STUDIES ON MOLECULAR WEIGHT CHANGES OF SULFUR MONOCHLORIDE' W. A. PATRICK AND N. HACKERMAN Department of Chemistry, T L Johns Hoplcins University, Baltimore, Maryland REcEivEd January 96, 10$8
The following investigation waa undertaken with a view to studying the anomalous behavior of sulfur monochloride in connection with its boilhig-point elevation as contrasted with its depression of the freezing point. Raoult (13), near the end of the last century, measured the freezingpoint lowering of glacial acetic acid and of benzene due to the addition of sulfur monochloride. The molecular weight of the monochloride as calculated from these data was 135, which corresponds to the formula S2C12. This value agrees with those Dumas (9) and Marchand (9) found independently by vapor density measurements. Beckmann and Geib (3) and Beckmann and Junker (4) obtained similar results by the cryoscopic method, using a number of low-boiling solvents, e.@;.,ethyl chloride, sulfur dioxide, and phosgene. The earliest mention of an investigation by a boiling-point method came in 1899. Odd0 and Serra (12), using the Beckmann boiling-point apparatus, found the molecular weight of sulfur monochloride in benzene and carbon tetrachloride to be abnormal, i.e., 190 and 173, respectively. Two years later, however, Odd0 (11) repeated the work more carefully and a t the same time used a more rigorous method of calculation. This time he found the molecular weight in the same two solvents to be about 150, showing therefore no effect due to the solvent. However, he did find a slight increase in this value with increase in concentration. From that time on the system sulfur-chlorine has been studied in more or less detail, but it was not until recently that more work was done on the molecular weight of sulfur monochloride in solution. Jones (6), by an ebullioscopic method, obtained a value in excess of 160 for the molecular weight of sulfur monochloride in carbon tetrachloride. His experiments also indicate that there is actually an effect due to the nature of the solvent. These results when taken in conjunction with those of Odd0 indilBased on a dissertation submitted June, 1935, to the Board of University Studies of the Johns Hopkins University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 679
680
W. A. PATRICK AND N. HACKERMAN
cate strongly that there is an inconstancy in the niolecular composition of sulfur monochloride. I n view of the volatility of sulfur monochloride, with the resultant uncertainty in the composition of the solutions, it became necessary to limit the investigation to very dilute solutions. This limitation was advantageous from two standpoints. I n the first place,changeinthecornpositionof the solution during ebullition mas made negligible, and in the second placc, 110 reasonable doubt could be entertained in regard to the application of Raoult’s law. Raoult’s law may be applied in the following form if we take the vapor pressure of the solution a t the boiling point to be equal to the sum of the partial pressures of the two components, [i60
+ (dP.i/dTA) AT]zA+ [P; + (dPB, dTB) A T ] T =~ 760
The second term on the left is the correction due to the volatility of the solute, where P i is the vapor pressure of the solute a t the boiling point of the solution and dPB/dTBis the slope of the vapor pressure curve a t that temperature. T h e n the solute is non-volatile both of these terms equal zero and the familiar relationship AT = BzB remains. EXPERIMENTAL
Materials I n order to get somc information on the effect of temperature, solvents nith boiling points as far apart as possible were used. Because of the reactivity of the sulfur monochloride, it was rather difficult to obtain a large number of inert solvents. The liquids finally used mere benzene, carbon tetrachloride, chloroform, cyclohexane, and toluene. Benzene that had been purified by the method described by Greer ( 5 ) was available. Since it was known to be free of thiophene and carbon disulfide it was used without further treatment. Baker’s C.P. carbon tetrachloride was distilled on the 11-foot, modified Podbielniak still described by Zinc (14). The fraction which had the value for ni5’ of 1.4575 was used. The cyclohexane was of Kahlbaum’s “reagent” grade, and it was deemed unnecessary to purify it further. Baker’s C.P. chloroform, which contains 0.5 per cent of alcohol, was treated in the following manner. Several hundred cubic centimeters of the chloroform was refluxed with 1 to 2 per cent of sulfur monochloride for about thirty hours. The resulting solution was washed several times with a solution of pure sodium hydroxide and finally with distilled water. The chloroform was stored over calcium chloride for several days, and was then distilled in a closed system, Baker’s C.P. toluene was treated in the same manner as that described for chloroform.
MOLECULAR WEIGHT CHANGES OF SULFUR MONOCHLORIDE
681
Merck’s “technical grade” sulfur monochloride was distilled from 1 per cent of sulfur and 1 per cent of activated charcoal in the manner indicated by IClann, Vernon, and Pope (8). The final distillation was made in an all-glass still, using a &foot refluxing column filled with glass beads. I n order to prevent contact with the air on transferring the sulfur monochloride from the receiver t o the flask which was t o hold it, the still was filled with dried nitrogen. The nitrogen was run through a three-way stopcock placed just in front of the pump. Tlie sulfur monochloride was kept in a glass-stoppered Erlenmeyer flask from which the air had been displaced by dried nitrogen. The flask was then placed in a desiccator over calcium chloride.
R
C
FIG.1. The ebullioscope
Apparatus Many types of ebullioscope are described in the literature, but it was thought best to design one modelled along the lines of the original Cottrell flask, which would be best adapted to the present purposes. The flask, consisting of three parts, is shown in figure 1. The lower body, a, is merely a piece of glass tubing 4.5 cm. in diameter, closed off a t one end and ground a t the other as the female of a ground-glass joint. The upper body, B, carries, besides the male of the ground joint, a shield and thermocouple well sealed into the top by a double ring seal. A condenser is sealed into the top on the outside a t an angle of about 50”. The pump, C, is of a type used by Mair ( 7 ) , the main difference being in the use of shorter arms and the absence of the heating element which he has sealedinthebottom.
682
W. A. PATRICK AND N. HACKERNAN
The pump is flared at the bottom to cover a wider area which was a consequence of the use of external heating. Two identical flasks, one containing pure solvent and the other the solution, were used side by side in all the experiments in order to overcome the effects due to the fluctuation of the atmospheric pressure. Because of the corrosive character of sulfur monochloride it was impossible to use the more efficient method of internal heating. The heat was supplied from the outside by two small, identical, electric heaters. The cores of the ovens were copper rods, shaped as tacks with concave heads. These were wound with 6 feet of No. 24 B & S nichrome wire, each oven having a resistance of 16.6 ohms. The ovens were connected in series with each other, so that voltage changes in the line would affect both similarly. A variable resistance was also placed in the heating circuit, in series, t o control the current input. A differential method of measuring the boiling-point rise, which is fully described by Jones (6) and Zinc (14),was used. It consisted merely of a galvanometer used as a deflection instrument and a simple two-junction thermocouple. The thermocouple consisted of No. 36 B & S double silkcovered copper wire and No. 30 B & S double silk-covered constantan, both of which were obtained from the Driver, Harris Co.
Procedure The apparatus was calibrated by measuring the difference in temperature between boiling carbon tetrachloride and a solution of naphthalene in carbon tetrachloride of known composition. Baker’s C.P. naphthalene, which contains only 0.002 per cent of non-volatile matter, was sublimed three times before being used in the calibration. The actual measurements were made by first obtaining the galvanometer reading with no current flowing, merely as a reference point. Then with pure solvent boiling in each flask an initial reading was taken. After the flasks had cooled sufficiently the one which was to contain the solution was opened and a weighed amount of sulfur monochloride added. During the introduction of the solute a slow stream of dried nitrogen flowed through the flask continuously. The flask was then closed and both vessels were connected by pieces of rubber tubing, from the end of their condensers, to a large copper box containing calcium chloride. The box was open, a t the far end, to the air. The heating current was started again, and after the liquid in both flasks was boiling evenly another reading was taken. The difference in the two positions on the scale gave the boilingpoint rise directly. Several additions of solute were made in each experiment until the galvanometer mirror had swung over almost the whole scale.
683
MOLECULAR WEIGHT CHANGES OF SULFUR MONOCHLORIDE
TABLE 1 Sulfur monochloride and benzene (see figure 2) z
W
2
W
0.02O05 0.03733 0.04674 0.05057 0.06617
140.0 148.5 152.4 152.5 155.3
0.01240
136.6 143.6 148.4 153.7 153.9 154.4
0.02470 0.03704 0.04653 0.05945 0.06645
+ (in figure 2) represents values of x and W obtained February 27,1935
I
0
(in figure 2) represents values of x and W obtained March 21, 1935
TABLE 2 Sulfur monochloride and cyclohexane (see figure 9 ) 2
W
0.00491 0.02265 0.03697 0.04933 0.05946 0.06501
149.5 153.7 160.2 165.6 168.7 169.5 I
/I
values of x and W obtained March 4, 1935
W
0.01376 0.02640
152.9 158.2 165.7 167.8 168.8
0.04335 0.05078 0.06126
I/
0 (in figure 3) represents
l
II
2
+ (in figure3) represents values of x and W obtained March 6, 1935
z
W
0.01694 0.03465 0.04715 0.05670 0.06840
155.2 161.2 167.6 168.1 172.2
3) represents values of x and W obtained March 15, 1935
0 (in figure
TABLE 3 Sulfur monochloride and chloroform (see figure 4 ) z
0.00824 0.02332 0.03373 0.04540 0.OB6
I
/I
W
z
193.4 167.2 165.8 163.9 164.4
0.00943 0.02156 0.02692 0.04050 0.05685
0 (in figure 4) represents values of x and W obtained March 13, 1935
I
W
187.9 169.0 166.2 164.7 165.4
Ii +
(in figure 4) represents values of x and W obtained March 14, 1935
RESULTS
Calibrations were made at various intervals throughout the series of experiments and an average of all the values was obtained. Using 31.9
684
W. A. PATRICK AND N. HACKERMAN
TABLE 4 Sulfur monochloride and toluene (see figure 5 )
I
2
/I
IV
0.01542 0.02902 0.04851 0.06338 0,07340 0.09235 0,11277
89.4 102.5 107.1 112.6 113.5 109.6 99.8
X
W
0.01857 0.04707 0.06499 0.08621 0.11006
94.6 111.7 112.9 105.0 101.9
~1 +
0 (in figure 5) represents values of x and
(in figure 5) represents values of z and 1Y obtained March 29, 1935
I
1V obtained March 27, 1935
TABLE 5 Sulfur ??ZfJnGChlGTide and carbon tetrachloride (see figure 6 )
1 1 '
Li'
2
0.01749 0.02817 0.0425 0.05643 0,06334
_ _ _ _ _7 _ 143 5 0 01519 0 02345 147 2 153 9 0 03649 156 8 0 04730 158 0 0 05715
W
1V
146 8 151 4 157 4 162 6 166 2
I ,
0 0 0 0 0
01438 02425 03538 04677 05989
0 02599 169 9 172 5
0 04350 0 05013 0 06099
I
Values obtained February 20, 1933
,
Values obtained Rlarch 1, 1935
l
156 154 152
I1
Values obtained April 8, 1935
172 170
I68
150
f
Values obtained March 12, 1935
183.6 182,l 180.9 180.8 182 .0 183.0
166
2
148
9
146 144
5a 142 2 i40
164
162
3
160
?
156
158
u
5
138 136
154
,152
153
131 001
02
C33
CC4
M3LC TRACT'ON
OC5
036
007
COI
om
a03
004
005
a06
am
NCLE TRACTION
FIG. 2 FIG. 3 FIG.2. Sulfur monochloride and benzene. 0,February 22, 1935; +, February 27, 1935; 0 , Narch 21, 1935 March 4, 1935; March 6, 1935; FIG.3. Sulfur monochloride and cyclohexane. 0, 0 , March 15, 1935
+,
MOLECULAR WEIGHT CHANGES OF SULFUR MONOCHLORIDE
685
194
192 190 I88
g
I86
I14
184
I I2
182
110
180
108
178
4
9
106
176
$
I04
174
8
102
172
2 $
I70 I68
166
100
98
: 92 90
164
I62
aoi
002
a03
004
no2
005
004
006
008
010
012
MOLE FRACTION
MOLE FRACTION
FIQ.4 FIQ.5 FIG.4. Sulfur monochloride and chloroform. 0 , March 13, 1935; f, March 14, 1935 FIG.5. Sulfur monochloride and toluene. 0, March 27, 1935; March 29, 1935
+,
184
180
2
176 !72
p
168
= 1:
IW
J 160
2
156 152
I48 144
aoi
002
003
004
i M L E FRACTION
005
006
ao7
60
70
80
90
100
IlO'C.
TEMPCPATURC
FIQ.7 FIQ.6 FIQ.6. Sulfur monochloride and carbon tetrachloride. I, February 20, 1935, 11, March 1, 1935; 111, March 12, 1935; IV, April 8, 1935 FIG 7. Effect of temperature on the molecular weight of sulfur monochloride
as the boiling-point constant, it was found that each division was equivalent to 0.0393"C. I n tables 1 to 5 2 is the mole fraction of the solute and W is the molecular weight calculated from the boiling-point rise. Two or more series of
686
W. A. PATRICK AND N. HACKERMAN
measurements were made with each solvent in order to afford a check. The values of all the experiments in the same solvent are plotted on a single curve. I n the experiments listed the age of the sulfur monochloride was about the same with each measurement in any one solvent and at no time was it more than fifteen days old. I n table 5 is listed an experiment in which sulfur monochloride which had been distilled on February 19, 1935 was used throughout. One observation may be noted here in connection with the experiments in toluene. As the measurement progressed, the solution which had had the normal golden-yellow color of sulfur monochloride during the first hour of boiling, became gradually darker until at the conclusion of the experiment the liquid was a very definite pink. There WBS no indication of a color change in any of the other solutions. DISCUSSION
We are forced to the conclusion that the molecular weight of sulfur monochloride exhibits a rather puzzling series of changes. It is obviously necessary to consider the method of preparation and the age of the compound, as well as the concentration and temperature of the solution in which the molecular weight is being determined. It might be well to mention here that the toluene experiments do not properly form a part of this study, since it is obvious that the high temperatures brought about the formation of sulfur dichloride, as evidenced by the pink color of the solutions. This causes complications of a nature which the data at hand are inadequate in solving. Moreover, the region past the maximum in figure 5 is somewhat outside the realm of dilute solutions and Raoult’s law cannot be justly expected to apply. Two simple conclusions may be drawn from the remaining experiments. First, we are dealing with a polymerized molecule which dissociates at higher temperatures. Figure 7 shows the molecular weight of sulfur monochloride, when 2 is approximately 0.02 in the various solvents, plotted as ordinate against the boiling point of the solvent as abscissa. Although the cyclohexane value is not in very good agreement, the effect of temperature is still unmistakable. Second, the carbon tetrachloride experiments indicate that a progressive polymeiization is taking place in the pure sulfur monochloride. From these results it would appear that the real equilibrium is not being measured in the boiling liquids, but that the true equilibrium rests on the side of the polymerized molecule (note the carbon tetrachloride experiment of April 8, 1935). It then beconies necessary to discover the cause of the sluggishness and the anomalous behavior of the reaction. Furthermore the conditions of age and temperature are insufficient in
MOLECULAR WEIQHT CHANQES OF SULFUR MONOCHLORIDE
687
explaining all of the facts. For example, when using chloroform as the solvent we find that the molecular weight increases with dilution. Such behavior is hardly consistent with the law of mass action. If (AB12
+ 2(AB)
dilution will certainly cause an increase in the right-hand member. I n the chloroform experiments one finds the molecular weight approaching 200 in the most dilute solutions and falling to 164 at a mole fraction of 0.044. A double molecule of sulfur monochloride should exhibit a molecular weight of 270, but’such a condition should result in an increase in molecular weight with increasing concentration. The following assumption, however, offers some interesting possibilities:
2SzCla
*
SrCL
* SzCh +
Sz
JI
s, We have here a series of changes whereby sulfur monochloride breaks up to form S&lr and SS. From one mole of sulfur monochloride there would be formed a/8 moles of SB and a/2 moles of S,Cl,, where a is the fraction transformed. The total number of moles at equilibrium would be
1
- a + a/2 + a/8 = 1 - 3a/8
If a = 1 the molecular weight is 216, and if a = 0 it is 135. By such a mechanism it is possible to account for a variation in molecular weights from 135 to 216, which is approximately the range covered in the experiments. The effect of concentration before the final stage of equilibrium is reached is probably best explained by the effect of the solvent and temprature on sulfur itself. If we assume a reaction such as that just above we are permitted to postulate a great many forms of sulfur. Mellor (10) cites examples of the many different forms of sulfur existing in various solvents and at different temperatures. Furthermore the presence of the sulfur monochloride may alter the form of the sulfur markedly. Aten (2), for example, reported that in a solution of sulfur in sulfur monochloride there existed a ternary mixture of SZCIZ,SA, and what he called S., I n a previous paper (1) he showed that the molecule SA is Sa and that of 5, is S p . The latter form alone may be sufficient to explain the apparent change in molecular weight with concentration, but if it is not adequate, certainly with all the possible ramifications of sulfur there may well exist, to some extent at least, the molecule SZ. I n this connection it is important to remember that the nature of the solvent may be profoundly altered by the presence of the sulfur monochloride.
*
688
W. A. PATRICK AND N. HACKERMAN
SUMMARY
1. il modified Cottrell ebullioscope has been described. 2. The molecular weight of sulfur monochloride has been measured in various inert solvents a t different concentrations. 3. It has been shown that the molecular weight of the sulfur monochloride seems to be affected by its age, the temperature, the nature of the solvent, and the concentration. 4. It has been postulated that the abnormal molecular weight is due t o the presence of a polymer (S&lz)z, which breaks up into Ss and SZCl4; also that the change in weight with concentration is due to an equilibrium between large and small molecules of sulfur. REFEREKCES (1) ATEN: Z. physik. Chem. 88, 321-79 (1914). (2) ATEN: Verhandel. Akad. Wetenschappen Amsterdam 26, 813-9 (1918). (3) BECKMAKK AKD GEIB: Z. anorg. Chem. 61, 96 (1906). (4) BECKMANN AND JCNKER: Z. anorg. Chem. 66,371 (1907). (5) GREER:J. Am. Chem. Sac. 62,4191 (1930). (6) JOKES, H. R.: Dissertation, The Johns Hopkins University, 1934. (7) MAIR: Bur. Standards J. Research 14, 345 (1935). (8) MANN,VERXON,A N D POPE: J . Chem. SOC.119, 634 (1921). (9) Cf.MELLOR:Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. X, p. 635. Longmans, Green and Co., Kea. York and London (1930). (10) Reference 9, p. 23. (11) ODDO:Gazz. chim. ital. 31, 11, 222 (1901). (12) ODDOA N D SERRA:Gam. chim. ital. 29, 11, 318 (1899). (13) RAOTJLT:Cf. reference 12, p. 327. (14) ZINC: Dissertation, The Johns Hopkins University, 1934.
