[CONTRIBUTION
FROM THE CHEMICAL
LABORATORY, UNIVERSITY
OF
CINCII\"ATI]
SULFONIUM COMPOUNDS. I1 (1). DERIVATIVES OF NITRIC AND OF ORGANIC ACIDS1 FRANCIS EARL RAY
AND
GEORGE J. SZASZ
Received October 19, 1949
The sulfonium halides and sulfates can be prepared in many cases by direct combination of divalent sulfur compounds with alkyl halides and sulfates. For this reason they have been studied extensively. Other sulfonium salts can be prepared by treating the iodides with the desired silver compound. By this method Kruger (2) prepared methyldiethylsulfonium hydroxide, which he converted to the nitrate by treatment with nitric acid. He states that the nitrate is hygroscopic, but gives no further properties. Renshaw and Searle (3) prepared four crystalline sulfonium nitrates by treating the mercuric iodide double salt with silver nitrate, followed by the precipitation of the mercury with hydrogen sulfide. Compounds of the following formulas were obtained as solids and characterized: (C~HSCH~)~S(NO~)CHI, (C~HSCHZ)~S(NO~)CHZCOOCZH~, ( C ~ H ~ ) Z S ( N O ~ ) Cand H ~ , (CH~)ZS(NO~) CH2CH20C6H6. The last compound was somewhat hygroscopic and an attempt to prepare (CZH~)~S(NO~)CHZCHZOC~H~ resulted in "only a gummy semi-solid." A somewhat similar method had been used by Kehrmann and Sava (4)in 1912 to prepare sulfonium bases. Courtot and Tung (5) prepared tri (hydroxyphenyl) sulfonium nitrate from the hydroxide. Addy and Macbeth (6) prepared trimethyl- and triethyl-sulfonium nitrites by treating the iodides with aqueous silver nitrite. They report that these compounds are very deliquescent and have a bad odor. In the light of our work we believe that the odor they observed was due to impurities, chiefly sulfide or mercaptan, formed by the following decompositions: (CH3)3SN02 (CH3)3SN02
+ HzO ---+
+ CHSONO CH3SH + CHBOH + CHSONO
CHaSCH3
Of the organic acids, only picric acid has been used extensively to prepare and characterize sulfonium compounds. Two carbonates, however, were prepared by Kehrmann and Sava (4); o-anisyldimethylsulfonium carbonate and o-phenetyldimethylsulfonium carbonate. A number of organic thiuronium salts have been prepared by the direct combination of ester and thiourea (7) but these may be regarded as special cases. Libermann (8) reported the preparation of benzoates of the following compositions: (C6H6C00C6H4)3SOCOC6H6, [CH~(C~H~COO)CGH~]~SOCOC~HS, and [ C1(C6HsC00)CaH3]~sOCOC6H6. These were prepared by heating the corresponding arylchlorosulfites in equimolar quantities of pyridine, followed by the benzoylation of the resulting bases. They were, considering their high molecular weight and salt-like properties, extremely low-melting compounds 1
Presented before Section C, A. A. A. S., Columbus, Ohio, Dec. 29, 1939. 121
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FRANCIS EARL RAY AND GEORGE J. SZASZ
(35", 53") and 57"). They were also soluble in ether. From the physiological standpoint these properties are extremely important. Ferrer (9) prepared sulfonium xanthates by reacting carbon disulfide with the sulfonium bases. We undertook to determine if organic sulfides would react directly with alkyl nitrates. When dimethyl sulfide was allowed to stand with methyl nitrate in a sealed tube a t room temperature, white crystals could be observed after two days. After two months about ten per cent of the material had reacted. The solid, on separation from the mother liquor, proved to be very hygroscopic. Drying over phosphorus pentoxide removed the mater, but resulted also in a gradual decrease in the melting point from 9G-98" to 84-86". The percentage of sulfur was too high for trimethylsulfonium nitrate. This may have been due to methylsulfoxide formation or to the presence of methyl sulfide of crystallization. The explosive nature of methyl nitrate makes the use of sealed tubes inconvenient. Using pressure bottles with spring caps made tight with rubber washers, we obtained, a t the end of eight months, thick, hard prisms (1 X 3 mm.) that melted a t 129-131" without visible decomposition. On recrystallization from methanol the substance melted a t 133" and was analytically pure trimethylsulfonium nitrate. The crystals from methanol were smaller, less hard, and somewhat hygroscopic. The compound was soluble in water, methyl and ethyl alcohols, but insoluble in benzene and acetone. In the presence of water or excess sulfide it was difficult to isolate the pure nitrate but the trimethylsulfonium ion was isolated as the picrate. Trimethylsulfonium sulfate was then treated with a solution of barium nitrate. On evaporation, only an oil was obtained but the trimcthylsulfonium picrate mentioned above was obtained from this oil. It is obvious that anhydrous conditions are desirable and possibly necessary to the isolation of the lower sulfonium nitrates. When methyl nitrate and methylethyl sulfide mere heated together at 55" the liquid separated into two phases, In the m-ater-soluble phase the trimethylsulfonium ion was identified as its picrate. The formation of the trimethylsulfonium ion is explained by the extension of the mechanism of Ray and Levine (1) to include sulfonium nitrates as well as halides. C&Eo3
+ C&SCzH,
$
(CH3)2CzHbSN03
11 (CK3)zS -1- CzII,N03
+ C H ~ x 0 3lJ
(CH3)3SN03 While the presence of the other possible sulfonium compounds was not demonstrated, there can be little doubt that it was their presence that caused the formation of a liquid product. Ethyl nitrate and dimethyl sulfide gave only a trace of an oily reaction product soluble in water. No pure compound could be isolated.
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Ethyl nitrate and diethyl sulfide gave a small amount of crystals, which were soluble in water and were identified as containing the triethylsulfonium ion by conversion to the picrate. Hellstrom (10) has shown that the presence of acid inhibits the sulfonium reaction. Pyridine was added, therefore, to a mixture of ethyl nitrate and diethyl sulfide and the solution was heated on the water-bath. At first crystals formed, but these disappeared and a reddish oil separated. It was soluble in water, from which triethylsulfonium picrate was isolated on the addition of picric acid. A mixture of methyl nitrate and methylphenyl sulfide slowly turned green, the color deepening with time. This color change is characteristic of the anhydrous higher molecular weight sulfonium compounds (1). The color disappeared on the addition of water, but no definite compound could be isolated from the small amount of gummy residue obtained. The ether extract on distillation gave a small amount of dimethyl sulfide, which also indicates that some sulfonium formation probably took place.
+
CH~SC~HSCH3NOs
(CH3)&aHsSNO3
+ (CH&S f C6HsNO3
KO evidence was obtained for the direct reaction of methyl nitrate with diphenyl sulfide. One might well question whether the direct sulfonium reaction which takes place with the esters of inorganic acids also takes place with the esters of organic acids. Methyl formate and dimethyl sulfide, therefore, were heated in a pressure-bottle and after a few hours fine white crystals were visible. They melted indefinitely a t 90-100" and charred above 150". Methyl stearate and dimethyl sulfide also formed fine crystals which, however, could not be freed from the mother liquor. The aqueous extract gave a picrate which, while not pure, indicated that the trimethylsulfoniuin ion had been formed. A neutral cottonseed oil was then heated with an excess of dimethyl sulfide. Although there was no visible change, the aqueous extract yielded a small amount of a picrate which melted a t about 90" to the characteristic red liquid of a sulfonium picrate. EXPERIMENTAL
Trimethylsulfonium nitrate. Fifteen cubic centimeters (0.25 mole) of methyl nitrate and 23 cc. (0.3 mole) of dimethyl sulfide were kept in a pear-shaped pressure-bottle for eight months. Crystals began t o separate after two weeks and gradually increased in amount and size, a yield of 18% being obtained. Recrystallization from methanol resulted in a smaller, softer crystal which melted a t 133' and was analytically pure. The compound was soluble in water, methyl and ethyl alcohols, but insoluble in benzene and acetone. A n a l . Calc'd for CaHoNOaS: S, 23.0. Found: S, 23.1. It gave tests for the nitrate ion with both brucine and diphenylamine. The monopicrate melts at 199" while the dipicrate melts at 7&75". Methyl nitrate and methylethyl sulfide (0.1 mole of each) on standing gave a trace of oil. On heating t o 55" the amount of immiscible oil increased considerably. It could not be
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FRANCIS EARL RAY AND GEORGE J. S Z d S Z
crystallized. It was extracted with water and converted t o its picrate which, when purified, proved t o be identical with trimethylsulfonium picrate by a mixed melting point. Ethyl nitrate and dimethyl sulfide failed t o react a t room temperature. When heated for 200 hours a t 70" there seemed to be a trace of immiscible oil formed. We were unable t o get any evidence t h a t i t was a sulfonium compound. Trzethylsulfonium nitrate. Forty-one and five-tenths cubic centimeters (0.5 mole) of ethyl nitrate and 53.5 cc. (0.5 mole) of diethyl sulfide vere allowed t o stand in a pressurebottle a t room temperature for eight months. Crystals were formed, but they could not be freed from the adhering oil. The mixture was treated with ether and extracted with water. From the yellow aqueous solution was obtained triethylsulfonium picrate melting a t 115" (1). Pyridzne as a catalyst. To a mixture of 8.4 ec. (0.1 mole) of ethyl nitrate and 10.8 cc. (0.1 mole) of diethyl sulfide was added 0.8 cc. (0.01 mole) of pyridine. After two vieeks a drop of heavy oil was visible. This increased slowly a t room temperature. On heating on the water-bath the amount of oil rapidly increased until i t formed about 30% of the total liquid. The main body of liquid was yellow, but the oil was dark red. This red oil was a mixture of triethylsulfonium nitrate and pyridine. It was soluble in water and although i t could not be obtained crystalline, it yielded triethylsulfonium picrate. It is thus evident that organic bases have a strong catalytic effect on the sulfonium reaction. Whether the pyridine first forms an intermediate pyridinium compound or catalyzes the reaction by virtue of its alkalinity (10) remains t o be determined. Trzmethylsulfonium formate. Twelve cubic centimeters (0.16 mole) of dimethyl sulfide and 10.1 cc. (0.16 mole) of methyl formate were heated on the water-bath. After two hours fine white crystals were observed. They could not be isolated in a pure form but melted indefinitely a t 90-100". The mixture was evaporated on the steam-bath and a dark oily residue was obtained. Methyl stearate (0.5 mole) and dimethyl sulfide (0.15 mole) were heated for 200 hours a t 70" in a pressure-bottle. Some fine solid was observed in the liquid. The material was treated with ether and extracted with dilute hydrochloric acid. From the aqueous acid solution the low-melting trimethylsulfonium dipicrate was obtained. Cottonseed ozl (0.01 mole) and dzmethyl suZjide (0.06 mole) were heated on the water-bath for 200 hours in a pressure-bottle from which the air had been expelled. There was no visible change. The product was treated with ether and extracted with water. From the aqueous extract on treatment with picric acid there was obtained a picrate t h a t melted a t 90" t o a red liquid. In the case of the latter compounds higher temperatures and pressures would doubtless give good yields of the trimethylsulfonium salts of the long-chain fatty acids. SUMMARY
It has been shown that a direct reaction occurs between organic sulfides and the esters of nitric acid. The equilibrium theory of Ray and Levine has been extended to cover this reaction. A dircct reaction occurs also between organic sulfides and esters of organic acids, such as methyl formate, methyl stearate, and glycerol esters of cottonseed oil. The catalytic effect of organic bases on the sulfonium reaction has been demonstrated. CINCINNATI,OHIO REFERENCES (1) Part I, RAYAND LEVINE,J. Org. Chem., 2, 267 (1937). (2) KRUGER, J. prakt. Chem., 14, 207 (1876).
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(3) RENSHAW AND SEARLE, J . Am. Chem. Soc., 66, 4952 (1933). (4) KEHRMANN AND SAVA,Ber., 46, 2895 (1912). (5) COURTOT AND TUNG,Compt. rend., 200, 1537 (1935). (6) ADDYAND MACBETH, J . Chem. SOC.,109, 755 (1916). (7) NENCKI,Ber., 7, 780 (1874); TAYLOR,J . Chem. SOC.,111, 650 (1917); LECHERet al., Ann., 438, 169 (1924); Ann., 446, 35 (1925). (8) LIBERMANN, Compt. rend., 197, 1425 (1933); C A R RAND ~ LIBERMANN, Compt. rend., 196, 799 (1932). (9) FERRER,Anales SOC. espa2. fis. quim., 16, 724 (1918). (10) HELLSTROM, 2. physik. Chem., A177,337 (1936).
