Energy & Fuels 1991,5,419-423 for a more fundamental understanding of elementary reactions in supercritical water. In situ spectroscopicmeasurements may yield some insights regarding radical species and stable intermediates. The improvement in our understanding of the potential energy surface of the water molecule and other simple molecules together with the advent of high-speed computers opens up the possibility of modeling the reaction kinetics of a single elementary reaction in supercritical water at a molecular level. Kalinichee has demonstrated that a TIPS2 potential energy surface for water is able to reproduce the thermodynamic properties of water to within 10% in the supercriticalwater range. A realistic potential energy surface for supercritical water together with a potential energy surface of the reactants allows detailed trajectory calculations to be made. Such calculations could provide valuable qualitative information about the likely effect of supercritical water on gas-phase reaction rates and help in guiding future experiments. Currently, the rate of oxidation of simple organics cannot be predicted accurately enough from elementary re(46)Kalinichev, A. G. Int. J. Thermophys. 1986, 7(4), 887.
419
action models. For design purposes, a global model based on experimental data is more reliable. The global model presented in this paper for methane oxidation is based on a more consistent data set than reported earlier.12 In forthcoming experiments, other oxidantssuch as H202 and the effect of pressure on reaction kinetics will be studied so that mechanistic details can be deduced. Since fundamental kinetic measurements are needed to quantitatively characterize oxidation rates and verify mechanisms, we plan to enlarge our data base on oxidation of simple compounds in supercritical water to include hydrogen (because of its importance in elementary reaction pathways) and model compounds like glucose, acetone, acetic acid, methylene chloride, and salt-organic mixtures. Acknowledgment. We gratefully acknowledge the financial support of the National Aeronautics and Space Administration (NASA) for the work done on this project. We also thank Rick Holgate, David Stevenson, Richard Helling, Donald Price, Michael Modell, Professors Jack Howard and Adel Sarofim, William Killilea, Glenn Hong, and the personnel at MODAR for helpful discuasion, ideas, and assistance in experiments and computer modeling. Registry No. CHI, 74-82-8; HOz, 7732-18-5.
Estimates of Solution-Phase Bond Dissociation Energies for C-S and S-S Bonds in Radical Anions Derived from Aromatic Sulfides M. J. Bausch,* C. Guadalupe-Fasano, and R. Gostowski Department of Chemistry and Biochemistry, Southern Illinois University a t Carbondale, Carbondale, Illinois 62901-4409 Received August 24, 1990. Revised Manuscript Received November 27, 1990
Estimated with the aid of a thermochemical cycle are the strengths of C S bonds contained in radical anions derived from the isomeric species 9-phenylthiomethylanthracene (1) and lO-phenylthio-9methylanthracene (2). Bond dissociation energies (BDEs) for the weakest C-S bonds in the radical anions derived from 1 and 2 are 11 and 38 kcal/mol, respectively. 'BDEs for C-S bonds present in radical anions derived from phenyl sulfide (3) and benzyl phenyl sulfide (4) are about 7 and -15 kcal/mol, respectively. Additionally, BDEs for the SS bond in the radical anions derived from n-butyl disulfide and phenyl disulfide are about 30 and 18 kcal/mol, respectively. These estimates result from the use of various thermochemical cycles and suggest that reductive chemistries carried out on the aforementioned sulfur-containing coal model compounds result in substantial weakening of selected C-S and S-S bonds present in these species. Data are also presented that indicate that reduction results in a 15-20 kcal/mol weakening of the anthrylmethyl C-H bonds present in 1 and 2.
(1)
discussing the strength of a given bond is its homolytic bond dissociation energy. The fact that several reviews have been published on the subject of homolytic bond dissociation energies attests to the relevance of BDEs to chemistry in general and to fuel science in particular.' While much is known about the homolytic strengths of bonds in neutral closed-shell organic species, less is known about the strengths of bonds in radical ions2and, in par-
strength data are often utilized in efforts to understand and predict the course and likelihood of various radicalproducing reactions. The quantity usually cited when
(1) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,33, 493-532.
Introduction The process of fossil fuel conversion is generally thought to involve organic radicals. Free radicals result when the bond connecting fragments A and B in molecule A-B is cleaved in a homolytic fashion, as shown in eq 1. Bond
Q-9
hC+-mlYllC CWW-
A'
+ Bg
0887-0624/91/2505-0419~02.50/0 0 1991 American Chemical Society
Bausch et al.
420 Energy & Fuels, Vol. 5, No. 3,1991
ticular, radical anions. Several research efforts have focused on chemical methods that aim to depolymerize and/or desulfurize coal via reactions that involve the initial reduction of coal and coal model compounds.s Reduction of coal presumably results in the formation of radical anionic centers throughout coal (eq 2) and acta to energize
Scheme I
Q
"unknown"
fragments of the coal macromolecule. Mechanisms for the affected moieties to relieve themselves of their newly acquired "energy" include various bond cleavage reactions. Knowledge of homolytic BDEs has proven useful in evaluations of the chemistry involving several varieties of fossil fuel transformations. Little is known about the strengths of bonds in energy-relevantorganic radical ions, despite the fact that ongoing research aims to cleave chemical bonds in coal and coal model compounds via reductive and/or oxidative methods. By determining the strengths of carbon-sulfur (C-S) bonds in radical anions derived from coal model compounds, we are attempting to determine the likelihood of the occurrence, from a thermodynamic perspective, of various C-S bond cleavage chemistries. In essence, several varieties of C-S bonds must be selectively broken in the early stages of any procedure that attempts to loosen organic sulfur from coal. Other possible reactions for radical anions derived from coal model compounds include hydrogen donation. Therefore, also found in this article are estimates of the magnitude of the weakening in selected C-H bonds in radical anions derived from two coal model compounds. Finally, also discussed are the strengths of S-S bonds present in radical anions derived from two organic disulfides.
Results and Discussion C-S BDEs. The initial step in the chemical reduction of coal and coal model compounds is the addition of a single electron to the species in question. The technique of cyclic voltammetry (CV) can serve as an electrochemical aid in studies of the aforementioned chemically induced reductions. In addition to obtaining a value for the redox potential of the reaction under investigation, the observed degree of reversibility in a given CV experiment often attests to the stability of the electrochemically formed product. It is instructive to compare the CV traces for the dimethyl sulfoxide (DMSO) phase reductions of the isomeric species 9-phenylthiomethylanthracene (1) and 10phenylthio-9-methylanthracene(2). At sweep rates of 100 mV/s, 2 is reduced reversibly, while 1 is reduced irreversibly. Evidently, on the time scale of the CV experiment, the radical anion derived from 2 is stable, while the radical anion derived from 1 is unstable. The potentials at which 1 and 2 are reduced are very similar (about -1.25 and -1.3 V,vs the NHE reference electrode). We attribute the difference in the kinetic stabilities of the radical anions derived from 1 and 2 to the presence of the anthrylmethyl sps C-S bond found in 1; 2 possesses only aromatic C-S bonds. (2)Griller, D.; Martinho Simbes, J. A.; Sim, B. A.; Wayner, D. D. M. J. Am. Chem. SOC. 1989,111, 7872-7876. (3)(a) Stock, L.M.; Cheng, C.; Wolny, R. Technical Report in the 1987 Annual Report of the Center for Research on Sulfur in Coal, 1987;pp. 9-1-9-3.(b) Stock, L.M. Coal Science; Gorbaty, M. L., h e n , J. W., Wender, I., Eds.; Academic Press: New York, 1982;Vol. 1, pp 161-282.
& s,
\
\
1
Table I. Estimated C-S BDEs f o r the Radical Anions Derived from 9-Phenylthiomethylanthracene(l), 10-Phenylthio-9-methylanthracene (2), P h e n y l Sulfide (3), and Benzyl P h e n y l Sulfide (4), BDEslJ*a n d Reduction Potentials" f o r 1-4, and Oxidation Potentialsa f o r Thiophenoxide Anionla C-S BDE, kcfJ/mol Ed E, substrate (H-A) (H-A)'- H-A (H-A) (PhS-) 9-phenylthiomethylanthracene 11 47 -1.25 0.3 (1)
10-phenylthio-9-methylanthracene (2) phenyl sulfide (3) benzyl phenyl sulfide (4) a In
38
75
-1.3
7 -15
76 54
-2.712b 0.3 -2.7'" 0.3
0.3
volts vs NHE.
A thermochemical cycle that enables determinations of the C-S BDE in the radical anion derived from 9phenylthiomethylanthracene (1)is shown in Scheme I (the cycle for 2 comprises similar parameters). Thermochemical cycles that incorporate related data have been utilized by several groups in examinationsof solution-phaseacidities,' gas-phase radical cation acidities,s gas-phase BDEs? solution phase BDEs for radicals,' solution phase BDEs for neutral closed-shell species?8 solution-phase radical acidities! C-C and C-H BDEs in radical ions? and solution-phase heterolytic BDEs.l0 (4)Juan, B.; Schwarz, J.; Breslow, R. J. Am. Chem. SOC. 1980,102, 5741-5748. ( 5 ) Nicholas, A. M. De P.: Arnold, D. R. Can. J. Chem. 1982, 60, 2165-2179. (6) Brauman, J. I. Frontiers of Free Radical Chemistry; Pryor, W . A., Ed.; Academic Press: New York, 1980. Janousek, B. K.;Brauman, J. I. Gas Phase Ion Chemistry; Academic Prees: New York, 1979; Vol. 2, Chapter 10. (7)Friedrich, L.E. J. Org. Chem. 1983,48,3851-3852. (8)(a) Bordwell, F. G.; Cheng, J.-P.; Harreleon, J. A. J. Am. Chem. SOC.1988,110,1229-1231.Solution-phase BDE data in this article a g m nicely with literature gas-phase data. (b) Bauech, M. J.; Gostowski, R.; Jirka, G.; Selmarten, D.; Winter, G. J.Org. Chem., in prese. Bausch, M. J.; Selmarten, D.; Gostowski, R.; Dobrowolski, P. J. Phys. Org. Chem., submitted for publication. (9)Parker, V. D.;Tilaet, M.; Hammerich, 0. J. Am. Chem. SOC.1987, 109.7!405-7!xM . .- , . - - - . - - - . (10)Amett, E.M.; e t h , K.; Harvey, N. G.; Cheng, J.-P. Science 1990,247,423-430.Summanzedin this article are Amett's recent studiea of organic ions, along with a discussion of how one obtains homolytic bond strengths from ionic and redox data.
C-S and S-S Bond Dissociation Energies
Energy & Fuels, Vol. 5, No. 3, 1991 421
Estimates for C-S bond dissociation energies for C-S bonds present in radical anions derived from 9-phenylthiomethylanthracene (1) and 10-phenylthio-9-methylanthracene (2) obtained with the aid of Scheme I are found in Table I. Examination of the data in Table I reveals that the anthrylmethylsp3C S bond preeent in the radical anion derived from 1 (eq 3)lSis substantially weaker than
Q s,
S-
Table 11. C-H BDEs for the Radical Anions Derived from 9-Phenylthiomethylanthracene(1) and 10-Phenylthio-9-methylanthracene (2), Relative to C-H BDEEfor 1 and 2 (Eq 7); Oxidation Potentials for the Conjugate Bases Derived from 1 and 2, and Reduction Potentiale for 1 and 2 AEiDE E, Ed (H-A
From eq 7; in kcal/mol.
In volts.
E
1
+
le-
5
the indicated aromatic C-S bond present in the radical anion derived from 2 (cleaved as shown in eq 4). The fact
a
AH, + le
AH,
that the reduction of 2 is reversible, while the reduction of 1 (the isomer of 2) is irreversible, is readily explained by comparing the C-S BDEs for 1 and 2 and their corresponding radical anions (Table I). The 38 kcal/mol BDE for the indicated C-S bond in the radical anion derived from 2 is evidently substantial enough to prevent its near-instantaneousfragmentation. On the other hand, the 11 kcal/mol BDE for the C-S bond in the radical anion derived from 1 is of such a magnitude that it is not unexpected that the lifetime of 1 + le- [under the conditions of our experiment (room temperature; DMSO solution)] is very short. A likely mode of decomposition for the radical anion derived from 1 is C-S bond cleavage. These data clearly suggest that the reduction of species that contain C-S bonds substantially weakens selected C S bonds found in these species and that anthrylmethyl sp3 C-S bonds, compared to aromatic C-S bonds, are more susceptible to cleavage when coal and coal model compounds are subjected to conditions leading to their reduction. It is important to recognize that the BDE data in Table I are only estimates-uncertainties of at least 5 kcal/mol are probably warranted for the C-S BDEs in radical anions derived from 1 and 2. The DMSO-phase C S radical anion BDEs are based on the extrathermodynamic cycle shown in Scheme I, a cycle that comprises estimated C-S BDEs for 1 and 2, experimentally determined reduction potentials for 1 and 2, and oxidation potentials for the PhS(11)(a) These values have been estimated with the aid of Beneon's Tablee and related resulta,' and are believed to be accurate to i5 kcal/mol. (b) Colueei, A. J.; Beneon, S. W. Int. J. Chem. Kinet. 1977,9, 295-306. (c) Benson, S. W. Chem. Rev. 1978, 78, 23-35. (12) . la) . , Electrochemiatrv CV conditione lexceot where notad): DMSO solvent; 0.1 M EtdN'BFi hctrolyte; Pt workirig and Ag/AgI reference e l d e e Iferrocenelferrocenium = +OB76 Vas internal standard. valuea corrected &I NHE by subtracting 0.125V). The reduction potential for phenyl sulfide (3) wae obtained in dimethylformamide.12b(b) Griggio, L. J. Electmanal. Chem. Interfacial Electrochem. 1982,140,15b160.(c) Estimated value. (13)That the radical anion derived from O-phenylthiomethylanthracene 11 + le-]degradee ae shown in eq 3 can be deduced from examination of the CV trace obtained when 1 is reduced: the reversible couple at -1.6 V that appeare after the initial irreversible wave at -1.26 V (indicative of the irreversible reduction of 1 and formation of 1 + leh due to the reduction of Smethylantluacene, a species formed when the anthrylmethyl radical (5, formed when 1 + le-cleavee ae shown in eq 3) is reduced and then protonated. I
- H-A?'
substrate (H-A) O-phenylthiomethylanthracene (1) 10-phenylthio-9-methylanthracene (2)
15
(A?* -0.6
(H-A)b -1.21
21
-0.4
-1.3
anion. The C-S BDEs for 1 and 2 (47 and 75 kcal/mol, respectively) have been estimated with the aid of Benson's tables and related results" and are probably accurate to A3 kcal/mol. These estimated BDEs are for gaseous 1and 2. In order to incorporate them into the cycle shown in Scheme I, a cycle that yields DMSO-phase BDEs, we assume that the gas-phase BDEs for 1 and 2 are equal to the BDEs for these species, when dissolved in DMSO. This is likely to be a good approximation, in light of literature that points to a remarkable agreement for solution- and gas-phase homolytic BDE In addition, the lack of reversibility observed in the reduction of 1, as well as in the oxidation of PhS-, also adds some measure of uncertainty to the estimated BDEs in Table I. Since the E , value for PhS- is an essential part of the BDE cycle for both 1 and 2, any uncertainties in this parameter do not affect the difference in the BDEs for radical anions derived from 1 and 2. It is difficult to quantify the uncertainty associated with the lack of observed reversibility in the CV reduction of 1; our best estimate is that the E d value for 1 is shifted (in an anodic direction) from the true reversible potential by about 150 mV (ca 3.5 kcal/mol), based on a similar deviation of 1 from the line established when the reversible reduction potentials for 10 variously 10-substituted-9-methylanthracenesare plotted as a function of their DMSO-phase equilibrium acidities.l6 Nevertheless, listed in Table I is the irreversible peak potential for the reduction of 1. We therefore feel most comfortable in stating that our observations of the reversible CV reduction of 2, in contrast to the irreversible CV reduction of 1, are supported by our estimates of the C-S BDEs for the radical anions derived from 1 and 2. We are most certain in stating that the difference in the BDEs for the radical anions derived from 1 and 2 is about 30 kcal/mol. It is of interest to compare the difference in the C-S BDEh for 1 and 2 (ca. 30 kcal/mol) with the differences in the C-S BDEs for the radical anions derived from 1 and 2 (also ca. 30 kcal/mol). Evidently, addition of an electron weakens the C-S bond present in 1 and 2 to a similar extent. The data in Table I for the C-S BDEs for the radical anions derived from phenyl sulfide (3) and benzyl phenyl sulfide (4) are not as reliable as similar data for 1 and 2, because the reduction potential for 3 was collected in dimethylformamide (DMF), while the reduction potential for 4 is only an estimate. Nevertheless, the irreversibility observed in the DMF-phase reduction of 3 is easily understood: a C-S BDE of 7 kcal/mol is indicative of a bond strength comparable to that of many hydrogen bonds. The (14)(a) Kanabus-Kaminska, J. M.;Gilbert, B. C.; Griller, D. J. Am. Chem. Soc. 1989,111,3311-3314.(b) Mulder, P.; Saastad, 0. W.; Griller, D. J. Am. Chem. SOC.1988,110,4090-4092.
(16)Bausch, M.J.; Guadalupe-Faaano, C. J. Phys. Chem., in press. (16)Howie, J. K.;Houta, J. J.; Sawyer, D. T. J. Am. Chem. Soc. 1977, 99,6323-6326. Surdhar, P.S.;Armstrong, D. A. J. Phys. Chem. 1966, 90,5915-5917.
Bausch et al.
422 Energy & Fuels, Vol. 5, No. 3, 1991
Table 111. Estimated 5-8 B D f i for the Radical Anions Derived from n-Butyl Disulfide and Phenyl Disulfide, BDE1*lland Reduction Potential Data for n-BuS-S(n-Bu) and PhS-SPh, and Oxidation Potentials for the Conjugate Bases Derived from n -Butylmercaptan and Benzenethio1lk S-S BDE, kcal/mol substrate (H-A) (H-A)' H-A E..dH-A)O E,.(RS-)" n-BuS-S(n-Bu) 30 72 -1.6l' 0.2 PhS-SPh 18 55 -1.3 0.3 In volts vs NHE.
data are consistent in suggesting that, in radical anionic sulfides, benzylic spa C-S bonds are substantially weaker than aromatic C-S bonds. Relative C-H BDEs. Another mechanism by which incipient radical anions derived from coal model compounds may degrade is loss of a hydrogen atom, forming the resonance stabilized carbanion (asshown in eq 5). In
-
. . Jvv)
reactions of this type, the radical anion is acting as a hydrogen atom donor. Clearly, since the reduction of 2 is reversible, we would not expect the radical anion derived from 2 to react in this fashion (in DMSO solution). The homolytic BDE for reactions of the type shown in eq 5 (loss of hydrogen from a radical anion), relative to the BDE for reactions of the type shown in eq 6 (loss of hydrogen from
-
a related closed-shell species), can be estimated via eq 7. BDE(H-A A' + H') - BDE(H-A'A- + H') = 23.06[E0,(A-) - Ed(H-A)] (7) +
Equation 7, where Ed(H-A) and Eo,(A-) are in volts, therefore allows estimates of the change in the BDE (in kcal/mol) of a given species, a change tHat results when the molecule in question is reduced. The data necessary to determine the effect (due to reduction) on C-H BDEs for 1 and 2 are found in Table II. Despite the 21 kcal/mol reductive bond weakening effect, the magnitude of the C-H BDE for the radical anion derived from 2 is apparently sufficient to prohibit it from reacting via loss of a hydrogen atom (the gas-phase BDE C-H BDE for 9methylanthracene, forming the anthrylmethyl radical and a hydrogen atom, is about 80 kcal/mol'). Assuming minimal (Le.
