INDUSTRIAL AND ENGINEERING CHEMISTRY
882
Vol. 22, No. 8
Autoxidation of Lead Mercaptides'oz Emil Ott3 and E. Emmet Reid4 CHEMISTRY LABORATORY, JOHNSHOPKINS UNIVERSITY, BALTIMORE, MD.
T 1s generally assumed that the oxidation of lead mercaptides with oxygen Occurs according to the following equation:
I
2Pb(SR)2
+ Oa =
2(SR)*
\i
';
-
+ 2Pb0
M o r r e l l a n d Faragher (2) proved this reaction by shorning that some alkyl disulfide, (SR)z,isformed. However, as only a small proportion of a mercaptide is converted to the disulfide, they consider this reaction as slow and incidental. It is true that the oxidation occurs slowly, but our experiments show that the fact that only relatively small amounts of disulfides are formed must be explained otherwise; the oxidation is much more complex than is represented by the equation above. From lead mercaptides in solid form, as well as in benzene solution in presence of atmospheric oxygen, insoluble yellowish substances are obtained which strongly resemble in appearance the lead mercaptides, but which have peroxide qualities, as is shown by the formation of an intense blue spot when they are put on potassium iodidestarch paper, moistened with concentrated hydrochloric acid. Such peroxides were obtained for the lead mercaptides of all the mercaptans studied, the normal mercaptans from ethyl to nonyl and the secondary from propyl to nonyl. Thus farpo-Eroxide has b a a w e d f r o g LhcleacJderiya1 is only slightly soluble in benzene, wklZ that from eniyl mercaptan forms slowly. Generally pure compounds are not obtained, which is indicated by the non-integral atomic ratios of lead and sulfur. However, in one case, working with secondary butyl mercaptan an apparently uniform substance was isolated the lead and sulfur content of which correspond to the formula Pb(SR)z.08; Pb : S : 0 = 1 : 1.91 : 7.95. A possible constitution is:
tTvZ-ofSt,cL
0-0
\/
R+Pb--S-R
mation of these peroxides of which our experiments give abundant evidence. The formation of the peroxide goes on in the dark also, but apparently more slowly. If free mercaptan is present in the solution, the peroxide is formed very slowly and only in traces. In this case the formation of white lead hydroxide and sec-butyl disulfide represents the main reaction. In another case a comp o u n d corresponding v e r y closely to the formula Pbz(SR)04, where R = secondary propyl, was obtained, analysis giving Pb : S : 0 = 2.00 : 0.98 : 4.02. Accidentally an inhibited reaction was observed with a mercaptan (sec-butyl) solution which had been standing in contact with air for over 3 months. I n this case only traces of a whitish substance containing some of the peroxide were formed. When a new solution was prepared using the same mercaptan, which also had been in contact with air, a similar whitish product was obtained. The mercaptan was redistilled and after this the usual amount of yellowish peroxide was formed. It was not possible to determine the nature of the inhibitor involved. Secondary butyl disulfide, which was the compound suspected, did not cause any change in the rate of peroxide formation when added to the solution. Phenol, when added to the lead mercaptide solution, does not show inhibiting effect. The fact that lead mercaptides undergo an autoxidation is not so surprising if we coaqider that i E m Z c a p t i d e , Fe(SC?H&, is so easily oxidized by atmospheric oxygen that it begins to glow when exposed to air (1). If ethyl alcohol is present as an acceptor, aldehyde is formed, suggesting that the oxygen is bound in an active form to the iron mercaptide. Also, Szent-Gyorgyi (5) came to the conclusion that the SH-group of thioglycolic acid forms with molecular atmospheric oxygen a* very active peroxide. If the lead mercaptide peroxides are treated with neutral 3 per cent hydrogen peroxide solution, whitish, slightly yellowish substances are obtained; if, however, alkaline hydrogen peroxide is used brown substances are formed. When treated with concentrated nitric acid, these brown substances are transformed to white products, thus proving that they were not lead peroxide. The composition of these reaction products with hydrogen peroide is not known. Substances analogous to the peroxides may be obtained by direct action of hydrogen peroxide solution On lead merthe proper amounts are used, the ifpractically freed from lead mercap most of the sulfur is also removed. eroxides do not contain tetravalent potassium iodide-starch cid, but only with the with concentrated nitric easily be detected by its * this is in contrast to the behavior ,pounds. It is not likely that the
It is shown that the oxidation of lead mercaptides by atmospheric oxygen gives fairly stable peroxides of high but varying oxygen content. In the case of the sec-butyl derivative a peroxide, the lead and sulfur content of which approximated the composition Pb(SR)20s, was isolated. Such peroxides have been shown to exist for all members of the following complete series studied-normal, ethyl to nonyl; secondary, propyl to nonyl; and also for isobutyl. Some inhibited peroxide formations are discussed. Peroxides of similar appearance and reaction may be prepared by direct action of hydrogen peroxide solution on lead mercaptide solutions.
0-0
\/
/\
A
0-0
0-0
where R represents secondary butyl. I n this and in svlne other cases a satisfactory formulation of the peroxides seems to be possible; yet, in view of the complexity of the peroxide formation, we wish to disclaim adherence to any specific formulation. The important thing is the fact of the forPresented before the Division of Petroleum 1 Received April 21, 1930. Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ca.. April 7 to 11, 1930. 8 This paper contains results obtained in an investigation on a study of the Reactions of a Number of Selected Organic Sulfur Compounds listed as Project 28 of the American Petroleum Institute Research. Financial assistance 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 cobperation of the Central Petroleum Committee of the National Research Council. These results were communicated to the American Petroleum Institute on December 20, 1929. a American Petroleum Institute Research Fellow. 4 Director, Project 28.
August, 1930
INDUSTRIAL ALVDENGINEERING CHEMISTRY
oxygen is attached to the hydrocarbon radical, since similar compounds have been obtained which do not contain any carbon but possess peroxide qualities. Such products are obtained if elementary sulfur is added to solutions of basic lead mercaptide. They will be discussed more in detail in a subsequent paper on sulfur addition products of lead mercaptides (4). It seems, therefore, fair to assume that the oxygen is attached to the sulfur, which leads to the constitution given above. These peroxides have additional importance, since the study of their formation and properties may explain some of the phenomena encountered in the investigation of the physiological oxidations as catalyzed by organic sulfur compounds. They have been found to aid considerably in the oxidation of ethyl benzene by air. A series of lead mercaptides,6 which originally corresponded to the formulation Pb(SR)z and which had been left standing in a desiccator in contact with air for over a year, were examined. The ethyl, n-propyl, sec-propyl, n-butyl, and isobutyl mercap tides gave very strong peroxide reaction. A sample of namyl lead mercaptide showed no peroxide reaction and was, in accordance with this fact, soluble in benzene (all the peroxides are insoluble in organic solvents). It was suspected that an inhibitor was present. Actually this mercaptide did form a peroxide after it had been allowed to stand for a day in benzene solution. EXPERIMENTAL
The mercaptans, benzene-mercaptan solutions, and doctor solutions are identical with those described in a previous paper (3). Oxidation of Lead Mercaptides Solutions by Atmospheric oxygen
883
Another sample of the peroxide prepared in a similar way had the unusually high lead content of 74.5 per cent, which may have been due to its formation from the basic lead mercaptide. Treatment of this with hydrogen peroxide lightened its color and increased its weight by 13.8 per cent. The peroxide reaction of this was slight when it was moistened with water but strong when hydrochloric acid was used. A peroxide formed in the dark had a lead content of 69.8 per cent. Other samples of sec-butyl lead mercaptide peroxide contained 72.2 and 71.1 per cent lead. The latter had 7.8 per cent sulfur, giving a ratio P b : S = 1 : 0.71. The product obtained in the inhibited reaction mentioned gave a lead analysis of 72.4 per cent. The peroxide was prepared from lead n-butyl mercaptide. The insoluble product was of deep yellow ocher color just as for the secondary compound, but analysis gave 74.3 per cent Pb and 7.1 per cent S; Pb : S = 1 : 0.62. The following results were obtained by shaking a solution of 0.4 gram sec-propyl mercaptan in 200 cc. of benzene with 200 cc. of doctor solution e. The light yellow precipitate formed a t first was filtered off ( 3 ) and the clear filtrate was left standing in contact with air for 1 day. A yellow precipitate was formed which possessed strong peroxide qualities. It was analyzed and the composition calculated as for previous compounds. The presence of propyl radical was indicated by charring when heated with concentrated sulfuric acid. Calculated for Pbz.SC3H7.04: Pb, 74.87; S, 5.79; 0, 11.56. Found: Pb, 74.94; S, 5.74; 0, 11.61 (assuming that the lead still carries the propyl groups), Pb : S : 0 = 2 : 0.98 :4.02. After removing the above peroxide, the filtrate was left standing in the dark for 3l/2 days. Again a precipitate was formed which was very pale yellow, nearly white, and had peroxide properties. Although the lead-sulfur ratio was identified with the one found above, the oxygen content was lower: P b : S : 0 = 2.00 : 0.98 : 3.44, the analysis being Pb, 76.33, and S, 5.78 per cent. In a new set of experiments carried out in the same way a peroxide was obtained which was of light yellow and behaved like the above substance. The composition, however, was different, being nearly Pb3(SR)205. This may be written as Pb0.Pbz(SC3H7)z04. Calculated: Pb, 72.97; S, 7.53; 0,9.39. Found: Pb, 73.21; S, 7.33; and 0,9.62; P b : S : 0
If a clear solution of secondary butyl lead m e r c a p t i d e prepared, for instance, with doctor solution c as described in another paper (3)-is left standing in contact with air, no reaction is apparent for several hours, but usually after about 12 hours cloudiness occurs, leading in a day or two to the formation of a yellow-orange fluffy precipitate. If the solution is kept free from air, no such precipitation occurs even after weeks, but instead lead sulfide is formed after long standing. A 55-cc. portion of a benzene solution containing sec- = 3.00 : 1.75 : 5.10. butyl mercaptan was shaken with 50-cc. of doctor solution Oxidation by Atmospheric Oxygen of Neutral Lead Merc for 30 minutes. Then 50 cc. of the benzene layer were captide Solution in Presence of Free Mercaptan carefully separated and left standing in a 300-cc. bottle. Doctor solution c was shaken with about its own volume After 2 days the yellow-orange precipitate was filtered off of a benzene solution of secondary butyl mercaptan. Exand washed with benzene. An analysis for lead and sulfur, according to the procedure described elsewhere (S), gave: periments in connection with a previous paper ( 3 ) showed P b 40.4 and S 11.9 per cent; Pb': S, 1 : 1.91. It is of interest that under such conditions only the neutral lead mercapto note that, although the original lead mercaptide solution tide, Pb(SR)Z, would be formed, while some mercaptan SR would remain as such in the benzene solution. The benzene layer was separated in the usual way and left standing contains the two types of molecules, Pb(SR)n and Pb< OH . in contact with air. A precipitate was formed surprisingly in the approximate molar proportion 4 : 7 (according to slowly and its color was whitish yellow. After 1Fj days previous experiments), the lead-sulfur ratio points to Pb (SR)2 the very small amount of it was washed with benzene and as the one that forms the insoluble peroxide. Since the lead left in a desiccator, containing calcium chloride, overnight. analysis is more reliable than that of sulfur, the former is The product had strong peroxide qualities and was suspected used as basis for the further calculations. Assuming a com- to be a mixture of the yellow peroxide and a white substance pound of the type Pb(SR)20,, the oxygen is calculated by (lead hydroxide). The lead content was 77.3 per cent. difference as 24.8 per cent z = 7.95. Qualitatively the S o carbonate was present, since when diluted hydrochloric presence of the hydrocarbon radical is shown by the fact acid was added under the microscope no gas bubbles formed that the substance chars when heated with concentrated at all. The benzene layer now contained no free mercapsulfuric acid. Calculated for Pb(SCJI&.O*; Pb, 40.4; tan, which therefore had been oxidized to sec-butyl disulfide. S, 12.5 per cent; 0, 24.9 per cent. Found: Pb, 40.4; S, 11.9; and 0, 24.8 per cent. Pb : S : 0 = 1 : 1.91 : 7.98. Oxidation of Lead Mercaptides by Hydrogen Peroxide A lead mercaptide solution was prepared by shaking 50 Prepared by P. Borgstrom and I ,. M. Ellis, Jr., in this laboratory as part of this croject. cc. of doctor solution c with 55 cc. of benzene solution of ~~
8
INDUSTRIAL AND ENGINEERING CHEMISTRY
884
sec-butyl mercaptan. A 50-cc. portion of the carefully separated benzene layer was shaken with 50 cc. of 3 per cent hydrogen peroxide solution, which was added in small quantities a t a time. The insoluble product is a t first brownorange, then paler and, when dry, of a yellow-orange color, which is common for the peroxides. The analysis (constant weight) gave 70.5 per cent Pb and 5.0 per cent S; Pb : S = 1 :0.45. Thus the substance is not chemically uniform. With this procedure the benzene, after all the insoluble products have been filtered off, remains slightly colored. If, however, only half the above volume of hydrogen peroxide solution is used, the benzene layer is obtained colorless. When some of the yellow oxidation product is evaporated to dryness to destroy all hydrogen peroxide, it gives the per-
Vol. 22, No. 8
oxide reaction on potassium iodide-starch paper if moistened with concentrated hydrochloric acid, but not with water. I n this it resembles the peroxides formed by oxidation with atmospheric oxygen though its composition is different, If hydrogen peroxide is added in sufficient excess, the formerly yellow substance is transformed into a white substance, which is but little soluble in water. This may be basic lead sulfonate. Literature Cited (1) (2) (3) (4) (5)
Manchot and Gall, Ber., 60, 2318 (1927). Morrell and Faragher, IND. ENG. CHEM.,19, 1045 (1927). Ott and Reid, I b i d . , 22, 878 (1930). Ott and Reid, I b i d . , 2'd, 884 (1930). Szent-Gyorgyi, Biochem. Z.,146, 245, 254 (1924).
Reactions of Lead Mercaptides with Sulfur' Emil Ott2 and E. Emmet Reid3 CHEMISTRY LABORATORY, J O H N S HOPKINSUNIVBRSITY, BALTIYORE, MD.
F TO a solution containing lead mercaptide elementary sulfur is added, a well-known change takes placeleading to the formation of a dark-colored insoluble product, assumed to be lead s u l f i d e . T h e r e a c t i o n is commonly written Pb(SR)Z S =
I
+
PbS
+ (SR)2
The reaction of lead mercaptides with sulfur in Actually it has been possisolution has been studied. It was found that the ble to obtain c o m p o u n d s reaction is not simply formation of lead sulfide, as corresponding to RS.Pb.8.generally assumed, but is more complex, leading to Pb.SR in the case of secondcompounds of the type PbS, P ~ L S ( S R )P Z~, Z ( O H ) Z S ~ary ~ and normal butyl merPbz(OH)& as well as mixtures of these and others. captans. These products are It is shown that all the experimental data are most brown when dry and correreadily understood if the formation of unstable polyspond to the well-known sulfides is considered as a first step followed by dePb2SC12,which may be obcomposition of these intermediates. tained from lead c h l o r i d e
(1)
showing the formation of lead sulfide and alkyl disulfide (1). The present investigation shows that the reactions involved are by no means so simple as here represented. It has been possible to isolate intermediates indicating a more complex reaction. These decompose rapidly and it is not always possible to duplicate results. I n some cases analyses have indicated definite compounds, but in most cases they point to mixtures. Emphasis is put on the existence of such compounds rather than on any particular formula. It appears reasonable to compare the lead mercaptides with other hydrogen suliide derivatives, and iherefore to assume that the reaction with sulfur may lead to intermediate formation of unstable polysulfides. The following reactions may be imagined : Pb/SR S 'R
+
P b < F +
S-SR S +Pb+P
