Separation of Primary and Secondary Thiols from Tertiary Thiols in Liquid Ammonia R. L. HOPKINS and H. M. SMITH Petroleum Experiment Station, Bureau o f Mines, Bartlesville,
A method is described for separating a thiol extract in the C S - C ~range ~ into two groups, one containing the primary and secondary alkane- and cycloalkanethiols; the other containing the tertiary alkanethiols. Compounds of the first group form insoluble ammonium mercaptides in liquid ammonia, whereas those of the second group do not. Separation efficiencies ranged from 60 to 98%, depending upon molecular weight and structure of the carbon chain. 5-Ethyl-2-nonanethiol could not be separated under the general conditions chosen for the method.
T
HE problems of identifying the sulfur compounds present in petroleum distillates become more complex as the boiling point of the distillate increases. BelolT 111" C. the sulfur compounds can be identified and quantitatively estimated after concentration by adsorption and distillation ( 2 ) . Above this temperature additional methods of separation, chemical and physical, must be used t o simplify the character of the material sufficiently for i t to be successfully identified. A method for extracting thiols from a distillate using sodium arninoethoxide in ethylenediamine has been reported (1). When a distillate boiling between 111" and 150" C. was treated by this method and the thiol extract fractionally distilled, a series of fractions \vas obtained in n hich seven thiols \Yere identified and two tentatively identified. Additional simplification of such mixtures is desirable and can be realized by using the method described in this paper. This should result in a more rapid and accurate characterization, particularly in the higher boiling ranges. This paper dexribes a method of separating primary and secondary alkane- and cycloalkanethiols from tertiary alkanethiols in the C6 through CIo range, with limited application through C12. The separation is based on the reaction of the primary and secondary thiols with liquid ammonia to form sparingly soluble ammonium mercaptides (S), whereas the tertiary thiols either react incompletely or form noncrystalline soluble mercaptides. There is evidence t h a t low reactivity, high solubility, and the noncrystalline nature of tertiary ammonium mercaptides are all involved in the failure of the tertiary thiols to yield a precipitate. DEVELOPMENT OF METHOD
Okla.
thiols form readily soluhle mercaptides, and phenyl alkanethiols form mercaptides whope solubility depends on the proximity of the ring to the mercaptide group. Thus phenylmethanethiol and 2phenylethanethiol form soluble mercaptides, whereas 3-phenyl-lpropanethiol forms a mercaptide which readily crystallizes from solution. Cyclohexanethiol is very reactive with liquid ammonia and the mercaptide is very sparingly soluble. It was found that the first two members of the tertiary thiol serieR give a t least partial yields of mercaptide; the method is therefore not applicable below c6 or 100" C., the boiling point of 2-methyl-2-butanethiol. Higher tertiary thiols seldom produce any crystalline mercaptide and if so the amount is very small. N o gem-alkyl cycloalkanethiols were available for testing, so their behavior cannot be predicted. The use of a mutual solvent is necessary in this procedure because of the low solubility of thiols in liquid ammonia Approximately 80 solvents have been screened, but only a few have any practical value. Methylal and tetrahydrofuran are satisfactory for the thiols of low molecular weight but lack enough solvent power for the C , and C12 thiols. Several C,, Cq, and Cj alcohols show fair solvent power but exhibit an inverse temperature-solubility relationship in that their solvent pon er in liquid ammonia increases with decrease in temperature. Crystallization is more difficult under these conditions, and especially difficult crystallizations may fail completely. The glycol ethers-n-heuyl Cellosolve, n-hexyl Carbitol, and 2-ethylbutyl Crllosolve-have the highest solvent po1ver of all compounds tested. They also show the inverse temperaturesolubility relationship which is unfavorable for crystal formation from some thiols. Their high boiling point and lo^ solubility in water are also disadvantageous because of the difficulty of recovering the portion of sample remaining in the filtrate. Dimethyl ether has hcen selected as the mutual solvent for this method because it has good solvent pori-er and lory boiling point and generally allows good crystal formation. DESCRIPTIOY OF 3IETHOD
The sample is dissolved in 50 volumes of dimethyl ether and added s l o ~ . l ya i t h stirring to 100 volumes of liquid ammonia (50 volumes for CI2). The mixture of sample, dimethyl ether, and ammonia is allowed to stand in a dry ice bath for 30 minutes and filtered on a precooled, vacuum-jacketed filter. Before the filtration is begun, some sodium aminoethoxide i n excess of that required for neutralization of the tertiary thiol is prepared in the flask which is t o be used as a receiver by dissolving sodium in liquid ammonia and adding monoethanolamiTe until t h e blue color is discharged. The filtrate is collected a n J the solvents are evaporated. Khen evaporation is complete
Various thiols 1% ere tested qualitatively to determine the molecular types and molecular weight range which yield precipitates with liquid ammonia. The following generalizations were inferred from these tests: Straight-chain ammonium mercaptides are less soluble than branched-chain mercaptides of the same molecular weight; primary mercaptides are less soluble than secondary mercapTable I. Application of Procedure to Thiol lllixtures tides of the same chain strucAnalysis Volume Recoi ered, ture; the n - 1-alkane mercapComponent "c Theor. A 4 (pri.-sec.) B (tert.) Ppt. Fllt. I n ppt. I n filt. In tides decrease in solubility tert-Hexy!mercaptan 110 70 1-Hexanet hiol with increase i n m o l e c u l a r ferf-Hexylmercaptan 2-Hexanethiol 100 86 weight with the exception of tert-Hexyl mercaptan 120 72 Cyclohexanethiol tert-Heptylmercaptan 140 50 1-Heptanethiol methylammonium mercaptide (Blank) terl-Heptylmercaptan 10 78 tert-Octylmercaptan 102 100 I-Octanethiol which is very sparingly soluble tert-Octylmercaptan 90 92 2-Octanethiol 80 90 2-Et hyl- 1-hexanet hiol tert-Octyl nierraptan in ammonia, whereas ethyl5-Ethyl-2-nonanet hiol tert-Dodecylmercaptan Unsuccessful (see text) 98 ammonium m e r c a p t i d e is tert-Dodecylmercs pian 100 1no I-Dodecanethiol readily soluble; and aromatic
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7 ppt.
I n filt. GO 79 83 76 87
91
77 2
9.5
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V O L U M E 2 7 , NO. 1 1 , N O V E M B E R 1 9 5 5 water is added and the solution is acidified n.ith dilute sulfuric acid. A trap ( 1 ) is attached to the flask and the tertiary thiol fraction is collected by steam distillation and measured. The precipitate is warmed on the filter by blowing a stream of air through a long test tube inserted into the filter to dissociate the ammonium mercaptide to thiol and ammonia. Dissociation is ordinarily complete before room temperature is reached. The regenerated primary and secondary thiol fraction is washed through the filter with pentane and recovered by distilling off the pentane. The receiver for the washings consists of a 15-m1., conical, graduated centrifuge tube sealed to the bottom of a roundbottomed flask. This p m n i t s diqtillation of the pentane and measurement of the residue in the same vessel. In a few instances the ammonium mercaptides have been difficult to dissociate. In this event a small amount of dry ice is placed on the filter, allon-ed to stand a fen- minutes, and washed with pentane. A residue of ammonium carbamate remains. The procedure should be carrizd to conclusion without interruption after the filtration has bzen made to prevent oxidation of the thiols after the ammonia has evaporated and air gains access to the sample under basic conditions. If t'he filtrate residue is diluted with acidified water immediately after the ammonia has evaporated, there is little chance for oxidation. Table I sunimariees the results of tests on a nuniber of binary mixtures of primary or sccondary thiols with a tertiary thiol. A mixture of 0.5 ml. of each component was dissolved in 50 ml. of dimethyl ether and added to 100 ml. (50 ml. for C1+& mixtures) of liquid ammonia. The precipitate was filtered after about 30 minutes' standing in a dry ice bath. The fractions from the precipitate and filtrate were recovered as described above. -411 of the tertiary thiols were commercial; each is a mixture of isomers of unknown structures. The high results for component d in the case of the 1-heptanethiol-tert-heptylmercaptan mixture
are attributable to a substance in the tert-heptylmercaptan which yields a precipitate, but this substance has not yet been identified. A second determination using the filtrate from a blank run on tert-heptylmercaptan gave normal yields of precipitate and filtrate. The fractions were analyzed by infrared spectroscopy to determine the amounts of primary or secondary and tertiary thiols in both the filtrate and precipitate. The absolute accuracy of analysis is thought to be &5%. The availability of pure standards would better this value considerably. CONCLUSIOXS
Primary and secondary alkanethiols may be separated from tertiary alkanethiols as an aid in identification of individual compounds in the mixture. The efficiency of the separation increases with increase in molecular weight for those mixtures on which data are given. On the other hand the mixture of 5-ethyl-2-nonanethiol and tert-dodecylmercaptan can be eeparated only under ideally chosen conditions. The separation failed under the conditions adopted for the method. LITERATURE CITED (1) Hopkine, R.L.. and Smith, H. lI.,ASAI. C H E x . , 26, 20B (1954). (2) Thompson, C. J . , Coleman, H. J.,Rall, H. T , and Smith. H. AI., Ibid.. 27. 175-85 (1955). (3) Williams, F. E., with Gebauer-Fuelnegg. E., J . Am. Chem. Sac.. 53, 352 (1931). R E C E I V Efor D review May 18, 1955.
Accepted August 26. 1955.
Absorption of Organic Vapors by Anhydrous Magnesium Perchlorate A. L. BACARELLA, DAVID F. DEVER, and ERNEST GRUNWALD Chemistry Department, Florida State University, Tallahassee, Fla.
Anhydrous magnesium perchlorate has been used as an absorbent for various organic vapors from mixtures of these with inert gases. For all the polar compounds tested, the absorption was quantitative. No explosions have occurred in 4 years, but adequate safety precautions are recommended.
Table I.
Compound Mrtlianol Ethyl alcohol Acetone 1 ,I-Dioxane Pyridine Acetonitrile Ammonia Kitromethane Chloroform
D
URING the past 4 years, anhydrous magnesium perchlorate has been used as a quantitative absorbing agent for a number of organic vapors. The procedure is completely analogous to that used in the determination of water vapor in air or other permanent gases, and appears to be equally quantitative for all but two of the vapors so far tested. A sample of inert gas containing organic vapor is passed through a Sesbitt absorber containing ea. 50 grams of magnesium perchlorate, and the increase in weight of the absorber is measured. Gas flow rates ranged up to approuimately 2 liters per hour a t 1 atmosphere, n i t h the organic vapor usually present a t its saturation pressure. Some sample data illustrating the quantitative nature of the absorption are shown in Table I. In experiments with methanol, e t h l l alcohol, acetone, and dioxane, air Tvas first bubbled through the liquids and then passed through two Nesbitt absorbers connected in series. In experiments with pyridine, acetonitrile, nitromethane, and chloroform, the second Kesbitt absorber was replaced by a cold trap a t dry-ice temperature. Ammonia was dralvn directly from a commercial cylinder and was not diluted with an inert gas. In this case, the second absorber contained an aqueous solution of bromothymol blue. For dioxane and chloroform, absorption is not quantitative. On the basis of these data, i t is reasonable to suppose that magnesium perchlorate could be a general reagent for the vapors of alcohols, aldehydes, ketones, amines, nitriles, and nitro compounds, or, more generally, polar rather than nonpolar compounds.
Absorption of Organic Vapors by Anhydrous 3Iagnesium Perchlorate at 25" f 2" C. Weight Increase, Granis Absorber A Absorber B 1.3890 - 0 , on02n 0.i180 - 0.0003a 0 4968 -o.OOO6= 2,4497 0.0367& n.oooib 0.00;Ob I . 408 5 .ii4
0 . i7.i 1.osn
n
051
n:0002 0.4418b
Magnesium perchlorate absorber. Cold trap. Aqueous bromothymol blue solution, colored yellow by dissolved CO2. S o color change during experiment. a
b
C
As magnesium perchlorate is k n o m to give explosive mixtures with organic materials ( I ) , i t is significant that the authors have not had a single explosion. The adsorbent was always kept at room temperature, and no attempts were made to regenerate it after use. This, however, should not be taken to imply that there is no explosion hazard under these conditions, and adequate precautions should be taken when this absorbent is used for organic vapors. LITERATURE CITED
(1) Stross, AI. J. and Zimmerman, G., Ind. Eng. Chem.. S e w s Ed., 17, 70 (1939). RECEIVEDfor review April 27, 1955. Accepted August 25, 1955. Work supported in part under Contract Nonr 988 (021, Project N R 055-330, betmeen the Office of S a v a l Research and Florida State University. Reproduction in whole or in part is permitted for a n y purpose of the United States Government.
