878 0
INDUSTRIAL A N D ENGINEERING CHEMISTRY
a negligible rate in air at ordinary temperature will assume enormous proportions with high pressure and temperature. This is certainly the case when the rubber approaches the kindling temperature. High oxygen pressure even a t lower temperatures can have an important effect on the oxidation of various materials which may have been added to the rubber. It is known that many organic materials, especially some accelerators and antioxidants, are easily oxidized. Since these oxidation reactions are of the first or higher order, the rate is greatly increased by increase in oxygen pressure, while the rate of oxidation of the rubber remains almost unchanged. Under such conditions an antioxidant which is quite effective for natural aging would be rapidly destroyed and appear worthless. Conclusions
IT,is apparent that an artificial aging test must consider the concentration of oxygen existing in the rubber. The concentration of oxygen in rubber suspended in air a t 70" C. will be less than that required for normal oxidation at the maximum rate. This should limit the use of an air atmosphere to the highest quality of rubber in which the rate of oxidation is extremely small in comparison with the rate of diffusion of oxygen, and in this case tests should be confined to thin slabs.
Vol. 22, No. 8
The solubility of oxygen in rubber suspended in oxygen at 70' C. and atmospheric pressure appears to be slightly greater than that required for uniform oxidation. Under these conditions thin slabs of good or medium-quality rubber should oxidize in a satisfactory manner. The fact that strips 4 mm. square of a rubber extremely sensitive to oxidation, when tested in an oxygen atmosphere a t 26" C., produce deviation from a straight line would make doubtful the accuracy of tests conducted on easily oxidized rubber under these conditions. This type of rubber would require 2 or 8 atmospheres pressure in order to maintain the required oxygen concentration. The type of deterioration appearing a t higher temperatures should have little influence unless high oxygen pressure is used. The use of both high temperature and high pressure may emphasize types of oxidation which are suppressed a t ordinary temperature. Since little is known about this type of oxidation, it is believed that the simultaneous use of high temperature and high pressure should be avoided. Literature Cited (1) Bierer and Davis, IND. END.CHEM.,16, 711 (1924). (2) Rohman, J . Phrs. Chem., 83, 226 (1929). (3) Marzetti, Rubber Age, 18, 433 (1923). (4) Morris and Street, IND. END.CHEW.,21, 1216 (1929). (5) Ostwald, J . SOC.Chem. I n d . , 82, 179 (1913). (6) Venable and Fuwa, J. IND. END. CHEM., 14, 139 (1922).
Reactions of Some Mercaptans with Alkaline Sodium Plumbite Solutions''z Emil Ott3 and E. Emmet Reid' CHEMISTRY LABORATORY, JOHNS
HOPKINSUNIVERSITY,
BALTIMORE,
MD.
It is shown that during the reaction of mercaptan normal series from ethyl to nonyl mercaptan. Marked solution with alkaline sodium plumbite (doctor) solu- differences were found between corresponding members of tion both neutral and basic lead mercaptides, Pb(SR)* the secondary and normal series. The lead mercaptides studied behave like organic compounds and are mostly soluble in organic solvents. /SR Both the neutral and the basic lead derivatives of nand P b , are formed. octyl mercaptan were isolated. The latter are more solu'OH ble and of deeper color than the former. The basic Quantitative measurements were made with normal derivatives from isopropyl and sec-butyl mercaptans butyl, hexyl, and octyl and secondary propyl, butyl, and were obtained in nearly pure form. The decomposition of lead mercaptides accompanied octyl mercaptans, and qualitative measurements with the complete secondary series from propyl to nonyl and the with formation of lead sulfide is discussed.
. . . .. . HE doctor solution (sodium plumbite in sodium hydroxide solution) has long been used in testing the removal of mercaptans and has been extensively investigated (1, 2, 3, 4 , 6, 9, 10, l l ) , but the studies that have been made have been limited to a few mercaptans and to certain concentrations of doctor solution. It seemed de-
T
1 Received April 21. 1930. Presented before the Division of Petroleum Chemistry a t the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 t o 11, 1930. 2 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 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 coaperation of the Central Petroleum Committee of the National Research Council. These results were communicated t o the American Petroleum Institute on December 20, 1929. American Petroleum Institute Research Fellow. 4 Director, Project 28.
sirable to obtain a wider knowledge of the possible reactions using a greater number of mercaptans under different conditions. When a mercaptan, dissolved in benzene, naphtha, or a similar organic solvent is shaken for a short time with the doctor solution, the organic solvent layer takes on a greenish yellow color. I n many cases, depending upon the nature and concentration of the mercaptan, this solution is clear a t first, but upon standing for several hours or days it deposits an insoluble yellow substance, which might be taken for the lead mercaptide. This suggests an irreversible coagulation of a colloidal solution of insoluble lead mercaptide. Morrell and Faragher (6) speak of lead mercaptide dispersions in naphthas. For mbutyl lead mercaptide they give one instance where the product settled out completely from the solution upon standing and another where it stayed in solution. However, in their next paper (4) they state that
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
August, 1930
lead mercaptide dissolves readily in benzene. From our results there can be no doubt that the latter statement is the correct one. All the lead mercaptides examined give true solutions in benzene and other organic solvents; the above mentioned insoluble substances represent products of subsequent reactions. The reactions between the mercaptan and the sodium plumbite (considered as Pb(OH)? and NaOH) may be of the following types: Pb(0H)z
+ HSR = Pb/"" + HzO
\OH Pb(0H)z + 2HSR = Pb(SR)*
2"
2Pb
=
\
+ 2H20
(SR)Pb-C--Pb(SR)
+ HzO
(1)
(2) (3)
OH
Pb
\
+ NaOH = Pb7" + HzO
OH NaOH HSR
+
\
=
ONa NaSR HzO
+
(4)
(5)
Of these (1) and (2) are the chief reactions, while ( 5 ) is usually unimportant though it may take place to an appreciable extent, particularly with secondary mercaptans. It is generally assumed that only neutral lead mercaptides are formed and, as far as we are aware, the basic lead mercaptides (reaction 1) have never been considered. Our results leave no doubt as to the existence of such compounds or of the importance of their role in the reactions of mercap tans with the doctor solution. Faragher, Morrell, and Monroe (4) used a suspension of basic lead acetate for the formation of the lead mercaptides in their titration method for mercaptans. They say: The use of aqueous sodium plumbite solution instead of basic lead acetate for the formation of the mercaptides gave high results when the relative proportion of sodium plumbite solution to the sample was increased beyond a definite value.
As shown by our results, a more concentrated doctor solution produces more of the basic lead mercaptide, HO.Pb.SR, which, when calculated according to their method in which the lead is assumed to be present as Pb(SR)z, would give high results. Our results explain their observations. Their low results when lead acetate solution is used in place of the basic lead acetate suspension are explained by the fact that the formation of the lead mercaptides is incomplete in presence of dilute acids. GENERAL DISCUSSION
I n the present investigation two series of mercap tans have been used, both derived from the normal hydrocarbons ethane to nonane, the S H group being on the end carbon atom in the primary series and on the second carbon in the secondary series. The secondary mercaptans h a w been found to behave quite captans corresponded to 0.0 the range of concentrations products. Five doctor sol
layers separated. The compl
879
aliquot of the benzene solution was taken out and evaporated. The residue was analyzed for lead, the sulfur content being known from the original composition of the mercaptan solution, since experiments have shown that all of the mercap tan reacts with the doctor solution provided sufficient lead is present. The results obtained usually indicated mixtures of lead mercaptide and basic lead mercaptide, which are accounted for by reactions 1 and 2 above. The proportions of these are different in the two classes of mercaptans and, in fact, change within a series. I n some cases i t is possible to obtain practically pure basic or neutral lead mercaptides, as the analyses show. Unfortunately these compounds cannot be recrystallized and obtained in a state of purity. Lead can remain in the hydrocarbon layer only as a sulfur compound; the Pb:S ratio shows whether i t is present as neutral or basic mercaptide. The presence of any free mercaptan in the residue is readily detected by the odor. Only in cases in which doctor solution b, which contained little lead oxide, was used was such odor apparent. A part of the mercaptan reacts with the caustic soda according to (5) and thus passes into the water layer. The amount of this was determined in typical cases by separating, acidifying, and extracting the water layer and determining the mercaptan in the extract. When the volume of a certain doctor solution was 1.1 of that of the benzene solution, 1.39 per cent of the wbutyl mercaptan was found in the water layer. Under the same conditions 18.9 per cent isopropyl, 4.61 per cent see-butyl, and 1.4 per cent see-amyl passed into the water layer. For the higher mercaptans the amount is insignificant. Of the two mercap tans of the same carbon content a larger amount of the secondary than of the primary goes into the water layer. This is in agreement with preliminary results on partition coe6cients which are now being determined in this laboratory. By evaporating the benzene layer, after shaking with the doctor solution, the mixture of the neutral and basic lead mercaptides is obtained as a yellowish mass, solid, semisolid, or oily according to the mercaptan used. The product from n-butyl mercaptan appears partly amorphous, but b e comes entirely crystalline on standing, while that from the n-hexyl is always crystalline. The neutral lead woctyl mercaptide is only slightly soluble in cold benzene and separates as a pale yellow precipitate. It is readily obtained as a crystalline powder, quite soluble in hot benzene. It dissolves in the common organic solvents to give highly colored solutions. When purified and dry it is quite stable, but when moist or in solution it readily turns dark under the influence of light. The basic lead n-octyl mercaptide is produced when a doctor solution containing an excess of lead hydroxide in suspension is shaken with a benzene solution of n-octyl mercaptan. The benzene layer is separated, filtered, and let. stand overnight in the ice box. The compound comes down as a yellow-orange powder (deeper colored than the corresponding neutral mercaptide), which is filtered off, washed with alcohol, benzene, and ether, and let stand in a vacuum desiccator. It is remarkable that this basic mercaptide is much more soluble in benzene than the corresponding neutral mercaptide. The lead Palts of the secondary mercaptans are more soluble in benzene than those of the primary mercaptans and are usually obtained as gums which sometimes become crystalline, as observed in the case of see-butyl, where distinct spherulite formation may occur. I n the primary series the solubility of the lead compounds decreases with increasing molecular weight, while in the secondary series the reverse seems to be true. The least soluble of this series is the basic derivative of isopropyl mercaptan. This separates in fairly pure form as a pale yellow crystalline powder; the neutral,
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
880
~
compound, which is darker, is obtained hy evaporating the mother liquor. I n the secondary series the basic mercap tide is less soluble than the corresponding neutral compound, while in the normal series the reverse is true. The decomposition of lead mercaptides into lead sulfides and alkyl sulfide has long been known (,5, 7, 8) and has been frequently observed in the present investigation. The lead mercaptides, both neutral and basic, darken, the first one particularly, in the light, especially when in solution or in a moist condition. When they are kept in a desiccator, the decomposition is slow and starts from distinct centers. The basic n-octyl lead mercaptide is more stable in solution than the corresponding neutral compound, but when spread on filter paper exposed to the air it darkens both in the dark and in the light. The presence of free mercaptan appears to accelerate the decomposition of the lead mercaptides, more so with those of the normal series than with the secondary. The solutions of lead mercagtides obtaineAtg. the use of doctor solution b and which contain unreacted mercaptan, decompoz’-mpm& particularly on slight heating. This decomposition, when once started, continues even when the substance is freed from solvent. The solid lead mercaptides obtained from these solutions continue to decompose in a vacuum desiccator. As the free mercaptan itself does not decompose, and since
SR /
solutions containing mainly Pb(SR)z and Pb
and alkyl sulfides, the decomposition of a basic lead mercaptide possibly gives an alcohol as the other product.
/SR Pb
/””
Pb
EXPERIMENTAL
Preparation and Characteristics of Doctor Solutions
The doctor solution is usually prepared by dissolving lead oxide in caustic soda solution. It has been found more convenient to use the more quickly soluble lead hydroxide in place of lead oxide. This was prepared by adding excess of ammonia to a solution of lead nitrate and washing the precipitate by repeated decantation. After preparation the solutions were left standing for a few days, the clear liquid siphoned off and analyzed for lead and caustic soda. Table I-Characteristics DOCTOR
SOLUTION
Pb(0Hh Mols Oer liter
0.0262 0.0155 0.0464 0.0651 0.1374
’a b C
d
e
of Doctor Solutions NaOH
Mols be7 liter 0.53 1.11 1.05 2.12 3.74
RATIO NaOH :P b (OH);
20.2 71.6 22.6 32.5 27.2
Solvents and Mercaptans Used
The mercaptans were weighed in sealed glass containers which were broken in a known volume of chemically pure benzene. The mercaptanss used were better than 99 per cent pure except the sec-butyl, which contained only 93 per cent of the mercaptan, the rest being the corresponding alcohol and bromide. The work with this mercaptan was done before the final purification of the material. As the results obtained with this are consistent with the rest of the series, they were not repeated. Determination of Sulfur
\OH
This hydrolysis has already heen observed by Klason (.’). We found it nicely demonstrated in the following way: To the yellow Pb(SCaH!7)2, free from mercaptan odor, water was added. A white product, naturally lead hydroxide, was obtained and a t the same time mercaptan odor could be observed. Since water is more volatile than n-octyl mercaptan, upon warming the reaction should reverse. This was actually the case and the yellow lead mercaptide was formed again. The process was repeated, with the same results. The activation of the darkening due to hydrolysis was more carefully studied in the case of sec-butyl m A lead mercaptide solution (free of mercaptan) was in contact with a lead acetate and a sodium acet tion and also with water. All three solutions turnec after one day, whereas the original solution kept the yc color. Since one of the hydrolysis products is lead hyc , v . i \ i r it was thought that hydrolysis and the successive blac > c might be checked by using, instead of water, a sat aqueous solution of lead hydroxide. In this case a no blackening occurred, proving the assumptions. As the neutral lead mercaptides bro-’: 1’1: b + n lead
+ ROH
Generally the decomposition to lead sulfide was not studied quantitatively, but in one case, where the solution contained both neutral and basic lead n-butyl mercaptides, 29 per cent of lead sulfide was found in the precipitate that separated. The lead sulfide which separates carries down lead mercap tides either by adsorption or by the formation of complexes.
OH
+ 2 H - 2 0 ~Pb (0H)z + 2HSR + H20 Pb(OH)2 + HSR
PbS
OH
but no free
Pb(SR)z
--f
\
\
mercaptan (solutions prepared with doctor solution a have these characteristics) do not decompose to any similar extent& seems that !he free mercaptan has a catalytic effert. This ma3eXG”To the formation of A loose double compound. If to a mercaptide solution which is stable and free from excess mercaptan some of the latter is added, decomposition will follow, showing that free mercaptan catalyzes the forming of lead sulfide. For t h e - s a m e z y o n hydrolysis will favor the decomposjtion sert_cction.
Vol. 22, No. 8
The sulfur was determined with the Parr bomb, the metallic hydroxides formed from the bomb material being filtered off after boiling the alkaline aqueous solution to obtain barium sulfate free from occluded matter. The small amounts of lead present in the solution do not cause any precipitation of lead sulfate when the solution is acidified. To show that no appreciable amount of lead salt was occluded in the barium sulfate, a series of analyses was made removing the lead in some and not in the others. The results of both series were practically the same.
.” c ~ La
5.
Preparation of Lead Mercaptide8
0
:\, rt‘,>?
,he mercaptan solution was shaken
,sated by warming in a current of
.
L
1
I
/
L
-,
prepared and purified by L M Ellis, Jr., for Pm-+a* Petroleum Institute Project No 28.
INDUSTRIAL AND ElVGINEERING CHEMISTRY
August, 1930
air. Strong heating leads to decomposition accompanied by darkening, owing to formation of lead sulfide; if no heat a t all is applied, water condenses and may cause hydrolysis. In the case of sec-butyl mercaptan double volumes were used and the shaking continued 30 minutes. When dry, the beaker is put in a vacuum desiccator for 15 to 30 minutes, weighed, returned t o the desiccator, and re-weighed until a constant weight is obtained. There was usually little change in weight after the first weighing. The lead mercaptides were sometimes redissolved to study the solubility and recovered by reconcentrating the solution. Finally, the substance was decomposed with concentrated sulfuric acid, the organic matter being destroyed by heating with more sulfuric acid, then heated to dryness and brought to constant weight in a desiccator over calcium chloride. The substance was then transferred to a Gooch filter and washed carefully with a little water and 50 per cent ethyl alcohol to remove a small amount of sodium sulfate. The Gooch crucible was ignited to constant weight, which gave the lead sulfate content of the sample, the small amount of sodium sulfate being found by difference. As the small values obtained for the sodium content were distributed a t random, it was assumed that these small amounts of sodium came from traces of the doctor solution suspended in the benzene layer. Hence an amount of lead corresppnding to the sodium was subtracted from the lead found in each case to give the corrected weight of lead. Table 11-Neutral and Basic Lead Mercaptide8 Formed b y Shaking 50 cc. of Benzene S o l u t i o n of Mercaptan w i t h 25 CC. of Doctor S o l u t i o n DOCTOR SOW. HSR PbO Milli- Millimols mols Q
c
d Q
b c d e
a c
d e a
c
d e
0.928 0 . 6 6 0.928 1 . 1 6 0.928 1 . 6 3
Pb RATIO Pb/SR FOUND S : P b Pb(SR)z \OH MilliMilliMillimols mols mols %-ButylMercaptan 0.481 1 . 9 3 0.447 0.034 0.661 1.40 0.267 0.394 0.643 1.44 0.285 0.358
0.816 0.816 0.816 0.816 0.816
0.66 0.39 1.16 1.63 3.44
sec-Butyl Mercaptan 0.527 1.55 0.289 0.302 2 . 7 0 0.302 0.606 1 . 3 5 0.210 0.578 1.41 0.237 0.750 1.09 0.066
0.909 0.909 0.909 0.909
0.66 1.16 1.63 3.44
1.008 1.008 1.008 1.008
0.66
1.16 1.63 3.44
WEIGHT Calcd. Found
Gram
Gram
0.1847 0.18FO 0.2277 0.2247 0.2230 0.2195
0.238 0 0.397 0.341 0.686
0.1874 0.1837 0.1216 0.1228 0,2064 0,2046 0.2008 0,2017 0.2408 0.2366
n-Hexyl Mercaptan 0.497 1 . 8 3 0.412 0.668 1.36 0.241 0.679 1.34 0.230 0.755 1.20 0.154
0.085 0.427 0.449
0.2126 0.2538 0.2563 0.2773
0.2170 0.2562 0.2574 0,2792
sec-Octyl Mercaptan 0.541 1 . 8 6 0.467 0.728 1.38 0.280 0.754 1.34 0.254 0.962 1 . 0 5 0.046
.0.074 0 448
0.2624 0.3088 0.3137 0.3856
0.21382 0.3156 0,3178 0,3669
0.601
0.500
0.916
The results are given in Table 11. The last column gives the weight of the residue obtained by evaporating down the 50-cc. portion of the benzene solution. The lead in this was determined and its weight corrected as described above for any sodium present. The lead so found is given in column 4. As all the mercaptan reacts when a sufficient excess of lead is present, the figures in columns 6 and 7 for the neutral and basic mercaptides are calculated from the lead in column 4 and the mercaptan originally present as shown in column 2. I n the case of sec-butyl double volumes of the solutions were actually used hiit the figures obtained were halved to make them comi The third column gives the lead 0x1 of doctor solution used. The eight], 'calculated from the data in the r into account any sodium found. It is evident from the figures p ~ : ? ~ . ' I C I ~ L e prod:I * baptides ucts are mixtures. The nearest to ; . I t YL-! Li
881
are those obtained from the two primary mercaptans with doctor solution a, while those from the secondary mercaptans with the strong doctor solution e are largely the basic compounds. These results showed the way to the preparation of basic lead mercaptides in nearly pure condition. 'I
Preparation of a Neutral Lead Mercaptide
*he results given in Table I1 point to the use of doctor solution a. This was shaken with n benzene solution of n-octyl mercaptan. A yellow precipitate soon formed and increased when the benzene layer was strongly cooled. This was filtered off, washed with 95 per cent alcohol, twice with 50 per cent alcohol, and with ether, and dried in a vacuum desiccator. These operations were carried out as quickly as possible and with minimum exposure to light. Analysis gave Pb 41.55 and S 12.95; calcd. for Pb(SCSH1,)2,Pb 41.64 and S 12.89 per cent. Another preparation contained 12.70 per cent sulfur. Preparation of a Basic Lead Mercaptide
A benzene solution of n-octyl mercaptan was shaken with doctor solution e. The product, which is much more soluble than the above neutral mercaptide, was obtained by evaporation of the benzene. It contained 52.77 per cent lead instead of the calculated 56.09 per cent, indicating the presence of considerable of the neutral mercaptide. The experiment was repeated with the modification that excess of lead hydroxide was added to the doctor solution. The benzene layer was separated, filtered, and concentrated. The product thus obtained contained 55.93 per cent of lead. Another preparation gave 56.41 per cent of lead and 8.53 per cent of sulfur, Pb : 8 = 1 . 0.98; while the calculated values for PbSCsH1,OH are Pb 56.09 and S 8.68. A number of attempts were made to obtain the basic niercaptide from isopropyl mercaptan in pure form. Of all the basic mercaptides of this group, this one is the least soluble in benzene. It separates a t once when a solution of 0.410 gram of isopropyl mercaptan in 200 cc. of benzene is shaken with 100 cc. of doctor solution e. It was filtered off, washed with benzene, 95 per cent alcohol, and 50 per cent alcohol, and ether. The product is not of constant composition, but approximates the basic mercaptide. Two preparations gave 71.35 and 71.12 per cent lead and 9.76 and 10.25 per cent of sulfur, while the calculated percentages are 69.22 and 10.71. That the hydrocarbon radical was present was shown by the fact that the material charred when heated with concentrated sulfuric acid. The basic lead mercaptide from sec-butyl mercaptan was obtained by shaking a benzene solution of the mercaptan with doctor solution e containing suspended lead hydroxide. The benzene layer was filtered and evaporated. The mercaptide was left as an amber-colored gummy mass containing 65.23 per cent of lead instead of the calculated 66.13 per cent. Literature Cited Brooks, IND. ENG.CHEX.,16, 58 (1924). Dow, Bur. Mines, R p f s . Investigations 2191 (1920); Re.finer Yalural Gasoline M f r . , 2, No. 5 , 12 (1923); Nafl. Pefroleum Y e i e s , 15, iYo. 20, 99 (1923). Egloff and Morrell, Refiner Natural Gasoline M f r . , 2, No. 7 , 5 (1923). Faragher, Morrell, and Monroe, IND.ENG.C H E Y . ,19, 1281 (1927). Klason, Ber., 20, 3410 (1887). Morrell and Faragher, IND. ENG. CHEX.,19, 1045 (1927). Otto, Ber., 13, 1290 (1880). I Stenhouse, A n n . , 149, 260 (1869). (9) Wendt and Diggs, IXD. E N G .CHEX.,16, 1114 (1924). Wood, Lowy, and Faragher, I b i d . , 16, 1116 (1924). (11) Wood, Sheely, and Trusty, I b i d . , 16, 169 (1926).
