Znd. Eng. Chem. Res. 1995,34, 981-986
981
RESEARCH NOTES Synthesis and Preliminary Evaluation of Aryl Arenesulfonate Esters as Potential Intermediate Temperature Fluids Bassam S. Nader," Chester E. Pawloski, Cynthia L. Powell, Colleen F. O'Brien, and Mark P. Arrington Organic Chemicals and Polymers Laboratory, Central Research and Development, The Dow Chemical Company, Building 1707, Midland, Michigan 48674
The synthesis and preliminary evaluation of sixty aryl arenesulfonate esters for potential use as intermediate temperature lubricant fluids (225-300 "C) is described. Thsse compounds were readily accessible synthetically from the reaction of phenols and arenesulfonyl chlorides. This class appeared to exhibit good methanolytic stability for extended periods under neutral or acidic conditions, even when heated a t reflux in methanol. However, methanolysis occurred readily under basic conditions, particularly in the presence of electron-withdrawing substituents. Many of the aryl arenesulfonates studied were fluids at ambient temperature, but no obvious correlations between structure and fluidity could be discovered. Monosulfonates had a greater likelihood of being fluids than disulfonates. Also, all of the monosulfonates studied which were based on 4-phenoxybenzenesulfonicacid were fluids at ambient temperature. Aryl arenesulfonates as a class were, in general, significantly more oxidatively stable than a leading commercial pentaerythritol ester fluid, as determined by pressure differential scanning calorimetry (PDSC). Fluoro substitution (F, CF3, CF30) seemed to enhance oxidative stability. However, the PDSC data did not lead t o the identification of any obvious trends or correlations between structure and oxidative stability. 1. Introduction
Since World War 11,carboxylate ester and phosphate ester synthetic lubricant base stocks and additives have received considerable attention from the lubrication industry (Randles, 1993; Marino, 19931, and have become major players in a number of lubricant markets (Ranney, 1976). As part of a program aimed toward the discovery and development of new, readily accessible lubricant base stocks and additives for intermediate temperature applications (225-300 "C),such as in advanced commercial aircraft engines, we became involved in the synthesis and evaluation of aryl arenesulfonate esters (Nader, 1993; Nader and Pawloski, 1993). The current state-of-the-art commercial aircraft lubricants, which are typically fluids based on pentaerythritol tetraester (PET) or related carboxylate ester
0
Aryl ArenesulfonateEster
0
Pentaerythritol Tetraester (PET)
materials, have maximum upper use temperatures on the order of 200-235 "C. In recent decades, this class of fluids has witnessed extensive improvements in oxidative stability at elevated temperatures through formulation and additives. However, this strategy is not expected to lead to further significant enhancements.
* Author to whom correspondence should be addressed. Present address: DowElanco, Discovery Research, Building 306/E2, 9330 Zionsville Road, Indianapolis, IN 46268.
Mention of arenesulfonate esters in the lubrication literature has been very sparse, which is somewhat surprising, considering the major role that related sulfonate salts have occupied in the lubrication industry, particularly as detergent and dispersant additives (Ran1993). ney, 1973;Klamann, 1984;Abou El Naga et d., To our knowledge, the only literature mention of sulfonate esters as having potential utility in lubrication has been in a 1944 U.S.patent (Knutson and Graves, 1944). Several factors contributed to our rationale for studying aryl arenesulfonates. Firstly, from an economic standpoint, it was expected that these materials would be readily accessible chemically by simple and straightforward reactions between phenols and arenesulfonyl halides, many of which are commercially available at relatively low cost. Secondly, these materials were expected to possess higher thermal and oxidative stability relative to carboxylate esters or alkyl arenesulfonate esters, provided that unstable, easily oxidizable substituents were absent. This notion was reinforced by preliminary thermooxidative stability studies in our laboratory, as well as by some literature references which indicated that certain aromatic polysulfonates possessed thermal stability of 2300 "C (Thomson and Ehlers, 1964; Podgorski and Podkoscielny, 1987). Thirdly, from the standpoint of hydrolytic and methanolytic stability, aryl arenesulfonate esters were expected t o exhibit higher stability than alkyl arenesulfonate esters or carboxylate esters. AZkyZ sulfonate esters are well-known in the chemical literature, and are widely used in activation of alkylhydroxy compounds toward nucleophilic displacement (e.g.,via tosylates, mesylates, triflates, etc.), a process which normally occurs through
0888-588519512634-0981$09.00/00 1995 American Chemical Society
982 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
Scheme 1 0
!-R
1 -O-CH*
-R'
b r t an sN2 type attack by the nucleophile on the a' carbon. Consequently,alkyl sulfonate esters usually possess low hydrolytic stability, and are readily cleaved by water, hydroxy, amino, or other nucleophilic groups, particularly in the presence of basic catalysts. Aryl sulfonate esters, on the other hand, would not be expected to undergo similar nucleophilic cleavage a t the a' carbon since that carbon is part of an aromatic ring, unless an activating group is present (e.g., ortho or p a r a nitro, carbonyl, or sulfonyl). Consequently, aryl arenesulfonate esters would be expected t o exhibit higher stability toward nucleophiles than alkyl sulfonate esters (Scheme 11, and by the same token, they would be expected to possess greater relative stability toward nucleophiles than carboxylate esters, the latter being readily cleavable by nucleophiles through direct attack at the carbonyl carbon. In this paper, we present our preliminary results on the synthesis and evaluation of the oxidative stability and fluidity of a significant number of aryl arenesulfonates in comparison with EMERY 2939 PET fluid,
c c ~ ~ , - c o ~ c - ctIc , c , , It 0
0
EMERY 2939 PET Fluid
which is a leading pentaerythritol tetraester fluid. We also discuss these results and their implications for further development of this class of potential lubricants.
2. Experimental Section 2.1. General. Melting points were determined in open capillary tubes, and are uncorrected. All preparative reactions were monitored by HPLC on an HP 1090 instrument using an ODS-Hypersil reverse-phase column (HzO-CH~CN).lH, 13C, and 19FNMR spectra were obtained on a Varian VXR300 instrument operating at 299.96, 75.43, and 282.20 MHz, respectively, using tetramethylsilane as an internal standard for lH and 13C NMR spectra and hexafluorobenzene as an internal standard for 19FNMR spectra. 2.2. Materials. The aryl arenesulfonates evaluated in this study are shown in Tables 1-3. Most compounds were readily synthesized by reaction of the appropriate phenols and arenesulfonyl chlorides using the general transformation depicted in eq 1.A few of the aryl Cg%Nor l 3 f i
R
i
c
DMAP (cat.)
arenesulfonateswere purchased from commercial sources (compounds 5,8,9,10, and 17). All of the phenols and arenesulfonyl chlorides were obtained from commercial sources.
Pyridine or triethylamine were used as solvent and acid scavenger, and a catalytic amount of 44dimethylaminolpyridine (DMAP) (Scriven, 1983; Hoefle et al., 1978) was used in most cases to speed up the reaction. Purification of the compounds was accomplished by a variety of methods, including bulb-to-bulb vacuum distillation on a Kugelrohr apparatus, column chromatography on flash-grade silica gel (Still et al.,1978),and/ or crystallization as needed. Experimental details for each compound are not included in this note due to the large number of compounds studied, but a representative procedure is provided below. Detailed procedures for several of the compounds have been previously published (Nader, 1993;Nader and Pawloski, 1993).No procedural optimizations were attempted, since our main concern at this stage was to obtain pure materials for preliminary evaluation rather than optimizing processes and maximizing yields. The characterization and purity of all compounds were satisfactorily determined by lH and 13C NMR spectroscopy and HPLC. Additionally, 19F NMR spectra were obtained for fluorinated compounds. 2.3. Typical Procedure for Preparation of Aryl Arenesulfonate Esters. The appropriate phenol and an equimolar amount of the appropriate arenesulfonyl chloride are mixed together in anhydrous pyridine (approximately 15-fold molar excess) in a predried flask fitted with a CaClDrierite drying tube and a reflux condenser. A catalytic amount (approximately2 mol %) of 44dimethylamino)pyridine (DMAP)is added, and the mixture is stirred and the temperature is gradually raised until a gentle reflux is attained. After the reaction is complete, as indicated by monitoring the reaction with thin-layer chromatography and/or HPLC, the mixture is cooled, and the product is isolated and purified by standard organic chemistry methods. 2.4. Methanolytic Stability Evaluation. Compound 13 was used as a prototype molecule for assessment of the methanolytic stability of aryl arenesulfonates bearing electron-donating groups. A 25 mL flask equipped with a magnetic stirring bar and reflux condenser was charged with compound 13 (0.5 g) and methanol (4 mL). The mixture was stirred at ambient temperature for 1 h and then was heated at reflux for 4 h. Small aliquots were withdrawn periodically and analyzed by HPLC. No change in 13 could be observed. Stirring at ambient temperature was continued for 65 h, but no change in the material could be observed. KOH (approximately 30 mg) was added, and stirring at ambient temperature was continued; no methanolysis could be observed after 1 h. The mixture was lastly heated at reflux for 3 h, during which gradual formation of the phenol could be observed, but the reaction proceeded only until the base was consumed. In another experiment, compound 13 (0.51 g) and methanol (4 mL) were stirred and treated with concentrated H2S04 (10 drops). The mixture was stirred at ambient temperature for 5 h and then was heated at reflux for 15 h. No change in 13 could be observed by HPLC analysis of aliquots during and at the end of the procedure. Compound 36 was used as a prototype molecule for assessment of the methanolytic stability of aryl arenesulfonates bearing substituents that are strongly electronwithdrawing. A mixture of compound 36 (0.42 g) and methanol (4 mL) was stirred at ambient temperature for 6 days. Small aliquots were withdrawn periodically and analyzed by HPLC. No change in 36 could be
Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 983 Table 1. Aryl Arenesulfonates Studied, Their Physical States, and Their PDSC Data in Comparison with a Leading Commercial PET Fluid
compound number 1
compound structm EMERY 2939 PET Fluid
2
3 4 5 6 7
and m.u. PC) oil
)xidative stability by PDSC (02); mw I exrrau (OC) 205 1223
solid, 34-35 solid, 68-71 oil solid, 48-50 oil solid, 71-73 solid, 96-97 solid, 105.5-106.5 solid, 52-53.5 solid, 66-67 solid, 121-123 solid, 68-71 oil oil oil solid, 101.5-102.5 solid, 62-63 viscous oil oil viscous oil viscous oil solid, 4143 oil solid, 57-60 oil oil solid, 66-67 oil viscous oil viscous oil oil oil viscous oil
280 I318 298 I317 319 I339 273 I299 270 I300 314 I336 2841342 270 1294 2881310 284 1 303 301 I320 28 1 1 320 286 I327 262 I300 243 I282 264 1 286 309 I325 2961331 282 I322 261 J 287 265 I320 231 I306 2641294 266 1279 246 1277 266 278 263 / 319 315 I354 259 I326 238 I291 337 I362 340 1 360 2681294
physical state
3-CH3(CH2hCH20 3-PhO
8 9
10
11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29
30 31 32 33 34
observed. The mixture was heated a t reflux for 8 h, but no change could be observed. The mixture was allowed t o cool, KOH (approximately 90 mg) was added, and the mixture was stirred at ambient temperature. After 1 h, complete disappearance of 36 had occurred. In another experiment, a mixture of 36 (0.44 g) and methanol (4 mL) was treated with concentrated HzS04 (5 drops), and stirred at ambient temperature. No change could be observed after 4 days. The mixture was heated at reflux for 8 h, but no change could be observed. 2.5. Oxidative Stability Testing. Pressure differential scanning calorimetry (PDSC)(Levy et aZ.,1970; Blaine, undated; Blaine, 1976;Wesolowski, 1981; Zeman, 1984)was used to determine the oxidative stability of the materials described in this study. The PDSC runs were performed on a DuPont Instruments 910 differential scanning calorimeter with a DuPont Instruments 1090B thermal analyzer control unit, using aluminum pans under 1.38 x lo6 Pa (200psi) oxygen pressure and a flow rate of 50 cm3 min-I. The instrument was programmed t o raise the temperature at a
rate of 15 "C min-l. Two data points were reported: (1)the onset of oxidation, which is obtained by noting the temperature at which the curve begins t o depart upwards from the baseline; (2) the extrapolated temperature of major oxidation exotherm, which is obtained by extrapolating the major oxidation exotherm curve tangentially a t the point of maximum slope, extrapolating the baseline, and noting the intercept temperature of the two.
3. Results and Discussion For the sake of clarity, the aryl arenesulfonate esters evaluated in this study were classified into three categories: monosulfonates, disulfonates, and heterocyclic sulfonates. These are shown in Tables 1,2, and 3,respectively. Monosulfonates in Table 1were organized on the basis of the nature of substitution on the arenesulfonate moiety, following the order H, alkyl, alkoxy, phenoxy. 3.1. Synthesis of Aryl Arenesulfonates. A variety of synthetic methods exist for the preparation of sul-
984 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 Table 2. Disulfonates Studied, Their Physical States, and Their PDSC Data compound number
commund structure
physical state and m.0. ("C)
)xidative stability by PDSC (02); Inset I extrap I"C)
R 35 36 37
R = 3-CH30 R = 3-CF3 R = 3-PhO
solid, 93-94 solid, 59-60 g-
293 1337 336 I360 288 1338
R R = 4-C(CH3)2CH2C(CH3)3 R = 3-CF3
solid, 123-125 viscous oil
297 I324 334 1358
40 41 42 43
solid, 82-84 solid, 82-83 solid, 59-60 viscous oil
300 1361 315 1330 320 1345 286 1292
44 45 46 47
48
solid, 92-93 solid, 125-127 solid, 99-101 solid, 133-134 solid, 177-180
49 50 51 52 53
solid, 129-130 glass solid, 136-137 solid. 138-140
3141332 311 1337 234 1307 338 1367 304 1357 236 I312 313 / 322 3061318 338 1375 291 I362
38 39
fonate esters, and these are well documented in the chemical literature (Hoyle, 1991; Anderson, 1979). Our compounds were conveniently synthesized following the most common sulfonate preparative method, namely, esterification by reaction of the appropriate arenesulfonyl chloride with the appropriate hydroxy compound (eq 1). We also chose to carry out our preparative reactions in either pyridine or triethylamine, which act as both base and acid scavenger. D W was used as catalyst in most cases, in analogy with its well-known utility in enhancing the esterification of carboxylate systems (Scriven, 1983; Hoefle et al., 1978). However, we are not certain whether or not the use of D W was necessary, since all the reactions, in general, proceeded smoothly and uneventfully, and afforded the desired sulfonate esters in good to excellent yields. 3.2. Preliminary Methanolytic Stability Evaluation. One important issue with new lubricant base stocks and additives is their compatibility with metha-
B-
nol. In order to assess the methanolytic stability of aryl arenesulfonates, two representative compounds were chosen: (a) compound 13, bearing electron-donating substituents; (b) compound 36, bearing strongly electronwithdrawing substituents. These compounds were examined under neutral, acidic, and basic conditions, as described in the Experimental Section. Both of these compounds showed complete stability in the presence of a large excess of methanol for extended periods at room temperature or under reflux conditions. They also showed complete stability toward methanol in the presence of concentrated HzS04, even under reflux conditions for extended periods. In the presence of KOH, 13 showed stability toward methanol a t ambient temperature, but did undergo methanolysis when heated at reflux. Compound 36 readily underwent methanolysis at ambient temperature in the presence of KOH.
Ind. Eng. Chem. Res,, Vol. 34,No. 3, 1995 985 Table 3. Heterocyclic Arenesulfonates Studied, Their Physical States, and Their PDSC Data
xidative stability by PDSC (02); nset I extrap ("0
number
physical state and m.p. PC)
54
oil
266 I287
55
oil
270 1286
56
solid, 35-37
281 1310
57
solid, 66-67
270 1296
58
solid, 114- 115
251 I296
59
solid. 107-109
227 I251
60
solid, 192- 197
296 1327
61
oil
295 I310
compound
This is not unexpected, since 36 is activated toward methanolysis by the presence of electron-withdrawing groups. 3.3. Fluidity. One desirable feature in new lubricants is fluidity at, or preferably below, ambient temperature. Our study resulted in the identification of many aryl arenesulfonates which exist as fluids under ambient conditions, as Tables 1-3 show. Attempts at developing an understanding of the factors that lead to fluidity and/or to lowering of melting points in these compounds were unsuccessful, and no obvious trends or correlations could be identified. However, examination of the data may indicate that monosulfonates, in general, have a better likelihood of being fluids than disulfonates. Another generalization that may be inferred from studying the data is that monosulfonates based on 4-phenoxybenzenesulfonic acid tend to be fluids. This becomes apparent from examining the data for compounds 29-34 and 55, all of which are oils. We did not attempt to assess the relative fluidity of fluid aryl arenesulfonates. Further consideration of these materials as potential lubricant base stocks would require that this assessment be carried out, such as by pour point and/or viscosity determinations. 3.4. Oxidative Stability Evaluation. Differential scanning calorimetry (DSC) is a quick and convenient
means of assessing the oxidative stability of materials. Additionally, it is inexpensive and requires only a very small amount of test sample. Carrying out the DSC experiment under pressure (PDSC) (Levy et al., 1970; Blaine, undated; Blaine, 1976; Wesolowski, 1981; Zeman, 1984) helps to alleviate potential problems or complications resulting from sample volatility. Examination of the PDSC data displayed in Tables 1-3 readily reveals that aryl arenesulfonates as a class are, in general, significantly more stable than carboxylate ester materials as represented by PET. However, analysis of our data with the purpose of developing a rational understanding of the factors leading t o enhanced oxidative stability in this class were unsuccessful, and no obvious trends or correlations could be uncovered. Nevertheless, it may be stated in general that fluoro substitution appears to offer improvement in oxidative stability. Indeed, compounds 32, 33, 36, 39,42, and 47, all bearing fluoro substitution, exhibited some of the highest oxidative stability profiles among our compounds. The fact that the oxidative stability data of fluorinated compounds 21,22, and 49 were not as good is possibly a result of the other aliphatic chain substituents on these molecules.
986 Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995
4. Conclusions
Aryl arenesulfonates are readily accessible synthetically from the reaction of phenols and arenesulfonyl chlorides. Compounds bearing either electron-donating or electron-withdrawing substituents appear to be methanolytically stable for extended periods under neutral or acidic conditions, even when heated at reflux in methanol. However, they undergo methanolysis readily under basic conditions, although electron-donating substituents seem to offer some resistance toward methanolysis. Many aryl arenesulfonates were identified which exist as fluids at ambient temperature, but no obvious correlations between structure and fluidity could be discovered. It may be stated, in general, that monosulfonates have a greater likelihood of being fluids than disulfonates. Also, all of the monosulfonates studied which were based on 4-phenoxybenzenesulfonic acid turned out to be fluids at ambient temperature. Aryl arenesulfonates as a class are, in general, significantly more oxidatively stable than carboxylate ester fluids, as determined by PDSC. Fluoro substitution (F, CF3, CF3O) appears to enhance oxidative stability. However, the PDSC data displayed herein did not lead to the identification of any obvious trends or correlations between structure and oxidative stability.
Acknowledgment The authors wish to acknowledge the Dow Chemical Company for support of this work and for permission t o publish the results described herein. We thank Dr. Ted A. Morgan of Dow for valuable discussions and Dr. Wendell L. Dilling (formerly of Dow) for obtaining some of the PDSC data.
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Hoefle, G.; Steglich, W.; Vorbruegeen, H. 4-Dialkylaminopyridines as Highly Active Acylation Catalysts. Angew. Chem., Znt. Ed. Engl. 1978, 17, 569-583. Hoyle, J. Preparation of Sulfonic Acids, Esters, Amides and Halides. In The Chemistry of Sulfonic Acids, Esters and Their Derivatives; Patai, S., Rappoport, Z., Eds.; Wiley: New York, NY,1991; Chapter 10; and references therein. Klamann,D. Lubricants and Related Products: Synthesis, Properties, Applications, International Standards; Verlag Chemie: Deerfield Beach, FL, 1984. Knutson, A. T.; Graves, E. F. Lubrication. U S . Patent 2,340,331, 1944, assigned to the Lubrizol Corporation. Levy, P. F.; Nieuweboer, G.; Semanski, L. C. Pressure Differential Scanning Calorimetry. Thermochim. Acta 1970, 1 , 429439.Marin0, M. P. Phosphate Esters. In Synthetic Lubricants and High-Performance Functional Fluids; Shubkin, R. L., Ed.; Marcel Dekker: New York, NY,1993; Chapter 3. Nader, B. S. Lubricants Containing Aryl Arenesulfonates as Lubricity Additives. U.S. Patent 5,204,011, 1993, assigned to the Dow Chemical Company. Nader, B. S.; Pawloski, C. E. Aryl Arenesulfonates and a Method of Lubrication Using the Aryl Arenesulfonates. World Patent 93 25,640, 1993, assigned to the Dow Chemical Company. Podgorski, M.; Podkoscielny, W. Linear Polysulfonates. I. Products of 4 4 - (1-Cyc1ohexylidene)diphenoland 4,4'-(2-Norbolidene)diphenol with Some Aromatic Disulfonyl Chlorides. J . Appl. Polym. Sci. 1987, 34, 639-650. Randles, S. J. Esters. In Synthetic Lubricants and HighPerformance Functional Fluids; Shubkin, R. L., Ed.; Marcel Dekker: New York, NY,1993; Chapter 2. Ranney, W. M. Lubricant Additives; Chemical Technology Review No. 2; Noyes Data Corporation: Park Ridge, NJ, 1973. Ranney, W. M. Synthetic Oils and Greases for Lubricants; Chemical Technology Review No. 72; Noyes Data Corporation: Park Ridge, NJ, 1976. Scriven, E. F. V. 4-Dialkylaminopyridines: Super Acylation and Alkylation Catalysts. Chem. SOC.Rev. 1983, 129-161. Still, W. C.; Kahn, M.; Mitra, A. Rapid Chromatographic Technique for Preparative Separations with Moderate Resolution. J. Org. Chem. 1978,43,2923-2925. Thomson, D. W.; Ehlers, G. F. L. Aromatic Polysulfonates: Preparation and Properties. J . Polym. Sci., Part A 1964, 2, 1051-1056. Wesolowski, M. Thermal Analysis of Petroleum Products. Thermochim. Acta 1981,46, 21-45. Zeman, A. Newer Examination Methods for Lubricants. Part I. PDSC. Maschinenschaden 1984,57, 9-51. Received for review July 6 , 1994 Accepted December 13, 1994@
IE940414Z Abstract published in Advance ACS Abstracts, February 1, 1995. @
