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Energy & Fuels 2002, 16, 1134-1140
Model Studies Examining the Use of Dicyclohexylphenylphosphine to Enhance the Oxidative and Thermal Stability of Future Jet Fuels Bruce D. Beaver,*,† Li Gao,† Mitchel G. Fedak,† Michael M. Coleman,‡ and Maria Sobkowiak‡ Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, and The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received February 15, 2002
A model study is described that tests the feasibility of using dicyclohexylphenylphosphine (DCP) as an additive to provide oxidative stability for future jet fuels. In addition, the mechanism of autoxidation of DCP in dodecane is examined.
Introduction To achieve optimal performance capabilities for the next generation of jet engines significant improvement in jet fuel oxidative and pyrolytic stability are necessary. This objective has led the U.S. Air Force to initiate an effort with the goal of producing a fuel stable to 480 °C (900 °F).1 Such a fuel is commonly referred to as JP900. This objective is very challenging since it is a giant step beyond the current fuel precombustion temperature limit of JP-8 + 100 of 263 °C (425 °F).2 A JP-900 fuel will have to achieve both oxidative and pyrolytic stability, properties which seem to be inversely related. 3,4 We define oxidative instability in terms of deposit formation on engine surfaces shortly after the fuel reacts with its dissolved oxygen. Pyrolytic instability, on the other hand, is an anaerobic deposit formation on engine surfaces that occurs at temperatures around 400 °C. Current jet engine technology has minimized fuel thermal stress prior to combustion. Consequently, in today’s engines only a fraction of the fuels dissolved oxygen reacts in deposit forming reactions prior to combustion. However, the thermal stress of JP-900 will result in complete consumption of the fuel’s dissolved oxygen and will thus produce significantly more oxidative deposit. Recent developments in the understanding of the oxidative degradation of conventional jet fuels and model systems can facilitate the development of new approaches to enhancing oxidative stability. Specifically, Hardy et al.,5 Heneghan et al.,6 and Jones et al.7 have noted an inverse relationship between the temperature †
Duquesne University. The Pennsylvania State University. (1) Edwards, T. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45, 436. (2) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison, W. E., III. J. Energy Resour. Technol. 1996, 118, 170. (3) Edwards, T.; Liberio, P. D. Prepr. Pap.s-Am. Chem. Soc., Div. Pet. Chem. 1994, 39, 92. (4) Edwards, T.; Liberio, P. D. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40, 649. (5) Hardy, D. R.; Beal, E. J.; Burnett, J. C. 5th Int. Conf. Stability Handling Liq. Fuels, Washington, DC, 1992, 260. ‡
at which a fuel absorbs oxygen and the amount of deposit formation during the oxidative degradation of various jet fuels. Fuels that tend to absorb the dissolved oxygen at lower temperatures (such as highly hydrotreated fuels) tend to produce smaller amounts of oxidative deposits, and vice versa. Heneghan and Zabarnick8 proposed that this observed inverse behavior is consistent with a peroxyl-radical chain mechanism for fuel degradation with free radicals derived from indigenous antioxidants (i.e., phenols, and various sulfur and nitrogen compounds) being deposit precursors. If the Heneghan and Zabarnick proposal is correct, then significant oxidative stability should be obtained by use of vigorous oxygen scavengers in conventional jet fuels. Presumably, the thermal oxidative stability would result from the direct reaction of the oxygen scavenger with molecular oxygen before the fuel is degraded. It is here assumed that the oxidized oxygen scavenger remains in solution. The predicted decrease in the oxidative deposits has been achieved in several systems when oxygen has been removed by physical displacement (e.g., gas purging, vacuum degassing, ultrasonic cavitation). 6,7,9 However, only partial oxygen removal (to as low as 5 PPM) may not be helpful in reduction of oxidative deposits.10 Shown in hypothetical reaction (A) below, an oxygen scavenging additive (Sc) might be designed to react with oxygen to form an innocuous product (ScO). Thus, reaction (A) would divert the oxidation of the fuels indigenous antioxidants to the scavenger molecule.
Sc + O2 f 2 ScO
(A)
The factors involved in the design of reaction (A) such (6) Heneghan, S. P.; Williams, T. F.; Martel, C. R.; Ballal, D. R. Trans. ASME J. Eng. Gas Turb. Power 1993, 115, 480. (7) Jones, E. G.; Balster, W. J.; Post, M. E. Trans. ASME J. Eng. Gas Turb. Power 1995, 117, 125. (8) Heneghan, S. P.; Zabarnick, S. Fuel 1994, 73, 35. (9) Zabarnick, S. Ind. Eng. Chem. Res. 1994, 33, 1348. (10) Ervin, J. S.; Williams, T. F.; Heneghan, S. P.; Zabarnick, S. ASME Paper No. 96-GT-132, 1996 ASME Turbo Expo., Birmingham, U.K., June 1996.
10.1021/ef020028r CCC: $22.00 © 2002 American Chemical Society Published on Web 07/10/2002
Enhancing the Oxidative and Thermal Stability of Jet Fuels
that it can function as an oxygen scavenger have already been discussed.11,12 Briefly, molecules that can be induced to undergo electron-transfer-initiated oxygenation (ETIO) should be amenable to oxygen scavenging functions. We define the simplest case of ETIO as an oxygenation reaction in which the rate-limiting step involves an electron transfer from the substrate to molecular oxygen (or an activated form of oxygen such as a hydroperoxide). We have postulated that electronrich molecules that have low oxidation potentials, such as dialkylarylphosphines, can be induced to undergo ETIO reactions. We believe coal derived liquids are excellent potential future sources of JP-900. Generally, jet fuels derived from the deep catalytic hydrogenation of coal are composed of hydrocarbon structures that are more resistant to pyrolytic degradation when compared to petroleum-derived jet fuels.13 The pyrolytic stability of such fuels is due to a much higher concentration of naphthenic and hydroaromatic compounds compared to petroleum-derived jet fuels. Presumably, hydrogen donation by the hydroaromatic compounds acts as an inhibitor toward pyrolysis. Unfortunately, the same benzylic structure which renders hydroaromatics reactive hydrogen donors also makes these compounds prone to autoxidation14 and subsequent deposit formation.15 Thus, these compounds are analogous to the indigenous antioxidants in the Heneghan and Zabarnick hypothesis8 for deposit formation in conventional jet fuels. The oxidative instability of such coal-derived jet fuels must be addressed before these fuels can serve as JP-900. In this report we update our efforts to develop phosphorus-based oxygen scavenger additives for JP900.12 We report experimental tubing bomb results at 250 °C and 425 °C with DCP as a potential oxygen scavenger with dodecane as a model fuel. We view this model system as representing a current heavily hydrotreated jet fuel (i.e., JP-7). In addition, this system may adequately represent future coal-derived FischerTropsch jet fuels. In a similar manner we examined a dodecane/tetrahydronaphthalene mixture as a model jet fuel from direct coal hydrotreatment. In addition, we report a mechanism study of the oxygenation of dicyclohexylphenylphosphine (DCP) in dodecane. This study allows us to further examine the validity of the ETIO hypothesis in the design of new additives for JP-900. Experimental Section Chemicals. Dodecane, tetradecane, tetrahydronaphthalene, dicyclohexylphenylphosphine (DCP), butylated hydroxy toluene (BHT), N,N-bis (salicylidene)-1,2-propanediamine (MCA), and o-dichlorobenzene were purchased from Aldrich in the highest purity available, and used as received. Tricyclohexylphosphine oxide (TCPO) was purchased from Alfa Aesar. (11) Beaver, B. D.; De Munshi, R.; Sharief, V.; Tian, D.; Teng, Y. 5th Int. Conf. Stability Handling Liq. Fuels, Rotterdam, The Netherlands, 1995, 241. (12) Beaver, B. D.; De Munshi, R.; Heneghan, S. P.; Whitacre, S. D.; Neta, P. Energy Fuels 1997, 11, 396. See also, Alfassi, Z. B.; Neta, P.; Beaver, B. J. Phys. Chem. A 1997, 101, 2153; Beaver, B.; Rawlings, D.; Neta, P.; Alfassi, Z. B.; Das, T. N. Heteroatom Chem. 1998, 9, 133. (13) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 1992, 37, 540. (14) Coleman, M. M.; Sobkowiak, M.; Fearnley, S. P.; Song, C. Prepr. Pap.-Am. Chem. Soc., Div. Pet. Chem. 1998, 43, 353. (15) Hazlett, R. N. Thermal Oxidative Stability of Aviation Turbine Fuels, ASTM, Philadelphia, PA, 1991; Chapter 6.
Energy & Fuels, Vol. 16, No. 5, 2002 1135 Instruments and Parameters. Gas chromatography was performed on a Varian 3700 with HP 3396A Integrator. A HP35 cross-linked 35% phenyl methyl silicone (30 m × 0.53 mm × 1.0 µm film thickness) column was used. The column was kept at an initial temperature 100 °C for 4 min. Then the temperature was raised with the program rate of 30 °C/min, to a final temperature of 240 °C for 9 min. The attenuation of the integrator was adjusted from 6 to 3 to observe the small peaks. GC-MS analysis was performed on a Varian 3410 coupled to a Varian Saturn II ion trap mass spectrometer. The GC column was 30 m × 0.25 mm i.d. Supelco Simplicity-5 (PDMS 5% phenyl, 0.25 µm film thickness) which was installed between a 5 m × 0.25 mm i.d. deactivated capillary guard column and a 5 m × 0.25 mm i.d. deactivated capillary transfer line. The column was kept at initial temperature 100 °C for 3 min, followed by a 20 °C/min temperature ramp to a final temperature of 260 °C. Then it was held at 260 °C for 7 min. The transfer line was held at 280 °C and the injection port was held at 260 °C throughout the separation. The carrier gas was He. For electron ionization (EI) the ion trap was operated using electron impact ionization at the energy of 70 eV. The ion trap was run at one scan per second at a temperature of 260 °C. The mass scan range was 50 to 350. General Procedure for Kinetic Runs. Into a 150 mL (14/ 20) three-neck round-bottom flask, was added 30 mL of a dodecane solution of dicyclohexylphenylphosphine (DCP) and 0.5 mL of tetradecane as an internal standard. To this solution was added 1 mL of o-dichlorobenzene as a co-solvent, and 0.025 g of N,N-bis (salicylidene)-1,2-propanediamine as a metal chelating agent (MCA). Finally, if desired, one equivalent (based upon DCP) of butylated hydroxy toluene (BHT) was added as an antioxidant. The flask was equipped with a reflux condenser on the inner neck and fitted with a gas adapter, which is connected to an oil bubbler. Both the outer necks were fitted with rubber septa, with a pipet inserted through one of them to function as a gas inlet. The reagent gas was first bubbled into the solution for 5-10 min. Then the three-neck flask was immersed up to its neck in a large oil bath capable of maintaining the temperature within 0.1 °C. The first sample was immediately pulled for quantitative GC analysis. Subsequent samples were taken at appropriate time intervals. GC/ MS analysis after 10% reaction reveals the formation of four phosphorus oxidation products: dicyclohexylphenylphosphine oxide, EIMS 291 (M+), and a trace amount of phosphinates, 307 (M+) and 391 (M+). In addition, a trace amount of tricyclohexylphosphine oxide was formed from tricyclohexylphosphine that was a trace impurity in the DCP. A trace amount of an unknown compound, 371 (M+), was present throughout the experiment. Calculation of Reaction Rates. The known molarity of the initial DCP solution was assumed to be the mean of the results of the initial GC analysis. After time t (min), between 5 and 10% reaction, the peak area ratios (DCP/C14) for 3-4 points were determined and assumed to be the fraction of DCP still present. The initial rate was derived from the slope of a plot of molar DCP loss as a function of time. Occasionally, an induction period was observed. For rate calculations only the linear portion of the plot was used. Preparation of Dicyclohexylphenylphosphine Oxide (DCPO). A stirred solution of 0.30 g of DCP in 3 mL of acetone, held at 40 °C, was treated with 0.41 mL of 10% aqueous hydrogen peroxide solution over a 2 h period. The temperature was maintained at 40-50 °C throughout the addition. The mixture was then heated at reflux temperature for 2 h, cooled to 30 °C, and poured into 6 mL of water. The product was extracted with ether and the ether layer was dried with anhydrous MgSO4. The crude product obtained after removal of the ether was purified through recrystallizations from hexane. The approximate yield was 63%. The melting point of DCPO is 159-160 °C. The IR shows the strong absorption
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Beaver et al.
Figure 1. (a) Temperature versus rate study for the oxidation of DCP in oxygen-saturated dodecane in the presence of 0.005 M MCA. (b) Rate of DCP loss as a function of time for the above conditions. Table 1. The Reaction Rate for DCP Oxidation in Dodecane at 150 °C in the Presence of MCA (0.005 M) and BHT (1 eq based on DCP)a [DCP] mol/L
O2%
0.010 0.020 0.040 0.020
100 100 100 20
a
reaction rate mol/L‚min × 10-5 0.54 1.17 3.31 0.17
0.53 0.90 3.36 0.20
0.55 1.16 3.71 0.18
0.54 1.08 2.98 0.20
0.53 1.18 2.81 0.22
3.12
AVER × 10-5
STDEV × 10-5
95% CONF × 10-5
0.53 1.10 3.22 0.19
0.01 0.12 0.32 0.02
0.01 0.14 0.33 0.02
R ) k[O2]1[DCP]1.
at 1165 cm-1 for PdO, and 1448 and 1431 cm-1 absorptions for P-C (aromatic), and P-C (alkyl), which are consistent with the literature.16 EIMS 291 (M+). The mass spectrum of this compound was identical to that of the DCPO formed in the kinetic DCP oxygenation studies. Tubing Bomb Studies. Thermal stressing of pure dodecane and dodecane mixtures containing 5% (v/v) tetrahydronaphthalene with or without the addition of 200 ppm of a dicyclohexylphenylphosphine were performed as a function of time on 10 mL samples at 250° and 425 °C in 25 mL type 316 stainless steel microreactors under 100 psi (0.69 MPa) of air. The microreactor containing the sample was purged with UHP-grade N2 five times at 1000 psi (6.9 MPa) to minimize the presence of dissolved oxygen before final pressurization (16) Thomas, L. C. Interpretation of the Infrared Spectra of Organophosphorus Compounds; William Clowes & Sons Limited, London, 1974; pp 14, 90.
with air. It was then placed in a preheated sand bath at 250° or 450 °C for required reaction time, followed by quenching into cold water and depressurizing. For the purposes of these “scouting” studies only visual observations of the products of thermal stressing were made.
Results Reactions of Dicyclohexylphenylphosphine (DCP) in Oxygen-Saturated Dodecane. At room temperature DCP solutions slowly oxidize. The presence of 0.005 M N,N-bis(salicylidene)-1,2-propanediamine (metal chelating agent, MCA) was found to prevent loss of DCP. Presumably, our experimental system contained a metal ion contaminate. All subsequent experiments contained MCA. At 150° C a 0.02 M solution of DCP in oxygensaturated dodecane is totally oxidized in 1 h or less.
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Energy & Fuels, Vol. 16, No. 5, 2002 1137
Figure 2. Schematic representation of experimental tubing bomb study.
However the same conditions, except for the presence of 1 equivalent of BHT (or greater), require 4 h of heating to observe a 10% loss in DCP concentration. GC/ MS analysis of this reaction suggests the formation of three DCP derived products: a phosphine oxide and two phosphinate esters as shown below. A mass balance was obtained for oxygenation of 0.01 M DCP at 150° C in oxygen-saturated dodecane. Ninety percent of the consumed DCP is converted into the corresponding phosphine oxide (DCPO). At higher phosphine concentra-
tions (0.04 M) a good mass balance could not be achieved. We suspect at the higher concentrations DCP insolubility becomes problematic. The phosphinate esters are present in only trace amounts. Presumably, the missing DCP is phosphinate esters that are retained on
the GC column. Nevertheless, the mass balance results are consistent with DCP functioning as an oxygen scavenger. Figure 1 presents estimates of the rate of DCP oxygenation in the absence of BHT at various temperatures. Extrapolation of these data to 250 °C suggests that in air-saturated dodecane DCP is consumed in minutes, while at 425 °C, it is consumed in seconds. Finally, initial rate studies are presented in Table 1 for DCP oxygenation in dodecane at 150 °C in the presence of BHT. These data suggests that in the 0.010.02 M range the oxidation reaction has a first-order dependency in both oxygen and DCP. Experimental Tubing Bomb Studies. Experimental tubing bomb studies at 250 °C and 425 °C were performed at Penn State with dodecane as a model for a heavily hydrotreated jet fuel such as JP-7. In addition, this system may adequately represent future coalderived Fischer-Tropsch jet fuels. In a similar manner we examined a dodecane/tetrahydronaphthalene (THN) mixture as a model jet fuel from direct coal hydrogenation. THN is considered a good model for the hydrogen donors present in a jet fuel derived from deep hydrogenation of coal.17
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Beaver et al. Scheme 1
In this initial “scouting” study our results are reported schematically in Figure 2 as qualitative descriptions of the chronological appearance of the model jet fuels after thermal oxidative stressing. DCP was used as a model oxygen scavenger additive. Ideally, sacrificial oxidation of DCP minimizes deposit formation two ways: First, by minimizing THN oxidative deposit and second by ensuring a higher concentration of the hydrogen donor present in the fuel when the pyrolytic regime is reached at 425 °C. Pure dodecane and dodecane containing 5% v/v THN were thermally stressed at 250 °C under an initial pressure of 100 psi air. As schematically shown in Figure 2a, pure dodecane is stable as there is no evidence of appreciable development of color or formation of solids or gels for periods of up to at least 12 h.
However, stressing the dodecane/THN mixture results in the formation of a yellow solution with a small amount of carbonaceous deposit observed at 12 h (Figure 2b). Sometime between 6 and 12 h, deposit was formed and is presumably the result of the autoxidation of the THN.15,18 Next, the effect of adding 200 ppm of DCP as an oxygen scavenger was examined in both model systems. As shown schematically in Figures 2c and 2d, the model fuels take on a yellow coloration. However, there are no gels or carbonaceous deposits present for up to 12 h of stressing. These results are consistent with the DCP stabilizing the THN towards oxidative deposit formation. Next, both model fuel systems were stressed at 425 °C under 100 psi air. As schematically shown in Figure 2e, neat dodecane produces some carbonaceous solids
(17) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7, 234.
(18) Corporan, E.; Minus, D. K. AIAA-98-3996, 36th Aerosoace Sciences Meeting & Exhibit, Los Angles, January 1998.
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Energy & Fuels, Vol. 16, No. 5, 2002 1139
Scheme 2
after 3 h and above 4 h the mixture has transformed to a black liquid containing a heavy deposit. Similarly, stressing the dodecane/THN mixture results in the formation of a black solution with a large amount of carbonaceous deposit between 4 and 6 h (Figure 2f). Thus, the presence of THN in dodecane results in an increase in pyrolytic stability at 425 °C. Figure 2g suggests that the presence of 200 ppm of DCP in dodecane results in a surprisingly significant improvement in stability. While the mixtures had a yelloworange coloration, there were no gels or carbonaceous deposits present for up to at least 6 h of thermal stressing. This is similar to that observed when dodecane is thermally stressed under an inert (oxygen free) atmosphere (N2 or argon) at 425 °C (data not shown). Similarly, in the dodecane/THN/DCP mixture, carbonaceous solids are not detected until sometime after 6 h. Discussion Significance of Bomb Study. Clearly our results suggest that DCP provides a degree of oxidative and pyrolytic stability for the model fuel solutions examined. It is recognized that there may be environmental, engine incapability, and economic issues militating against the use of phosphines as fuel additives. However, we believe this work represents a verification of the concept that fuels can be developed that will be stable for significant amounts of time in the pyrolytic regime. The lowemission combustors for advanced subsonic turbines currently being investigated by NASA will require fuel cooling that exceeds that of JP-8 + 100.19 Our study suggests that, with model future coal-derived jet fuels, oxidative and pyrolytic stability to 425 °C (797 °F) can be achieved for significant periods of time. Such an accomplishment significantly increases the currently available jet fuel heat sink. Encouraged by the tubing bomb studies we will soon begin examining a suite of coal-derived jet fuels with a flowing stress system. Mechanisms of DCP Oxygenation. Table 1 reveals a DCP oxygenation rate of roughly 10% reaction in 4 h when heated at 150 °C in oxygen-saturated dodecane in the presence of a large concentration of BHT (1 equivalent based upon BHT). Since a similar rate for DCP oxygenation is observed in the presence of two equivalents of BHT (data not shown) it is unlikely that peroxyl radical chain chemistry is operative.20 We also have observed complete phosphine oxygenation in less (19) http://www.grc.nasa.gov/WWW/AST/propulsion.htm. (20) Floyd, M. B.; Boozer, C. E. J. Am Chem. Soc. 1963, 85, 984.
than 1 h in the absence of BHT. Thus, there are at least two different reaction mechanisms for phosphine oxygenation: presumably these mechanisms operate concurrently with only one mechanism being inhibited by BHT. The more rapid phosphine oxygenation pathway most likely involves a peroxyl radical chain mechanism that can be suppressed by the presence of a high concentration of BHT. Such a mechanism is proposed in Scheme 1. Benson and Nangia 21 have observed the direct formation of organic hydroperoxides when oxygen-saturated hydrocarbons are heated at 150°C. Subsequent reduction of these hydroperoxides can occur via either a nucleophilic pathway (step 1) or an electron-transfer pathway (step 2), vide infra. Step 2 is proposed to initiate a radicalchain oxygenation of DCP as shown in steps 3-8 in Scheme 1. These steps are well-known since the work of Bentrude and colleagues.22 Rapid oxygenation of DCP by a similar mechanism is also expected in the tubing bomb study, which was done in the absence of BHT and is presented in Figure 2. In addition, this pathway may account for the traces of phosphinate formed in the presence of BHT reported in the Results section. We are not aware of any precedent for step 2 in Scheme 1.23 To test the feasibility of single-electron transfer from DCP to an organic hydroperoxide the following experiment was performed. To a 0.01 M solution of tert-butylhydroperoxide in oxygen-saturated dodecane at 60 °C was added dropwise a 0.02 M dodecane solution of DCP. After addition, immediate GC analysis suggests reduction of all the hydroperoxide to tert-butyl alcohol and approximately 50% of the DCP was converted into DCPO. This result can be simply explained by the phosphine reducing the hydroperoxide by a nonradical mechanism as shown in step 1 in Scheme 1.24 However, the same experiment at 76 °C results in the total conversion of all the DCP into DCPO. Clearly, an oxidant other than tert-butylhydroperoxide was generated in this experiment. This result is consistent with (21) Benson, S. W.; Nangia, P. S. Acc. Chem. Res. 1979, 12, 223. (22) For a detailed study of this mode of reaction see, Bentrude, S. W. Acc. Chem. Res. 1982, 15, 117. See also, Buckler, S. A. J. Am. Chem. Soc. 1962, 84, 3093, and Ogata, Y.; Yamashita, M. J. Chem. Soc., Perkin II 1972, 730. (23) Diisopropyl azodicarboxylate has been shown to promote single electron transfer from triphenylphosphine at room temperature. See, Camp, D.; Hanson, G. R.; Jenkins, I. D. J. Org. Chem. 1995, 60, 2977. (24) Hiatt, R.; Smythe, R. J.; McColeman, C. Can. J. Chem. 1971, 49, 1707.
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a portion of the hydroperoxide reduction occurring by electron transfer as shown in step 2 of Scheme 1.25 Subsequent oxygen atom transfer reactions rapidly converts DCP to its oxide as shown in the abbreviated chain reaction in steps 3, 4, 6-8 in Scheme 1.25 Repeating this experiment in the presence of five equivalents of BHT based upon DCP limits the DCP oxygenation to approximately 50%. Thus, a high concentration of BHT inhibits the atom transfer mechanism for phosphine oxygenation and only step 1 is operative. The additional phosphine oxygenation mechanism believed to be concurrently operative with the radical mechanism (Scheme 1) is shown in Scheme 2. Unlike the radical mechanism (Scheme 1) the ETIO mechanism depicted in Scheme 2 is not a radical chain mechanism and is thus not affected by phenolic antioxidants. The distinctive step in Scheme 2 involves rate-limiting single electron transfer (SET) from DCP to oxygen to form a triplet contact-radical ion pair. Both the phosphine radical cation12 and superoxide radical anion26 are wellknown species. Subsequent intersystem crossing27 to a (25) Two control experiments are necessary. The first experiment finds no direct reaction between DCP and oxygen at 76 °C. The second control addresses the possibility is that at 76 °C traces of the tertbutylhydroperoxide undergo homolysis without the involvement of DCP. In the absence of BHT this process could initiate DCP oxygenation. To test this hypothesis the same experiment was performed with triphenylphosphine (TPP) rather than DCP. In this experiment only 50% of the TPP was converted into its oxide. This result rules out the generation of oxidizing radicals by the thermal degradation of the tertbutylhydroperoxide. Presumably, at 76 °C tert-butylhydroperoxide is not a strong enough oxidant to promote single electron transfer from TPP.
Beaver et al.
singlet ion pair followed by radical coupling would presumably yield a phosphadioxirane.28 Rapid reduction of this intermediate by unreacted phosphine yields two equivalents of DCPO for each oxygen molecule consumed. This mechanism is consistent with the observed firstorder dependency in DCP29 and oxygen and with the inability of BHT to hinder the oxygenation reaction. Other examples are known of the ineffectiveness of chain-breaking donor antioxidants, such as BHT, to inhibit non chain SET mechanisms.30 Acknowledgment. This project was jointly supported by the U.S. Air Force Wright Laboratory/ Aero Propulsion and Power Directorate, WrightPaterson AFB and by the U.S. DOE under contract F49620-99-1-0290. EF020028R (26) Sawyer, D. T.; Valentine, J. S. Acc. Chem. Res. 1981, 14, 393. (27) Leffler, J. E. An Introduction to Free Radicals; Wiley-Interscience: New York, 1993; pp 46-49. (28) Phosphadioxiranes are believed to form intermediates during the oxygenation of arylphosphines with singlet oxygen: See, Gao, R.; Ho, D. G.; Dong, T.; Khuu, D.; Franco, N.; Sezer, O.; Selke, M. Org. Lett. 2001, 23, 3719. (29) At higher concentrations DCP insolubility in dodecane complicates the mechanistic investigation. For instance, Table 1 suggests that oxygenation of 0.04 M DCP under our standard conditions exhibits a one-and-a-half order in phosphine. (30) Zhang, X.-M.; Yang, D.-L.; Liu, Y.-C J. Org. Chem. 1993, 58, 224.
