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decreased a t about 36-38" C., indicating a melting of crys- Two adhering sheets, 3 by 3 inches thaw in 3 minutes; three tals. They evidently did not encounter frozen rubber of sheets of the same size thaw in 4l/2 minutes. This can be the type which has not been exposed to freezing temperature. carried on up to the dimensions of a bale, which thaws comLeblanc and Kroger (3) and Kroger (6) used sufficient pres- pletely in 24 hours if not surrounded by other bales. sure on rubber to cause a change of state (aggregation) which The thawing of temporarily frozen rubber is a different type they regarded as analogous to the effect produced by cold of change. Simple exposure to a moderate room temperature and, in a way, resembling vulcanization. will thaw sheets in a few minutes. The thawing of a bale is, Apparently, milled rubber undergoes the same change as of course, a longer process, but extreme temperature is not smoked sheets. Several bales of milled rubber were placed necessary except as a means of hastening heat transfer. in a local ice plant a t -5.6" C. and examined a t various interNone of the baled, milled rubber was found to be pervals during 38 weeks. The bales showed normal cooling manently frozen, even after 38 weeks at -5.6" C. However, curves and came to equilibrium in 4 to 6 days, depending upon a number of 1-pound samples which were deformed by a their original temperature. When removed from the ice rubber-cutting machine and later stored under slight pressure plant, the bales thawed in 5 to 7 days at room temperature for nearly a year a t room temperature (5-35" C.) assumed varying from 15" to 30" C. the opaque, horny condition of permanently frozen smoked Frozen rubber, whether in sheet or milled form, assumes sheets and did not thaw out a t room temperature. It would an opaque, creamy color and becomes very difficult to work. seemi therefore, that pressure is one cause of permanent This condition may be either permanent or temporary. By freeiing. the former is meant rubber which remains unworkable, Acknowledgment boardy, and opaque at room temperature; and by the latter, rubber which will thaw a t room temperature. Most frozen The writer wishes to express his thanks t o G . K. Hinshaw rubber is in the second class. Both usually have the same J. P. Maider for helpful criticism in the preparation of and appearance, although bales of smoked sheets may assume this article, and to R. P. Dinsmore for permission to publish their natural brown color and still remain boardy. By cutting a sheet the opaque, creamy color will be seen on the it. freshly cut edge. Literature Cited The thawing of frozen rubber is a very definite temperature effect. A single sheet of permanently frozen crude rubber (1) Grifiths, Trans. Inst. Rubber I n d . , 1, 308 (1926). (2) Kroger, Gummi-Ztg., 40,782-4 (1926). will remain boardy for months a t room temperature. If (3) Leblanc and Kroger, Kolloid-Zlg., 37, 205-14 (1926). placed in an oven at 50" C., it will thaw in a few minutes. A (4) Park, Carson, and Sebrell, IND.ENG.CHEM.,20, 478 (1928) piece of smoked sheet 3 inches square thaws in 2 minutes. (5) Rossem, van, and Dekker, Kaalschuk, 6, No.1, 2-5 (1929).
The Reduction of Carbon Disulfide'" J. A. M i t ~ h e l l Emil ,~ Ott,a and E. Emmet Reid' CHEMISTRY LABORATORY, JOHNS HOPKINSUKIVBRSITY, BALTIMORE, MD.
T
H E reduction of carbon disulfide has not received m u c h a t t e n tion. I n 1856, Girard (4) obtained thioformaldehyde by using zinc and d i l u t e d hydrochloric acid as r e d u c i n g agents, according to the reaction :
C&'+ 4H+ H&:S H2S The gas formed during the reaction had a strong odor of leeks but only hydrogen sulbeen fide and carbon disulfide were identified. In particular no volatile mercaptan was observed. It is probable that the thioformaldehyde always polymerizes as formed. Gawalowsky (3) reduced carbon disulfide with zinc and either sulfuric acid or a strong solution of potassium hydroxide and obtained in both cases gases having the odor of rotting cabbage. This description of the odor is in agreement with the present writers' observations. The gas obtained by the reduction with sulfuric acid immedi-
ately gives a pure black precipitate from lead nitrate solution, whereasthat from the reaction with alkali yields a gas giving a fiery orange-red p r e c i p i t a t e . Mittasch ( 6 ) found that when hydrogen a n d c a r b o n disulfide were conducted over nickel powder, which was heated t o a m o d e r a t e temperature, the disagreeable odor of certain organic sulfur compounds was apparent. Although no accurate analyses could be obtained, the presence of mercaptans and alkyl sulfides was assumed. The vapors were condensed together with unchanged carbon disulfide in an ice-salt mixture. Similar investigations were conducted by Sabatier and Espil (7). The nickel was heated to 180" C. and a product of very disagreeable odor was obtained, which they thought might be methylene dithiol, H2C (SH),. Its precipitated salts were yellow for mercury, white for cadmium, yellow-
Presented before the Division of Petro1 Received March 3, 1931. leum Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. 2 This paper contains results obtained in an investigation on a study of the Reactions of a Number of Selected Sulfur Compounds listed as Project 28 of the American Petroleum Institute Research. Financial asssitance in
this work has been received from a research fund of the American Petroleum Institute donated by John D. Rockefeller. This fund is being administered by the Institute with the co6peration of the Central Petroleum Committee of the National Research Council. 8 American Petroleum Institute Research Fellow for summer of 1930. 4 Director, Project 28.
+
A study of the reduction of carbon disulfide has shown that under favorable conditions methylene dithiol, HzC(SH)Z,is formed. Owing to the presence of excess of carbon disulfide it is impossible to isolate the methylene mercaptides since they react at once with carbon disulfide to form trithiocarbonates. Several such salts have been prepared. It is proved thatthey are salts of methylene ditrithiocarbonic acid, H2C(S.CS.SH)2, which is obtained free as a heavy brown oil. It is also possible that the lead salt of an interS.CS.SH mediate monotrithiocarbonic acid, H&' 7 has 'SH
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brown for lead and copper. Although these authors stated their intention of studying this reaction further, no reference has been found to any subsequent work. These investigators were unaware of the fact that mercaptides react with carbon disulfide to form thiocarbonates, which would make i t impossible to obtain analyses in agreement with the pure mercaptides. Theoretical Considerations
It thus appears that the reduction of carbon disulfide can occur in two fashions-namely, yielding thioformaldehyde with hydrogen sulfide when zinc and acids are used as reducing agents, and mercaptan compounds, presumably methylene mercaptan, during the catalytic reduction with hydrogen and nickel a t higher temperatures. However, the peculiar odor observed in the reduction with zinc suggests that also in this case a mercaptan is formed, at least in traces. I n particular the reddish lead product obtained from the vapors which were produced in Gawalowsky’s experiments when caustic was used in place of acid links this reaction up a t once with the one of Sabatier and Espil. The experiments of both Gawalowsky and Girard lead to the conclusion that hydrogen sulfide and thioformaldehyde are the chief products when acids are employed, though any mercaptan present could easily escape detection on account of the presence of considerable amounts of hydrogen sulfide. The reaction between formaldehyde and hydrogen sulfide as studied by Baumann (1) is of interest here, since he was able to show that in the course of this reaction methylene dithiol, as well as some other mercaptan compounds, is formed. The mercaptans become less stable when the acidity of the solution is increased, which is in agreement with the results summarized above. Baumann could not isolate methylene mercaptan as such, but he was able to show its presence in the solution by converting i t to the methylthio ether and oxidizing the latter to the sulfone. His work indicates that methylene mercaptan, in contrast to the corresponding hydroxyl compound , is a fairly stable substance, so that it is reasonable to expect its formation in the reduction of carbon disulfide. Reduction Procedure
I n the present investigation pure carbon disulfide (100 cc.) was reduced with chemically pure zinc dust (150 grams) and 50 per cent acetic acid (200 cc.) a t the boiling point of the mixture. The reaction product was condensed, together with unchanged carbon disulfide, by means of carbon dioxide snow. The apparatus consisted of a 1-liter balloon flask attached to a short-bulb reflux condenser. A piece of bent glass tubing immersed in a large Pyrex test tube cooled in carbon dioxide snow was connected to the top of the condenser. Mossy zinc is not so effective as zinc dust. Under such conditions hydrogen sulfide was formed only in negligible amounts. It was impossible to separate the mercaptan from the carbon disulfide by distillation; however, it could be shown that the mercaptan is stable under the conditions of such a distillation. The first portion of the distillate contained mostly the mercaptan. The replacement of zinc by amalgamated aluminum for the reduction of the carbon disulfide yields mainly hydrogen sulfide, resulting from the decomposition of the methylene mercaptan, according to the reaction: Cs2
+ 2Hz +HzC(SH)z +HiCS + H B
Effect of Adding Metal Salts to Mixture
To prove the presence of the methylene mercaptan, various metal salts, especially lead and silver, were added to the
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mixture of carbon disulilde and methylene dithiol. The lead salt so obtained was pure yellow, but when hydrochloric or sulfuric acid was used instead of acetic the lead derivative was yellow, orange, red, brown, or black a t first, though it changed to black on standing. Since carbon disulfide was present, any mercaptide formed would be immediately converted to the trithiocarbonate (2,6)according to the general equation: RSM
+CS = RS
Since methylene mercaptan has two thiol groups, two molecules of carbon disulfide will be added and the formula for the lead salt would be:
HE(
.s.cs.s, )Pb s.cs.s
This lead salt was isolated in a high state of purity. It was obtained as a yellow precipitate which gradually turned brown, generally in the form of a fine powder, but in some cases as flaky crystals. The corresponding silver salt was obtained in the same fashion as a light yellow, flocculent precipitate which changed slowly to orange and then brown. The salts were prepared in alcoholic solution since the presence of water seemed to hasten the decomposition. The copper salt, prepared from an alcoholic solution of copper ammonium chloride or copper butyl phthalate, is light brown in color. The nickel salt, when precipitated from aqueous ammoniacal nickel nitrate or dilute alcoholic nickel butyl phthalate, is light brown. The cadmium salt from alcoholic cadmium bromide is white. The mercury salt from alcoholic mercuric chloride is cream-colored, but gradually turns yellow. The iron salt from 50 per cent alcoholic ferric butyl phthalate is dark brown, as is the cobalt salt obtained either from aqueous ammoniacal cobalt chloride or alcoholic cobalt butyl phthalate. A11 these salts darken upon standing and are insoluble. The sodium salt which is obtained by using caustic soda is yellow and water-soluble. I n every way all these salts correspond to other lower alkyl thiocarbonates, such as the ethyl thiocarbonates. Heavy-metal salts of higher alkyl thiocarbonates containing a larger alkyl group are soluble in certain organic solvents, such as benzene and carbon disulfide. This begins to be quite marked with butyl thiocarbonates, as has been observed during this work. The lead thiocarbonate mentioned above was also prepared in a different way. From the reaction product the sodium salt of the thiocarbonic acid was prepared by means of aqueous sodium hydroxide. Since this salt is water-soluble, it could be separated from the excess of carbon disulfide. To this solution concentrated hydrochloric acid was added, yielding the free thiocarbonic acid, HzC(S.CS.SH)z, in the form of a brown oil heavier than water. This is characteristic of the thiocarbonic acids. By interaction of the free acid with lead acetate the lead thiocarbonate was formed. I n some cases the free thio acid suffered partial decomposition, as is characteristic of these compounds; carbon disulfide was split off, yielding free mercaptan. The free mercaptan must be quite volatile, since in such cases a small evolution of gas was apparent. When conducted into alcoholic copper ammonium chloride solution, a small portion of an almost white, but with traces of dark brown, precipitate was formed. The whitish color is characteristic of copper mercaptides in the first stage of formation. However, i t is also possible that another reaction occurs simultaneously. If only one molecule of carbon disulfide were
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split off per molecule of acid, we would obtain the intermediate acid :
Certain samples of the sodium salt yielded small amounts of a gaseous product when acidified with 6 N hydrochloric acid. Moist lead acetate paper, when held in the gas, was covered with a light yellow precipitate; the edges of the paper H2C
