i n the a n o m a l o u s p o s i t i o n o f arsenic with respect to p h o s p h o r u s a n d a n t i m o n y . A r s e n i c p e n t a f l u o r i d e is k n o w n , but dissociates readily. Despite a claim, (2~ the p r e p a r a t i o n o f arsenic p e n t a c h l o r i d e as a stable c o m p o u n d has n o t been a c c o m p l i s h e d . I n c o n n e c t i o n with o u r w o r k on i s o t o p i c e x c h a n g e r e a c t i o n s o f p e n t a - c o - o r d i n a t e d species, t3~ we h a v e m e a s u r e d the rate o f e x c h a n g e o f c h l o r i n e a t o m s b e t w e e n arsenic t r i c h l o r i d e and c h l o r i n e in c a r b o n t e t r a c h l o r i d e solution. It is easy to see that a very f a v o u r a b l e p a t h for e x c h a n g e w o u l d be via the f o r m a t i o n o f arsenic p e n t a c h l o r i d e : o u r results i n d i c a t e such a c o m p o u n d does not exist. EXPERIMENTAL In general, the procedures and precautions against the intrusion of moisture were similar to those used previously. '3) All operations which might introduce moisture were done in an all-glass system using high-vacuum techniques, or in a dry-box atmosphere having a dew-point of less than - 6 0 . Arsenic trichloride, reagent grade, was further purified by triple-vacuuna distillation, each time from room temperature to a trap cooled to ~-80 ~. Arsenic trichloride-C136 was prepared by the reaction of hydrogen chloride-C1 a~' and arsenic trichloride for 24 b at 100=. Equilibration was complete, and the arsenic trichloride was separated from hydrogen chloride by another set of vacuum distillations as above. A run was made as follows. Solutions of the reactants in carbon tetrachloride were prepared inside the dry box and mixed by pouring into calibrated reaction tubes. After a preselected time, a reaction tube was emptied into a flask, the chlorine evaporated by a stream of nitrogen, and the chlorine-nitrogen mixture conducted to a cold trap outside the dry box. Preliminary experiments showed that separation was essentially complete in a 30-sec evaporation ; no arsenic trichloride could be detected in the chlorine sample. A single run was made with hydrogen chloride (in place of chlorine) exchanging with arsenic trichloride. Exchange was complete in less than 6 rain, faster than we could conveniently measure. * From the M.S. ThesisofJ. H. O., Florida State University, 1955. Present address: Savannah Rive; Plant, Aiken, South Carolina. ~ Z. Z. HuGt s. J. Amer. Chem. Soc. '74, 1076 (1952) ~J C. BASKERWLL~and H. H. BENNET'r,J. Amer. Chem.°Sot. 24, 1070 (1902). ~3i j. j. DOWNSand R. E. JOHNSON,J. Amer. Chem. Soc. 77, 2098 (1955). f~'~Received on allocation from the U.S. Atomic Energy Commission. 260

The isotopic exchange of arsenic trichloride a n d chlorine

261

All runs were m a d e at r o o m temperature (24.5 q_ 0.5 ) . Decreasing the level of illumination by wrapping the reaction tubes in a l u m i n i u m foil or shielding the dry bbx with red cellophane had no effect on the speed o f the reaction. A n u m b e r of reaction tubes of different sizes were used without a n y apparent effect on the rate. RESULTS

AND

DISCUSSION

Each experimental run showed a straight-line graph of log (l - - F ) vs. time. indicating that a homogeneous isotopic exchange reaction was occurring. Within experimental error, the line extrapolated back to zero exchange at zero time, which means that the separation procedure was not inducing exchange. For each run, the exchange rate-constant R was calculated by

R-

3 [AsCla] 2 [C12]

0.693

3 [AsCla] + 2 [CIe]"

1~

I~)

and the resuhs are given in Table I. It was apparent that hydrogen chloride had a powerful catalytic effect on the reaction rate; the A, B sets of runs represent an effort to offset the catalysis. In each A, B set, the reactants were prepared simultaneously TAgLE

1.

C o n c e n t r a t i o n g.f.w. CI/I, x 10 ~ Run

1 2A 2B 3 4 5 6 7A 7B 8A 8B 9A 9B 10 IIA liB 12 13

CI 2

AsCI a

5'40 3"88 3.88 300 8.16 10.4 lO, I 7.36 7-36 7.48 5.02 7-40 3.54 7.08 ll.l I1.1 6.90 3.55

2.40 2.66 5.33 19-2 12.0 21 "3 15.0 11.3 5-66 12.5 12.5 12.1 12.1 11'6 16.6 4.76 6.40 12.0

t, (hours)

R >: 10 :*

2"5 15"0 2"5 8"9 7-3 ,~0'2 2"2 5-6 4'0 0.65 2'3 6'7 6'9 8"0 3.8 7-4

4"6 0'73 6"2 2"0 4"6

• :0.02 2.0

19"4 5.5 5.5 50 11"4 4.7 2"7 3.8 12-1 2"8 1200 ~~'~ 9"5

,a~ A small (unknown) amount of HCI present in the chlorine. Exchange complete in less than one hour ,hJ 0-020 g.Lw,/l, of HCI added to the reaction mixture. Exchange complete in six minutes.

and fi'om the same batches of arsenic trichloride, chlorine, and carbon tetrachloride. By comparing 7A,B and IlA,B, or comparing 8A,B and 9A,B, for example, it is seen that there is still a catalytic effect, due most probably to hydrogen chloride. Because of the catalysis, the exact mechanism of exchange cannot be studied by our methods. In the presence of any reasonable added amount of hydrogen chloride~

262

J. HARDING OWEN a n d ROWLAND E. JOHNSON

see Run 12, the rate is immeasurably fast. We suspect, in fact, that the mechanism of exchange m u s t involve hydrogen chloride, and maybe there is no direct exchange of chlorine atoms between arsenic trichloride and chlorine. As mentioned above, we tested the exchange of arsenic trichloride and hydrogen chloride in carbon tetrachloride solution, and found it immeasurably fast, in accordance with the results of CLUSIUS and HAIMERL.(5) We certainly expect that the hydrogen chloride-chlorine exchange also would be rapid, in accord with the work of JOHNSTON and LIBBY.(6) Two things, then, indicate against the direct formation of arsenic pentachloride by a combination reaction. The exchange reaction is not first-order in chlorine concentration and first order in arsenic trichloride concentration, as it should be to lead to arsenic pentachloride as a reaction intermediate. Secondly, the exchange is relatively slow. Since phosphorus trichloride and chlorine combine very rapidly, we might expect the arsenic reaction to be rapid also. This would lead to an exchange as rapid as the combination reaction, whereas the exchange reactions have halflives of the order of hours. Similarly, the experimental data obviate a mechanism involving a first step as follows: AsCl a ~_ AsC12 + .Cl

(2)

as postulated (v) for the exchange of arsenic between arsenic trichloride and arsenic tribromide. Such a step as eq. (2) would lead to a first-order dependence on arsenic trichloride concentration. We have considered a number of ways in which arsenic trichloride and hydrogen chloride may exchange chlorine atoms. Perhaps the most reasonable is the direct addition as follows: HC1 ÷ AsC1 a ~-HAsC14 (3) This compound is similar to the aryl tetrachloro arsenic compounds, 18~ and we would expect an equilibration of activity. The hydrogen tetrachloro arsenic would be very unstable and perhaps only tetra-co-ordinated. We are happy to acknowledge the assistance given by the Air Reduction Company, which donated the liquid nitrogen used in this work. This work was performed on Contract No. AT-(40-1)- 1317 with the Atomic Energy Commission. 15) K. CLUSlUS and H. HAIMERL,Z. phys. Chem. 5IB, 347 (1942). 161 W. H. JOHNSTON and W. F. LIBaY, J. Amer. Chem. Soc. 73, 854 (1951). (71 R. MUXART and R. DAUDEL, J. Chim. phys. 47, 610 (1950). (8) N. V. SIDGWICK, The Chemical Elements and their Compounds, Oxford University Press, London, 1950, p. 769