of (I, P’-Bis (Chlorocyclohexyl) Sulfides Duncan A. MacKillop Dunlop Research Center, Sheridan Park, Ontario, Canada OLEFINSREACT with sulfur monochloride to give @,P’-bis(chloroalkyl) sulfides, i.e., RlRKXXHzSxCHCClRlR2. The product of reaction is a mixture ofpolysulfides (X = 2, 3, 4, etc.) and monosulfide, resulting from rearrangement of sulfenium ion intermediates in the addition. Extensive research (1) has been conducted on the synthesis of bis(chloroalky1) polysulfides, and their subsequent reduction as a method of synthesis of alkylene sulfides. Cyclohexene was chosen as a model olefin compound for reaction studies to determine the factors controlling the yield of polysulfide us. monosulfide intermediate, such as catalyst, temperature, or diluents. Therefore, a rapid quantitative method of analysis was required to determine the monosulfidefpolysulfide compound ratio. Methods are available for the analysis of alkyl sulfides. Mixtures of alkyl sulfides are usually analyzed by simple reduction with reagents such as sodium sulfite (2), zinc and cadmium amalgam or lithium aluminum hydride (3, 4). The thiol groups generated upon reduction can be determined by colorimetry with potassium cyanide reagent (2), or amperometric titration (5) with silver nitrate, mercuric nitrate, or lead acetate. The hydrogen sulfide liberated can be assayed as a measure of the average sulfur link length. Alkyl monosulfides can be determined by direct oxidation with potassium bromate and potassium bromide (6). A combination of these two techniques has been used for analysis of phenylsulfide compounds (7). Polarographic techniques have been applied successfully to the analysis of alkyl disulfide compounds (8). These methods were tried and found inapplicable to the analysis of chlorosubstituted alkyl sulfide compounds. The products of reaction of cyclohexene and sulfur monochloride can be analyzed by a rapid infrared spectrophotometric method or a time-consuming procedure based on the hydrolysis of the monosulfide compound. Thin layer chromatgraphy (TLC) gives a semiquantitative estimate of monosulfide/polysulfide compound ratio. The infrared spectrophotometric method is based on the unique difference between monosulfide and polysulfide compounds in the intensity ratio of their infrared absorbance bands at 692 cm-l and 738 cm-l, due to the C-CI stretching mode for the axial and equatorial configuration of the molecule, respectively. EXPERIMENTAL
A purified sample of the bis(chlorocyc1ohexyl)monosulfide was obtained by crystallization from ethanol solution. Polysulfide compounds free of monosulfide compounds were ob(1) F. Lautenschlaeger and N. V. Schwartz, J. Org. Chem., 34, 3991 (1969). (2) A. Schiiberl and E. Ludwig, Ber., 70B, 1422 (1937). (3) R. C.Arnold, A. P. Lien, and R. M. Alm, J. Amer. Chem. Soc., 72, 731 (1950). (4) ha. L. Studebaker and L. G. Nabors, Rubber Chem. Technol., 32,941 (1959). ( 5 ) I. M. Kolthoff, D. R. May, P. Morgan, H. A. Laitinen, and A. S. O’Brien, IND. ENQ.CHEM.,ANAL.ED.,18, 442 (1946). (6) S. Siggia and R. L. Edsberg, ANAL.CHEM., 20, 938 (1948). (7) D. P. Harnish and D. S . Tarbell, ibid., 21, 968 (1949). (8) W. Stricks and S . K. Chakravarti, ibid., 33, 194 (1961).
tained by liquid chromatography through an alumina column using hexane as eluent. In the hydrolysis method approximately 4 grams of sample were weighed into a 250-ml conical flask to which 100 ml of 0.05N sodium hydroxide was added in order to neutralize the liberated HCl and accelerate the hydrolysis reaction. After several hours’ stirring, a 10-ml aliquot was neutralized with sulfuric acid or sodium hydroxide solution as required to the phenolphthalein end point. Distilled water, 40 ml, was added to the flask and the free chloride titrated with 0.1N silver nitrate solution using potassium chromate as indicator. The per cent chloride was plotted with time until a constant value was obtained. Fifty milliliters of distilled water was titrated as a blank. Thin-layer chromatography was used as a qualitative assay. Seven-microliter samples of 0.5 % w/v solutions of monosulfide/polysulfide mixtures in ethyl acetate were deposited on a 0.25-mm thickness substrate prepared from silica gel with gypsum and applied on glass. Full development with benzene, chloroform, or ethyl acetate carried the polysulfide compounds off the substrate. Hexane was found to be suitable for polysulfide presentation but didn’t move the monosulfide at all. Therefore, two-stage ascending development was employed for best results-Le., 4-cm frontal advance with ethyl acetate and 10-cm frontal advance with hexane. The spots were visualized by iodine absorption and benzidine spray. Spot area was estimated by counting squares through a grid. The infrared spectra were recorded on a Perkin-Elmer Model 521 spectrophotometer using 2.5 % w/v solutions of sample in carbon disulfide and the absorbancies measured as shown in Figure 1. RESULTS AND DISCUSSION
It has been shown that mustard compounds are readily hydrolyzed by a stepwise reaction with the formation of an intermediate ionic species (9). A simple argentimetric titration for liberated C1- ion gives a measure of the monosulfide compound concentration. However, these aqueous hydrolysis reactions on water insoluble compounds are very slow, requiring several days to reach completion. Attempts to accelerate this process by increasing alkalinity using mixed solvents, and raising the temperature failed to increase the rate of hydrolysis significantly. A simpler, more rapid analysis method was required to support the research program. The infrared spectra of the monosulfide and polysulfide compounds have one unique difference that provides a means for their analysis. The polysulfide compound exhibits strong absorption bands at 692 cm-1 and 738 cm-l. These absorptions are assigned to the C-C1 stretching frequencies for the axial and equatorial configurations of the molecule, respectively. The IR spectrum of the monosulfide compound exhibits a much lower intensity band for the C-Cl stretching mode in the equatorial configuration. Two bands of weak intensity (746 cm-1 and 724 cm-1) bracket the 738 cm-’ band. In monochlorocyclohexane the equatorial configuration i s the preferred state as evidenced by its infrared spectrum(9) “Organic Chemistry of Bivalent Sulfur,” VI ed., E. E. Reid, Chemical Publishing Co. Inc., New York, 1958.
ANALYTICAL CHEMISTRY, VOL. 42, NO, 14, DECEMBER 1970
0
1815
Table 111. TLC Analysis of Blends wt monosulfide Area, Z (from log w = kA1’2) 25 35 37 50 61 62 75 70 68 Wt %
Figure 1. Measurement of IR spectral absorbances at 692 cm-1 and 738 cm-l
Table I. IR Spectral Data on MonosulAde/PolysulRde Blends Wt A69~crn-’/ monosulfide A 6920rn-~ A7asom -1 Anaom-1 nil 0.460 0.459 1.oo 18 0.583 0.413 1.41 36 0.682 0.378 1.80 50 0.690 0.315 2.16 66 0.730 0,274 2.66 83 0.736 0.231 3.19 100 0.880 0.267 3.30
z
Table PI. Weight Per Cent Bis(Chlorocyclohexy1) Monosulfide in Samples Hydrolysis Sample method“ IR method Difference 1 16 17 1 2 20 21 1 3 24 27 3 4 34 31 3 5 38 38 0 Av 26.4 26.8 1.6 a 96-hr reaction time.
Le., the ratio of A692cm-1/Ai85crn-1 is much less than unity.
However, the bis(chlorocyclohexy1) polysulfide has A6920rn-1/ Ai38cm-1approximately equal unity, whereas the monosulfide compound has a ratio of approximately 3. The preference of the axial configuration in the monosulfide compound may be energetically favored since free rotation of the chlorocyclohexyl groups about the C-S bond is sterically hindered by the equatorial chlorine atoms. It has been suggested that there is some molecular restraint due to the overlap of the chlorine and sulfur atom nonbonding electron orbitals in the monosulfide compound, which inhibits rotation. This artifact of differing absorbance ratios was employed as
1816
e
z
the basis for an analytical procedure. The data of Table I show that the ratio A6920m-~/A,380m-~ is a good linear function of weight per cent monosulfide compounds in a blend of bis(chlorocyclohexyl) sulfides. Several samples of reaction products were analyzed by both the hydrolysis and infrared spectrophotometric methods. Analysis data are compared in Table II. There is reasonable agreement between these two methods for the few samples tested. The hydrolysis method was too time-consuming to establish statistical reproducibility data. The IR spectrophotometric method is reproducible to ca. & O S wt in the range of concentrations tested. The infrared spectrophotometric results m e considered to be the most reliable since the calibration curve is smooth and linear and the test compounds are placed in reasonable order of their expected composition. A weakness in the infrared spectrophotometric method is that all polysulfides are assumed to have the same ratio of equatorial to axial chlorine and the same values of C-Cl absorptivity coefficient. Also the calibration is made on a weight percentage basis whereas in fact the absorbance is a molecular function which may cause s5me error in that the mole concentration of C-Cl will decrease as the number of sulfur atoms increases in the polysulfide molecule. Three blends of the monosulfide and polysulfide compounds were analyzed semiquantitatively by thin-layer chroniatography. The data of Table I11 show the estimate of composition for these blends. The calculation of concentration by the simple area percentage is as good a measure of concentration as the relationship log w = kA1’2,where w i s the weight of material in a spot and A i s its area. The accuracy of the TLC method is relatively poor with average difference values of 10 wt on the blend samples. Although not quantitative, TLC is useful in providing an estimate of the number of compounds in a sample and their order of concentration. TLC has shown that the test samples contained mono, di, and tri sulfides as the major components with traces of only higher sulfides.
RECEIVED for review June 16, 1970. Accepted August 24, 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
