[CONTRIBUTION FROM
THE
NOYESCHEMICAL LABORATORY, UNIVERSITY OF ILLINOIS]
LEVINSTETN MUSTARD GAS. VI. T H E MODE OF FORMATION1 REYNOLD C. FUSON, ROBERT E. FOSTER,
AND
ROBERT D. LIPSCOMB
Received March 16, 1948
Mustard gas, prepared from ethylene and sulfur monochloride by the Levinstein process (in which the reaction is carried out at 35"), is composed chiefly of bis(2-chloroethyl) sulfide (approximately 70%) and polysulfides (approximately 30%). The polysulfides vary in composition and stability and have the general formula (ClCH2CH2)2S,. Three of the sulfur atoms in the polysulfides appear to be held in a stable linear trisulfide skeleton (1) and cannot b3 removed by stripping. The pentmulfide represents another stable level and has been assigned the following formula I. S
r
ClCH2 CHzSSSCH2 CH2Cl
-1
S I The amount of sulfur removed by stripping, the amount of pentasulfide which can be obtained, and the freezing point of fresh material indicate that the polysulfides in freshly prepared Levinstein mustard gas may have an average composition varying from that of the hexasulfidc to that of the nonasulfide. Usually the composition of the polysulfide corresponds to that of the heptasulfide. MECHAIGISM
In 1920 a mechanism for the formation of mustard gas from ethylene and the sulfur chlorides was postulated by Conant, Hartshorn, and Richardson (2). From the evidence then available thay assumed that 2-chloroett~ylsulfenyl chloride (11) was an intermediate in the process. CH2=CH2
SC12 or SICl,
ClCHzCHzSCl
CHZ=CHt
(CICH2CH2)*S
I1
This theory recently has been substantiated by the isolation of 2-chloroethylsulfenyl chloride in pure form and by its reaction with ethylene to produce bis(2-chloroethyl) sulfide (3). Further evidence that 2-chloroethylsulfeny1 chloride is an intermediate has been obtained by using an equimolecular mixture cjf ethylene and cyclohexene in the Levinstein process instead of ethylene alone. bis(2-Chloroethyl) sulfide, bis(2-chlorocyclohexyl) sulfide, and 2-chlorocyclohexyl 2-chloroethyl sulfide were obtained. 1 This paper is based on work done for the Office of Scientific Research and Development under Contracts Nos. OEMsr-300 and OEMsr-48 with the Board of Trustees of the University of Illinois.
504
LEVINSTEIN MUSTARD GAS.
505
VI
In a similar manner an equimolar mixture of ethylene and cyclohexene was passed into a solution of sulfur dichloride in mustard gas. The same three products were obtained, demonstrating that the reaction of ethylene withsulfur dichloride proceeds in two steps and involves the intermediate formation of 2-chloroethylsulfenyl chloride. Any mechanism which is to account for the formation of Levinstein mustard gas must also afford a satisfactory explanation for the formation of polysulfides derived from the linear trisulfide as well m the virtual absence of the disulfide and of free sulfur (1). The following mechanism appears to fulfill these conditions. Although sulfur monochlorjde may react only as such with ethylene (4),there is chemical and physical evidence to support the assumption that it may undergo disproportionation to form sulfur dichloride and sulfur tritadichloride (5). 2 s2c12
(8)
* SCl2 +
s 3 c 1 2
I n the Levinstein process, accordingly, the ethylene may react with the dichloride (b), the monochloride Cc), or the tritadichloride (d). (b) (c) (d)
2 C2H4 2 C2Hd 2 C2H4
+ SClz + S2Clz + S3Cl2
CICH2CH2SCH2CH2CI CICH2CHrSSCH2CH2Cl 4 CICH2CH*SSSCH&H2Cl
-+
4
If higher polythio sulfur chlorides such as S&lz were present they might condense in a manner analogous to that represented by equation (d). The reaction of sulfur dichloride with ethylene is known to be w r y rapid. Accordingly, the excess of ethylene which is aIways present should serve to keep the concentration of sulfur dichloride at a very low value, thus promoting the displacement of the equilibrium (a) to the right and increasing the rate of formation of the tritadichloride. This would bring about a corresponding decrease in the amount of sulfur monochloride which could react with ethylene to produce bis(2-chloroethyl) disulfide. However, it is to be expected that the reaction of ethylene with sulfur monochloride should proceed at least as rapidly as with sulfur tritttdichloridz (equations c and d). Therefore, the only way to account for the virtual absence of bis(2-chloroethyl) disulfide in Levinstein mustard gas is to assume that the disulfide is attacked rapidly by sulfur monochloride in the manner previously obeerved (4). (e)
+
CICH~CR~SSCH~CH2Cl3 S2C12 -+ 2 ClCHzCHzSCl
+ 2 Sac12
The formation of sulfur tritadichloride as a product of this reaction obviates the nece8sity of assuming the disproportionatj on of sulfur monochloride (equation a) to account for the formation of bis(2-chloroethyl) trisulfide. However, that is still considered a definite possibility. The trisulfide, formed according to equation d, has been shown (4)to be SUIfurized readily at 30-35" by sulfur monochloride to higher polysulfides (equations
506
FUSON, FOSTER, AND LIPSCOMB
f , g, and h); and sulfur dichloride is the other product of the reaction. It reacts immediately with ethylene to produce mustard gas. (f) (g)
(h)
+
+
C ~ C H ~ C H ~ S S S C H ~ C HS2C12 Z C ~ + ( C ~ C H ~ C H ~ ) ~SCls SI (ClCH&H2)2Sd SzCl2 + (ClCH2CH2)zSs SC12 (ClCH2CHz)zSs SzClz 3 (ClCH&H2)& SClz etc.
+ +
+ +
Because sulfur dichloride is generated in the reaction mixture only as a product of other reactions, and because an excess of ethylene is always present, the concentration of sulfur dichloride can never be very great. For this reason, overchlorination is not as serious a problzm as in the processes involving the use of sulfur dichloride directly (2). The net result of this series of reactions is the production of a mixture of bis(2chloroethyl) sulfide and the corresponding polysulfides. All these equations may be combined to give the over-all equation for the Levinstein process. (i)
2x C2H4
+ x S2C12
--$
(x
- ~ ) C ~ C H ~ C H ~ S C H ~+C H(C1CH2CH2)2S,+~ ZC~
The composition of the polysulfide as well as its concentration in fresh Levinstein mustard gas depends on the conditions under which the reaction was run, especially on the temperature, degree of agitation, amount of seed charge, rate of addition of ethylene, and very likely many other factors, such as previous history of the sulfur monochloride, have an important effect. So long, however, as there is no sulfur precipitated during the reaction, there must be according to the general equation above, a definite relationship between the composition of the polysulfide and its concentration. Empirically, one sulfur atom is produced each time a molecule of bis(2-chloroethyl) sulfide is formed and this sulfur must all be present as polysulfides. The higher the sulfur content of the polysulfide, the lower need be its concentration. The table which follows illustrates this relationship. The monosulfide-polysulfide ratios tabulated are those which would be required t o satisfy the general equation. The table lists other properties, based on theory, which the original mixtures should have. Also included are the calculated pentasulfide-monosulfide ratios in the products obtained by stripping or aging the original mixtures. The theoretical values which appear in the table are in excellent agreement with the great quantity of experimental data on Levinstein mustard gas compiled at Edgewood Arsenal and other Chemical Warfare Service laboratories (6). THF EFFECT O F TEMPERATURE ON THE LEVINSTEIN PROCESS
The mechanism for the Levinstein process which has just been outlined is further supported by results obtained in a study of the effect of temperature on the reaction. The amount of impurity (polysulfides) in the resulting Levinstein mustard gas depends on the rate of formation of the trisulfide and on the rate of its sulfurisation by sulfur monochloride. The increase in the rate of the reactions with increase in tempxature is partially offset for those reactions involving ethylene because of the decreased solubility, and therefore concentration, of
LEVINSTEIN MUSTARD GAS.
507
VI
ethylene in the warmer solvent. Thus, the sulfurization reactions (f, g, and h) would be expected to proceed relatively more rapidly than the other reactions which depend upon the concentration of ethylene. Each time an atom of sulfur is added to a polysulfide molecule, a molecule of bis(2-chloroethyl) sulfide must be formed concurrently in order to preserve the stoichiometric relationship. Thus, a t elevated temperatures the reaction between ethylene and sulfur monochloride might be expectsd to proceed more rapidly and to produce a greater yield of bis(2-chloroethyl) sulfide a t the expense of the polysulfide fraction. However, the polysulfides which were formed would have a higher sulfur content TABLE I RELATIONSHIP BETWEEN COMPOSITION AXD PROPERTIES OF POLYSULFIDES his (2.CELOROETBYL)
Mixture
F O L Y S U L F I D E - Y O P i O S ~ L ~ I MIXTURES D~ WHICH SATISFY TEE GENERAL EQUATION
Mo!e
Ratio
Mole %
Weight
7%
I
Calc'd F.P.
Weight % Strippable Sulfur
I ~ S ( ~ - C R L O R O E T EL) Y PENTASUL' IDEYONOSULFIDE IXTUBES PRODUCED R Y STRIPPING
Weight
%
1. Monosulfide Pentasulfide
3 1
75 25
62.5 37.5
5.6
0
62.5 37.5
2. Monosulfide Hexasulfide
4 1
80 2c
65.8 33.2
7.2
3.4
69.0 31 .C
3 . Moncisulfide Heptasulfide
5 1
83.3 16.7
69.5 30.5
8.5
5.5
73.6 26.4
4. Monosulfide Octamlfide
6 1
85.7 14.3
71.5 28.5
9.1
7.2
76.9 23.1
5. Monosulfide Nonasulfide
7 1
87.5 12.5
72.9 27.1
9.7
8.4
79.6 20.4
6. Monosulfide Decasulfide
8 1
88.9 11.1
74.1 25.9
10.4
9.3
81.7 18.3
than those produced at low reaction temperatures. These considerations have been borne out by the following experimental observations. When the sulfur monochloride-ethylene reaction was carried out a t 60" (2), the reaction proceeded rapidly to give high yields (80%) of bis(2-chloroethyl) sulfide and large amounts of free sulfur. If the rate of sulfurization was high, then as fast as bis(2-chloroethyl) trisulfide was produced it was sulrurized by sulfur monochloride to higher polysulfides. Since a molecule of bis( 2-chloroethyl) sulfide was produced each time a sulfur atom was added to a polysulfide molecule, the amount of bis(2-chloroethyl) sulfide was large also in this case. The higher polysulfides produced are unstable a t elevated temperatures and deposit sulfur readily.
508
FUSON, FOSTER, AND LIPSCOMB
When run a t 35", as in the Levinstein process, the reaction proceeded somewhat more slowly, the yield of bis(2-chloroethyl) sulfide was less (70xl), and sulfur was observed to precipitate only after the product had been 'allowed to stand for several weeks. Here, the rate of sulfurization was moderate, and the polysulfides formed were of lower molecular weight and consequently more stable. MacInnes and Belcher (7) carried out the reaction a t 27", and obtained a still lower yield of mustard gas and a polysulfide fraction of still lower molecular weight . When the sulfur monochloride-ethylene reaction was carried out at 20", the yield of bis(2-chloroethyl) sulfide was only 61%, and the polysulfide portion made up 39% of the total product. The polysulfide proved t o be nearly pure bis(2-chloroethyl) pentasulfide and contained only traces of higher polysulfides. Apparently the rate of sulfurisation was low, and the yield of bis(2-chloroethyl) sulfide correspondingly low. Indeed, if no sulfurisation a t all took place the yield of the sulfide would be only 50 mole-per cent (41.6% by weight), and bis(2chloroethyl) trisulfide would be the other product. It is likely that the latter is sulfurized to bis(2-chloroethyl) pentasulfide so readily that it would not be possible to carry out the reaction to produce polysulfides lower than the pentasulfide. This contention seems to be borne out by the results obtained when the reaction was run at a still lower temperature (12'). The polysulfide obtained had an average composition higher than bis(2-chloroethyl) pentasulfide. A s with the preceding papers in this series, valuable assistance has been afforded by cooperating groups. The authors wish to make special acknowledgment of the help of Dr. R. Macy of the Edgewood Arsenal. EXPERfMENTAL
Reaction between sulfur monochloride and a mixture of ethylene and cyclohezene. I n this reaction pure bis(2-chloroethyl) sulfide (157 g.) WBB used as the "seed charge" (8) because it could be removed easily in subsequent operations. Ethylene (1.35 moles) was bubbled through 1.35 moles (111 g.) of warm cyclohexene and the mixture of gases introduced into the well-stirred reaction mixture at 32" over a period of three hours. Sulfur monochloride was added simultaneously and kept in moderate excess until 1.35 moles (182.5 g.) had been added. Finally, ethylene was passed into the mixture for three hours longer to remove excess sulfur monochloride. The product (440g.) was distilled i n vacuo, yielding 285 g. of yellow oil, b.p. 60-135' (1-2 mm.) and 140 g. of a liquid residue. Redistillation through an efficient column yielded 183 g. of bis(2-chloroethyl) sulfide, b.p. 54-57" (0.25 mm.), and 59 g. (0.277 mole) of 2-chlorocyclohexyl 2-chloroethyl sulfide (3), b.p. 115-118O (1.35 mm.). If the original 157 g. of bis (2-chloroethyl) sulfide used as the seed charge is subtracted, then the yield of pure mustard gas from the reaction is 26 g. (0.165 mole). Furthermore, if it is assumed that the usual 30% yield of polysulfide is obtained then 85 g. of the non-distillable residue is polysulfides and the remainder (55 g., 0.207 mole) is the bis(2-chlorocyclohexyl) sulfide. Reaction between sulfur dichloride and a mixture of ethylene and cyclohexene.' The procedure used in the preceding experiment was followed, except that one-molar proportions ~~
~
2
This experiment was performed by Mr. Lester J. Reed.
LEVINSTEIN MUSTARD GAS.
509
VI
of sulfur dichloride and cyclohexene were used, and the temperature was maintained below 15”. The final distillation yielded 32 g. of 2-chlorocyclohexyl 2-chloroethyl sulfide. The e$ect of temperature on the Levinstein process. a. The reaction at 20’. The Levinstein reaction aa ordinarily carried out W M modified by the use of pure bis(2-chloroethyl) sulfide as a “seed charge” and a temperature of 20°, rather than 35’. Alcohol-free, dry ethylene was passed into the vigorously-stirred reaction mixture at such a rate that ethylene escaped freely. During the first four hours sulfur monochloride waa added slowly, maintaining a moderate excess; then, ethylene was passed in for five hours longer. The yield of pale yellow liquid was 96% based on the sulfur monochloride used. It did not darken or deposit sulfur during three months observation. A 542-g. sample of this product was hydrolyzed exhaustively by stirring vigorously with 1.6906. The water for several days. The clear, pale yellow, residual oil weighed 125 g.; material hydrolyzed had contained 38.2% of pure mustard gas (the seed charge); therefore, only 61.8% or 322 g. of the sample had furnished the 125 g. of residue. Thus the residue amounts to 38.8% by weight of the hydrolyzed Levinstein mustard gas. This weight of residue and its refractive index indicate its composition to be approximately that of bis(2chlorolethyl) pentasulfide. Sixt y-nine grams of the non-hydrolyzable residue was subjected to treatment with Cellosolve, etc., as described in the preparation of bis(2-chloroethyl) pentasulfide from Levinskein mustard (1). About 1g. of oil was insoluble in the Cellosolve, and a very small amount of sulfur precipitated. The final product weighed 48 g.; the refractive index, n: 1.6884, was in good agreement with the value obtained previously for the purified pentasulfide (1). b. The reaction at 19’. The experiment just described waa repeated except that the temperature of the reaction was 12’. The results of the hydrolysis indicated that the Levinstein mustard gas formed had contained 34.3% by weight of polysulfide with an average composition slightly less than that of bis(2-chloroethyl) hexmulfide. The non-hydrolyzable residue (106 g.), after stripping with Cellosolve, yielded an oil of refractive index, n: 1.6911. A considerable amount (22 g.) of Cellosolve-insoluble residue, consisting of sulfur and an amber oil, was obtained. The oil waa removed by filtration, washed with ether, and submitted for analysis. The results were in excellent agreement with values calculated for a polysulfide containing thirteen atoms of sulfur. Anal. Calc’d for C,HeCltSla: C, 8.83; H, 1.48; C1, 13.0; S, 76.6. Found: C, 8.71; H, 1.41; C1, 14.0; S, 76.3.
TZ:
SUMMARY
A mechanism has been suggested which accounts satisfactorily for the composition of mustard gas obtained by the Levinstein process. URBANA,ILL. REFERENCES (1) FUSON, PRICE,BURNESS,FOSTER, HATCHARD, AND LIPSCOMB, J . Org. Chem., Paper
IV of this series. (2) CONANT, HARTSHORN, AND RICHARDSON, J . A m . Chem. SOC.,42, 585 (1920). , BAUMAN, BULLITT,HATCHARD, A N D MAYNERT, J . Org. Chem., Paper (3) F u s o ~ PRICE, I of this series. (4) FUSON,BURNESS, FOSTER, AND LIPSCOMB, J . Org. Chem., Paper V of this series. (5) MELLOR,“ A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Longmans, Green and Co., New York, 1930, Vol. X, pp. 633-643. (6) Chemical Warfare Service Reports. (7) MACINNES AND BELCHER, O.S.R.D. Report. (8) GIBSONA N D POPE,J . Chem. Soc., 117, 271 (1920).
