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ation (RRD) in which the donor solvent (i.e., AnH2) itself exchanges hydrogen with the reactant (reaction 3). The application of this type of chemical...
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Energy & Fuels 1996, 10, 235-242

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Study of Hydrogen Shuttling Reactions in Anthracene/ 9,10-Dihydroanthracene-Naphthyl-X Mixtures Isabel W. C. E. Arends*,† and Peter Mulder Center for Chemistry and the Environment, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received July 6, 1995. Revised Manuscript Received October 31, 1995X

In a matrix of anthracene (An) and 9,10-dihydroanthracene (AnH2) (1:1) between 350 and 400 °C in pressurized liquid systems, the rates and mechanisms have been studied for desubstitution of a number of aromatic compounds: naphthyl-X, with X ) Cl, Br, F, D, CH3, NH2, CHdCH2, OCH3, OH, Ph, C(O)CH3. It appears that for these compounds hydrogen transfers via radical hydrogen transfer (RHT) by 9,10-dihydroanthracenyl radicals (9-AnH•) or reverse radical displacement (RRD) with 9,10-dihydroanthracene are the major desubstitution pathways. However, for bromo- and chloronaphthalene, desubstitution is much faster and naphthyldihydroanthracenes and naphthylanthracenes are the main products. In these cases Radical displacement (RD) by 9-AnH• is the predominant route. Also, condensation between An and naphthyl-X is noticed. During these experiments the hydrogenation of aromatic rings was observed as well: anthracene f 1,2,3,4-tetrahydroanthracene and naphthalene f tetralin. This process has been studied by employing naphthalene-d8. The rate of H/D exchange appears to be twice as fast as hydrogenation in the naphthalene molecule. A thermodynamic and kinetic rationale is presented to explain the change in mechanism as a function of the substituent.

Introduction The conversion of biomacromolecules into useful feedstock compounds is widely practiced. Pertaining to a liquid starting material like oil, the pyrolysis at elevated temperatures involves a free-radical chain process to yield the desired chemical building blocks. Solid precursors, however, first need to be converted into fluids. In this liquefaction process, coal is mixed with a hydrogen-donating solvent at 400 °C and high hydrogen pressures, often together with a catalyst. As to the mechanism, the coal structure was believed to break down exclusively through thermal homolytic bond cleavage, followed by scavenging of the radicals by the solvent. However, recently it has been suggested1,2 that another more important mechanism is operative during these coal-liquefaction operations. In order to explain that the best trapping solvents do not necessarily give the highest liquid yield1e and moreover that strong bonds in diaryls are ruptured as well, which given the reaction temperature and time would be thermally stable,3 a new process has been advanced which designates an active role to the solvent. With, e.g., 9,10dihydroanthracene as the solvent, the reactive species for radical hydrogen transfer (RHT) is the 9,10-dihy† Present address: Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. X Abstract published in Advance ACS Abstracts, December 1, 1995. (1) (a) McMillen, D. F.; Malhotra, R.; Chang, S-J.; Ogier, W. C.; Nigenda, S. E.; Fleming, R. H. Fuel 1987, 66, 1611. (b) Malhotra, R.; McMillen, D. F. Energy Fuels 1990, 4, 184. (c) McMillen, D. F.; Malhotra, R. Energy Fuels 1991, 5, 179. (d) Malhotra, R.; McMillen, D. F. Energy Fuels 1993, 7, 227. (e) McMillen, D. F.; Malhotra, R.; Hum, G. P.; Chang, S-J. Energy Fuels 1987, 1, 193. (2) (a) Billmers, R.; Griffith, L. L.; Stein, S. E. J. Phys. Chem. 1986, 90, 517. (b) Billmers, R.; Brown, R. L.; Stein, S. E. Int. J. Chem. Kinet. 1989, 21, 375. (3) Poutsma, M. L. Energy Fuels 1990, 4, 113.

0887-0624/96/2510-0235$12.00/0

droanthracenyl radical which persists in the solution through the fast equilibrium:

An + AnH2 a 2 9-AnH•

(1, -1)

A hydrogen atom is transferred directly from the donor solvent to an aromatic ipso-position in the coal structure (reaction 2). β-Cleavage results in the desired fragmentation of the matrix. However, the importance of RHT has also been questioned.4,5 An alternative hydrogen transfer mechanism is reverse radical disproportionation (RRD) in which the donor solvent (i.e., AnH2) itself exchanges hydrogen with the reactant (reaction 3).

9-AnH• + ArR f An + [ArRH]• f An + ArH + R• (2) AnH2 + Ar f 9-AnH• + [ArRH]• f 9-AnH• + ArH + R• (3) The application of this type of chemical conversion can be extended to other compounds and chemical matrices. For example, it might be utilized to transform lignin into smaller fragments or to detoxify high-boiling aromatic waste streams containing a variety of strongly bonded substituents to include amino and chlorine. In view of recent papers on this subject concerning the debate RHT vs RRD, we feel that more experimental information is necessary which may serve to disentangle these hydrogen exchange pathways. Therefore, we decided to study the hydrogen transfer mechanisms using a variety of mostly non-alkyl substituted naphthalenes as model compounds to elucidate the contribu(4) Autrey, T; Alborn, E. A.; Franz, J. A.; Camaioni, D. M. Energy Fuels 1995, 249, 420. (5) Savage, P. E. Energy Fuels 1995, 9, 590.

© 1996 American Chemical Society

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Arends and Mulder

Table 1. Product Formation in Mixtures of Naphthalene Derivatives and An/AnH2a temp conv naphthyl-X (°C) (%) HX 1-Cl 2-Cl 1-Clc 2-Clc 1-Br 2-Br 1-F 1-Me 2-Me 1-NH2 1-Ph 1-OH 2-OH 1-OCH3 2-OCH3 2-CHdCH2 1-C(O)CH3 2-C(O)CH3 d8

355 339 372 390 355 350 355 388 383 390 405 383 380 365 380 350 365 388 377

25 7 21 24 45 26 2.4 1.1 0.7 0.3 0.2 8 5 11 7 100 12 9 4

N

T

100 6 0.5 100 18 0.5 100 3 100 5 96 57 3 100 + + 100 ? 92 ? 31 ? 57 28 100 14 14 4 2 1

1′-/2′-N-AnH2 2 (only 1′) 8 (only 2′) -

product selectivitiesb (%) 1′-/2′-N-An X-tetralin 58 (4:1) 24 (only 2′) 39 (1:1.7)d 46 (only 2′) 23 (1.3:1) + 78 (1:11)

13 26

others

1 4 + 1 8 69 43 13 9

12 +

Ph-An (14), Ph-AnH2 (58) 1-tetralone (35), dinaphthofuran/dinaphthyl ethers (24) 2-tetralone (85), dinaphthofuran/dinaphthyl ethers (2) 1-OH-N (64), Me-1-OH-N (30), 1-tetralone (2), Me-N (2) 2-OH-N (71), Me-2-OH-N (16), 2-tetralone (6), Me-N (6) 2-ethyl-N (63), Me-2-Et-N (5), 2-Me-N (15), dimers (17) dihydro-1-acetyl-N (61), 1-Et-N (12), 1-Me-N (2) same products, selectivity unknown naphthalene-d7 (80), tetralin-d8 (20)

a Typical conditions, [An] ) [AnH ] ) 2.5 M; [naphthyl-X] ) 0.8 M. Overall conversion reached after 1 h thermolysis at the indicated 2 temperature. +, observed; - not observed. b Product selectivity determined as (moles of product)/(moles of converted naphthyl-X). For X ) Cl, Br, or F: (moles of converted naphthyl-X) ) (moles of HX plus X-tetralin formed). N ) naphthalene, T ) tetralin, N-AnH2 ) 9-naphthyl-9,10-dihydroanthracene, N-An ) 9-naphthylanthracene. c These experiments were performed with 5 M anthracene and 0.8 M reactant, so without 9,10-AnH2. d These data were extracted from an experiment with an intake of 2.7/1 mixture 1-Br vs 2-Br-naphthalene. Both isomers were converted equally fast and the resulting isomer distribution for the main product naphthylanthracene is as indicated.

tion of RHT and other pathways in high-temperature liquid-phase desubstitution. Experimental Section Experiments were performed in sealed Pyrex tubes of 3.65 mL, i.d. 0.54 cm, which can resist pressures up to 20 atm. The introduction of the reaction mixture in the tube was performed by weighing separately a defined amount (usually 3.2 mmol) of the hydrogen donor mixture (e.g. 1:1 AnH2/An) and the reactant, together occupying 15% of the volume. Subsequently, the tubes were degassed three times by freeze/pump/ thaw cycles and sealed under vacuum. Special care was taken to clean the upper end of the tube, so that no reactant could be pyrolyzed during sealing. Even at the highest reaction temperature applied (400 °C) > 90% of the reactants remains in the liquid phase. Total pressures reached amounted to ca. 6 atm. The sealed tubes are put in a stainless steel jacket for safety and placed in a thermostated oven. It took about 20 min for the tube to reach the desired reaction temperature (set value), as measured with a Cr-Al thermocouple inside the stainless steel jacket. Considering the slow reaction rates, this temperature ramp will not significantly change the overall reaction time which was usually 60 min. After the jacket was removed from the oven it was cooled in water, to ensure a rapid decline in temperature. The tubes were opened at liquid nitrogen temperature, with the addition of Milli-Q water in case of inorganic acid determination. Subsequently, toluene together with external standards (bromobenzene and fluoranthene) were added. The mixture was separated, the organic layer was extracted, and the water fraction was analyzed for HCl or HBr using a microcoulometer. Typical concentrations found in the water layer, diluted to a total volume of 100 mL, are 1 × 10-3 M. HF was analyzed by an ion-selective electrode. The organic layer (about 50 mL of solution in toluene) was dried with molecular sieves and analyzed by GC. Samples were analyzed at least in duplicate to minimize analytical errors (in general for high-boiling compounds the analytical accuracy was found to be (10%). A Chrompack 438 or HP 5790 gas chromatograph was used for quantitative analyses. Samples were injected on a CP-Sil-5-CB (50 m × 0.32 mm i.d., film 0.4 µm) capillary column, with ultrapure nitrogen or hydrogen carrier gas and FID detection; temper-

ature program 60 °C (5 min), 10 °C/min up to 280 °C (20 min), or 100 °C (10 min), 10 °C/min to 280 °C. Identification of the peaks was based on comparison of retention time (rt) of authentic samples and/or GC/MS analysis. Identification of naphthyl-AnH2 or naphthyl-An was made by GC/MS according to their parent mass (m/e ) 306 or 304) and fragmentation pattern (loss of naphthyl moiety). The isomer identification was as follows: experiments with mixtures of 1- or 2-chloronaphthalene and AnH2/An resulted in the formation of 9-(1′naphthyl)AnH2 or 9-(2′-naphthyl)AnH2. However, for 1-chloronaphthalene the two isomers of 9-(naphthyl)anthracene were observed (see Table 1). In this case, a (tentative) identification was based on the observation that in experiments with anthracene only, the 2-isomer predominates.6 The rt sequence has been found to be 9-(1′-naphthyl)-9,10-dihydroanthracene, 9-(2′-naphthyl)anthracene, 9-(2′-naphthyl)-9,10-dihydroanthracene and 9-(1′-naphthyl)anthracene. In some experiments, a peak with mass 308 was found with the same rt as 9-(2′-naphthyl)anthracene; a tentative identification is 9-(1′naphthyl)-1,2,3,4-tetrahydroanthracene. For quantification, molar response factors relative to the external standards (bromobenzene and fluoranthene) were used. The factors were derived from calibration mixtures or estimated based on the number of carbons present in the product.

Results Chloronaphthalene. The desubstitution of 1- and 2-chloronaphthalene was studied in liquid pressurized systems at temperatures of 340 to 400 °C. With a mixture of 1:1 9,10-dihydroanthracene/anthracene, 25 and 12% dechlorination (disappearance) for 1- and 2-chloronaphthalene, respectively, was observed in a competition experiment after 1 h at 355 °C. In Table 1, representative product packages are given at 355 and 339 °C, respectively, for 1- and 2-chloronaphthalene. The naphthyl parts ended up mainly as naphthylanthracenes and naphthyl-9,10-dihydroanthracenes, together with minor amounts of naphthalene. Also experiments have been performed with only anthracene (6) Stein, S. E.; Griffith, L. L.; Billmers, R.; Chen, R. H. J. Org. Chem. 1987, 52, 1582.

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Figure 1. Four pathways for conversion of 1-chloronaphthalene.

as the solvent and two illustrative examples are given in Table 1. As can be seen, the reaction is much slower and both isomers of 9-naphthylanthracene are formed for 1-chloronaphthalene. The naphthyl yield is only around 60%, which means 40% of the converted naphthyl parts cannot be accounted for. This could be due a combination of an unknown systematic error and further coupling reactions of the product (naphthyl(dihydro)anthracenes). A priori, this product distribution can be rationalized by the presence of four reaction pathways as illustrated for 1-chloronaphthalene in Figure 1: I. Radical hydrogen transfer. II. Reverse radical displacement: a hydrogen is transferred in a molecule-molecule reaction. III. Radical displacement: 9-AnH• radicals add to the carbon containing the chlorine substituent followed by elimination of chlorine to give naphthyl-9,10-dihydroanthracenes and naphthylanthracenes. IV. Condensation with anthracene to yield naphthylanthracene. As to radical displacement (route III), this has been previously described by McMillen1a for compounds like diphenyl ether. In the case of 1-chloronaphthalene, 9-(1′-naphthyl)-9,10-dihydroanthracene (A) is the primary product which is rapidly converted into 9-(1′naphthyl)anthracene (B) through hydrogen shuttling in the reaction matrix. The existence of condensation (route IV) was retrieved from experiments with anthracene only, to give both 9-(1′- and 2′-naphthyl)anthracene when starting with the 1-chloronaphthalene (see Figure 1). A similar condensation-type reaction was reported by Stein for pyrocondensation of anthracene and anthracene/naphthalene mixtures.7 The mechanism is thought to involve a biradical intermediate from the addition of chloronaphthalene to anthracene, preferably at the 2-naphthyl and 9-anthracene positions, followed by HCl elimination (see Figure 1). The importance of the four routes has been estimated on the basis of product distribution (see Table 2): The formation of naphthalene will occur via RHT or RRD; (7) Choi, C.-Y.; Stock, L. M. J. Org. Chem. 1984, 49, 2871.

Table 2. Pathway Distribution for Conversion of Naphthalene Derivatives in An/AnH2 Mixtures N-X scission side T RHT/ ring chain naphthyl-X (°C) %conv RRD RD condensn hydrog conv 1-Cl 2-Cl 1-Br 1-F 1-Me 2-Me 1-NH2 1-Ph 1-OH 2-OH 1-OCH3 2-OCH3 2-CHdCH2 1-C(O)CH3 2-C(O)CH3 d8

355 339 355 355 388 383 390 405 383 380 365 380 350 365 388 377

25 7 45 2.4 1.1 0.7 0.3 0.2 8 5 11 7 100 12 9 4

6 18 60 92 31 57 100 28 4 2 1 13 26 80

66 47 13 -

26 35 23 99

2 1 4 1 8 69 43 48 94 73 ? 20

24 2 98 99 100 14 ? -

the naphthyldihydroanthracenes originate from RD and naphthylanthracenes are formed via both RD and the condensation pathway. The relative contributions of pathways III and IV are derived from formation of the two isomers of naphthylanthracene in experiments with mixtures of chloronaphthalene and anthracene only, under the same conditions. An additional pathway is the hydrogenation of the aromatic ring to yield chlorotetralin (vide infra). For 2-chloronaphthalene, routes I, II, and III are slower than for 1-chloronaphthalene, but route IV is essentially equally fast. A series of experiments have been performed where instead of An/AnH2 mixtures either pure 9,10-dihydroanthracene or anthracene were used. Results show that the ratio of the rate constants for overall disappearance of 1- and 2-chloronaphthalene increases from unity in mixtures with anthracene, where route IV predominates, to around 2.5 in mixtures with 9,10-dihydroanthracene, where all four pathways are present. Other Substituted Naphthalenes. Under identical experimental conditions the reactivity of other substituted naphthalenes has been investigated. In Table 1 representative examples are given. As can be seen in

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most cases, naphthyl-X cleavage is relatively slow for compounds other than chloro, bromo, and fluoro. Hydrogenation of the aromatic ring as well as conversion of the side chain becomes relatively more important. With 1-bromonaphthalene, the conversion to HBr is found to be 45% at 355 °C, i.e., faster compared to chloronaphthalene. The analogy with chloronaphthalene is evident; radical hydrogen transfer, reverse radical displacement, radical displacement, and condensation with anthracene take place. For 1-fluoronaphthalene, slow desubstitution, 2.4% HF is formed at 355 °C, is observed, however, without any formation of naphthalene. From the products it can be inferred that condensation with anthracene (route IV) is the only desubstitution mechanism. For methyl-, amino-, and phenylnaphthalene the naphthyl-X cleavage seems, in contrast with Cl- and Brnaphthalene, to be not importantsless than 1% of naphthalene is formed at 380-405 °Csaccompanied by a similar degree of ring-hydrogenation. In case of hydroxynaphthalenes relatively large yields of their hydrogenated analogues are found. Especially 2-OHnaphthalene gives high yields of 2-tetralone (see Table 1). What appears to be a side chain conversion is encountered for methoxynaphthalene where naphthols are the main products: 1-methoxynaphthalene, 13% conversion after 1 h at 365 °C, with a selectivity of 67% to 1-naphthol, and 31% to 2- and 4-methyl-1-naphthol. Note that the rate for naphthyl-X cleavage is again very slow. A fast hydrogenation of the side-chain of 2-vinylnaphthalene into 2-ethylnaphthalene (100% at 350 °C) is encountered. In acetylnaphthalene the side chain is hydrogenated to give ethyl- and methylnaphthalene as products. Experiments were performed with deuterated naphthalene of 98.4 mol % D (88.1% d8, 8.1% d7, 3.7% d4, MS -analysis) and 1.6% perdeutero-tetralin (80% d12, 9.7% d11, 10% d10). After reaction of naphthalene-d8 for 1 h at 377 or 398 °C, the d7-content has increased from 8 to 11%, at the expense of naphthalene-d8. Again, on the basis of product formation, the degrees of conversion according to the mentioned desubstitution mechanisms I/II, III, and IV as well as hydrogenation of the aromatic ring and conversion of the substituent, have been calculated (see Table 2). Discussion This study shows that, when comparing the various substituted naphthalenes, two categories of naphthyl-X cleavage can be envisaged. For halogenated compounds (X ) Cl, Br) desubstitution is relatively fast and is accompanied by anthracene-naphthyl adduct formation, while for other substituents hydrodesubstitution is much slower (X ) CH3, OH, etc.). First, the mechanism for the N-X cleavage pathway, observed for all substituents, is addressed which leads to HX and naphthalene and that has been assumed in literature to occur via RHT or RRD. Selectivity of 1- vs 2-Isomer. In the classic example with 9-AnH• Malhotra1b found that the selectivity between attack at the 1 and the 2′ position of 1,2′dinaphthylmethane can reach as high as 7 (400 °C). We tried to elucidate this selectivity in mixtures of 1- and 2-chloronaphthalene. The ratio for overall disappear-

Arends and Mulder

(4)

ance of the 1- and 2-isomer of chloronaphthalene is only around 2, but in view of the multiple pathways of conversion the selectivity for hydrodesubstitution via RHT could well be higher. RHT vs RRD. Evidence for the existence of the RHT reaction has been presented in coal type systems by Stein,2 McMillen,1 and Savage.5 However, also for other matrices RHT has been suggested, for example, by intermediates during hydrogenation of benzophenone7 and hydrogenation of olefins by alkyl radicals.8 At present, the exact mechanism by which hydrogen transfer takes place is not fully resolved.4,9 Calculations9a have pointed out that direct hydrogen transfer from resonance stabilized dibenzocyclohexadienyl radicals to a naphthalene would require energy barriers over 30 kcal/mol. An alternative10 c has been advanced, involving a three-step process: first addition of a 9-AnH• to a neighboring position followed by hydrogen transfer and β-scission of the anthracene moiety. The other possibility is that 9-AnH• is not the reactive species but that AnH2 transfers a hydrogen to the ipso-position in the naphthalene in a molecule-molecule reaction: RRD. In our case already at 350 °C substantial levels of naphthalene formation are observed, while most studies dealing with RHT/RRD are performed at 400 °C. The contribution of I and II can be calculated in representative examples (parameters from Savage5):

RHT (9-AnH• + N-X): k/M-1 s-1 ) 108.1 exp(-26.1 kcal mol-1/RT)10 (I) RRD (AnH2 + N-X): k/M-1 s-1 ) 109.2 exp(-47.0 kcal mol-1/RT)10 (II) In this case an intrinsic barrier for RHT of 16.5 kcal/ mol is assumed. This is consistent with the experimentally determined barrier for hydrogen transfer by 1,2diphenylethyl radicals as studied in bibenzyl thermolysis,11 where hydrogen is transferred to and from a resonance stabilized radical, as in the presently considered reactions. Hence the ratio of v(RRD)/v(RHT) ) 12.3 exp(-20.9 kcal mol-1/RT)*[AnH2]/[g-AnH•] with [AnH2] ) 2.5 M (see Table 1), and for [9-AnH•] the equilibrium concentration (the rates for reaction 1 and -1 dictate that the equilibrium concentration is reached within seconds, while the total reaction time is 1 h!). With5 K1(eq) ) 1.3 exp(-31.6 kcal mol-1/RT),12 this results in [9-AnH•]eq (8) Metzger, J. O. Angew. Chem., Int. Ed. Engl. 1986, 25, 80. (9) (a) Camaioni, D.; Autrey, S. T.; Ferris, K. F.; Franz, J. A. Sixth International Symposium on Organic Free Radicals; Noordwijkerhout: The Netherlands; Book of Abstracts, 1992; p 75. (b) Camaioni, D.; Autrey, S. T.; Franz, J. A. J. Phys. Chem. 1993, 97, 5791. (c) Freund, H.; Matturo, M. G.; Olmstead, W. N.; Reynolds, R. P.; Upton, T. H. Energy Fuels 1991, 5, 840. (10) For RRD, from Savage,5 with log A ) 8.6, reaction path degeneracy (RPD) ) 4, E0 ) 9 kcal/mol, R ) 0.82, and ∆H ) 46.3 kcal/ mol. For RHT, log A ) 7.8, RPD ) 2; E0 ) 16.5 kcal/mol; R ) 0.65 and ∆H ) 14.7 kcal/mol. (11) Miller, P. E.; Stein, S. E. J. Phys. Chem. 1981, 85, 580. (12) The entropy contribution in the equilibrium constant is very small. The reaction path degeneracy for the forward and backward reaction and spin factors result in ∆S ) 0.6 eu. ∆H ) 31.6 kcal/mol.

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Energy & Fuels, Vol. 10, No. 1, 1996 239

Table 3. Heats of Formation of Species of Interest AnH2 An 9-AnH• 1-AnH• N T 1-NH• 1-Cl-N 2-Cl-N Cl• H• N-An N-AnH2

9,10-dihydroanthracene anthracene 9,10-dihydro-9-anthracenyl 1,2-dihydro-2-anthracenyl naphthalene tetralin 1,2-dihydro-2-naphthyl 1-chloronaphthalene 2-chloronaphthalene chlorine atom hydrogen atom 9-(1′-naphthyl)anthracene 9-(1′-naphthyl)-9,10-dihydroanthracene

∆Hf

ref

38.2 55.2 62.5 72.2 36.1 6.0 58.1 29 29 29 52.01 91.5 77

29, 30 29, 30 31 31 29, 30 30, 32 33 34 34 29 29 17, 18 17, 18

of 8.2 × 10-6 M at 350 °C and 2.1 × 10-5 M at 400 °C, leading to v(RRD)/v(RHT) is 1/5.7 at 350 °C and 1/4.1 at 400 °C. Because the activation energy for RRD is higher, this reaction will become less important at lower temperatures. This thermochemical kinetic evalution shows that at 350 °C, accepting a “low” intrinsic barrier of 16.5 kcal/ mol, RHT seems to be the major pathway. However, if we calculate the experimental desubstitution rate for, e.g. 1-chloronaphthalene at 355 °C, we observe that it is 5 times higher than the predicted RHT rate:13 Only the RHT parameters can be fitted to the data, since the RRD rate parameters are invariant since they comprise an activation energy close to the reaction enthalpy. This would mean that the intrinsic barrier will be even lower than 16.5 kcal/mol. Although speculative and beyond the scope of this paper, another desubstitution pathway, i.e., electron transfer, may be more important for reactions between halonaphthalenes and 9,10-dihydroanthracene. At 400 °C the situation for methylnaphthalene is quite different. The rate for naphthyl-methyl cleavage is 2.7 × 10-6 s-1, while the predicted rates for RHT and RRD are 8.8 × 10-6 and 2.1 × 10-6 s-1 (vide supra), respectively. Suppose RHT is the pathway, then a difference in reversibility could account for the actually lower observed route (vide infra).14 So at this point we must conclude that in the case of “slow” desubstitution at 400 °C, considering the fact that all rate parameters are obtained through estimation, both pathways, RHT and RRD, are contributing. Thermokinetic Rationale. Comparing Different Substituents. Without discussing the fate of each individual compound, it seems appropriate to derive general guidelines for the reactivity in the An/AnH2 environment by thermochemical assessment (Table 3). Radical Hydrogen Transfer. For RHT, the activation barrier for the initial H-transfer has been estimated to be at least 25 kcal/mol according to Malhotra1b (or 26 kcal/mol in our case with an 1.2 kcal/mol lower heat of formation for 9-AnH•). The overall rate constant for RHT is determined by three reaction steps: (a) initial H transfer, (b) the reverse reaction, and (c) elimination of X• (Figure 2b + reaction 5). (13) The k(RHT)[9-AnH•] at 355 °C amounts 0.104 × 9.0 × 10-6 ) 9.4 × 10-7 s-1 and k(RRD)[AnH2] ) 2.5 × 7.0 × 10-8 ) 1.7 × 10-7 s-1, whereas at 355 °C for naphthalene formation in 1-chloronaphthalene, with a selectivity for naphthalene formation of 6% and an overall conversion of 8.0 × 10-5 s-1 the rate is 4.8 × 10-6 s-1. (14) In this discussion the influence of the substituent on the BDE (C-H) in the adduct radical (e.g., I) is ignored; however, this is not likely to affect the RHT/RRD ratio.

a

9-AnH• + naphthyl-X a An + (naphthyl-H,X)•[I] b c

a An + naphthalene + X• (5) d Once the intermediate I is formed, the reverse bimolecular reaction must compete with a relatively fast unimolecular loss of X•. For chloronaphthalene, step a is therefore rate determining and the rate constant equals that for the initial RHT process. However, when for the second step the barrier for X elimination increases from 15 kcal/mol (X ) Cl) to 30 kcal/mol for X ) CH3,15 the initial addition becomes reversible and the overall rate constant is calculated to drop by a factor of 5. Therefore, it is evident that also RHT with 9-AnH• is not feasible for fluoronaphthalene (C-F bond ) 126 kcal/mol). Radical Displacement. Analogously the RD process involves three reaction steps (reaction 6).

9-AnH• + Cl-N a (naphthyl-Cl,AnH)•[Z] f N-AnH + Cl• (6) The energy diagram for this reaction is given in Figure 2a. In the case of 1-chloronaphthalene, the initial addition is only slightly exothermic based on an estimated heat of formation for the intermediate adduct radical, ∆Hf(Z) of 90 kcal/mol.16,17 The activation energy for this addition can be calculated as follows while accepting that RD is an irreversible process. Using the derived selectivity toward RD (ca. 65%) at various temperatures (340-400 °C), the equilibrium concentration for 9-AnH• (9.0 × 10-6 M at 355 °C,11 and assuming that the preexponential factor A is 108.2 M-1 s-1, Ea becomes 21 kcal/mol. In retrospect, the reverse reaction (decomposition of Z into 9-AnH• and Cl-N) is slower since the release of Cl requires ca. 16 kcal/mol only.18 With this information it can be predicted that the contribution of RD is substantial only if the C-X bond dissociation energy in intermediate Z is lower than the barrier, of approximately 23 kcal/mol, for the reverse reaction as is the case for chloro- and bromonaphthalene. Thus, RD is irreversible when the C-X bond dissociation energy (BDE) in the starting naphthyl-X is lower than 95 kcal/mol, e.g. with X ) Cl, Br. Hence with, e.g., CH3, Ph, and F no RD is expected as is indeed the case; see Table 1. The relative stability of (Z) (e.g., the difference between the 1- and 2-isomer) will also depend on the position of the newly formed carbon-carbon bond. Thermochemical calculations have shown that addition to the 1-position relative to the 2-position yields a bond which is 4 kcal/mol stronger and hence the Ea is likely to decrease. Thus, under the conditions that the first (15) The bond for X ) Me in intermediate I is 22 kcal/mol18 and with an estimated activation barrier for the addition of methyl of 8 kcal/mol as for Me• + benzene (see Holt, P. N.; Kerr, J. A. Int. J. Chem. Kinet. 1977, 9, 185) this adds up to an Ea of 30 kcal/mol. Note that additions to naphthalene and benzene have comparable activation energies. (16) The heat of formation is calculated from the heat of formation of naphthyldihydroanthracene (see Benson18) and adding the increment as observed for ∆Hf(chlorohydronaphthyl) - ∆Hf(naphthalene). (17) Stein, S. E. NIST structures and Properties Database, ver. 2.01; National Institute of Standards and Technology, Gaithersburg, MD, 1994. (18) Estimated with group additivities, Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley, New York, 1976.

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Figure 2. Energy diagrams for radical displacement and radical hydrogen transfer between 9,10-dihydroanthracenyl radicals and substituted naphthalenes. Enthalpies in kcal/mol.

Ru¨chardt22 for the transfer hydrogenation of R-methylstyrene with 9,10-dihydroanthracene to give cumene at 280-310 °C. Accordingly, acetylnaphthalene, in which the carbonyl group has an hydrogen affinity of 41 kcal/mol,18 is hydrogenated via RRD to ethylnaphthalene. A similar reactivity is encountered for benzophenone7to yield diphenylmethane. Interference by Free H Atoms. From the literature1b it is known that at 400 °C with a 1:1 AnH2/ An mixture the contribution of free H atoms is less than 10%. Free H-atoms exhibit a very low selectivity in conversion of 1 vs 2-chloronaphthalene, and minor contribution would also partially explain the rather low difference in conversion rates for 1- and 2-substituted naphthalenes reported here. Possible interference of H atoms in the present system can be assessed using a simple kinetic model consisting of reactions 8, -8, 9, and 10.

9-AnH• a An + H• step is irreversible conversion of the 1-chloronaphthalene will be faster than the 2-isomer. Reverse Radical Displacement. The following examples for RRD could be pointed out in the studied matrices: In the calculation for the equilibrium concentration of 9-AnH•, the existence of RRD is implicitly assumed. The exchange of hydrogens is fast in our time frame5 (k1/M-1 s-1 ) 109.5 exp(-34.9 kcal mol-1/RT) and v1 ) 5.0 × 10-3 s-1 at 350 °C, while overall conversion for 1-chloronaphthalene is in the order 8 × 10-5 s-1. As derived above, RRD could also play a role in the conversion of methylnaphthalene. Suppose the transfer of hydrogen from AnH2 to the ipso position is reversible like for RHT, then loss of methyl from intermediate I is competing with the backward reaction (Figure 2b). In the case of RRD, the loss of methyl (k/s-1 ) 1014 exp(-30/RT) competes with a radical disproportionation (k/M-1 s-1 ) 109.1 exp(-3 kcal mol-1/RT) with dihydroanthracenyl radicals19 ([9-AnH•] ) 2.1 × 10-5 M at 400 °C). Thus at 400 °C the net rate constant due to reversibility k′ ) 0.87kRRD. Another example of RRD is the hydrogenation of 2-vinylnaphthalene. Because of the high hydrogen affinity20 of the vinylic group (45 kcal/mol),18 this compound is very effective in a molecule-molecule hydrogen exchange reaction (reaction 7):

(7)

Rate parameters have been assigned to reaction 7 of k7/M-1 s-1 ) 109.2 exp(-31 kcal mol-1/RT) or k ) 5.3 × 10-2 M-1 s-1,21 meaning that with the reaction time of 1 h, >99.99% conversion is achieved (see Table 1). Similar observations have recently been made by (19) Arends, I. W. C. E.; Mulder, P.; Clark K. B.; Wayner, D. D. M. J. Phys. Chem. 1995, 99, 8182. (20) Defined as the exothermicity of hydrogen atom addition. E.g. the H affinity of anthracene at the 9-position amounts 45 kcal/mol, for naphthalene at the 1-position 30 kcal/mol and the 2-position 26 kcal/mol, and for vinylnaphthalene 45 kcal/mol; see also Stein.31 (21) The preexponential factor was taken from Savage5 and Ea is ∆H of the reaction, see Ru¨chardt.22

(8, -8)

9-AnH• + Cl-N f Cl• + N + An

(9)

H• + Cl-N f Cl• + N

(10)

Here RHT is assumed to be the major pathway as elucidated above and for [9-AnH•] the equilibrium concentration taken. k8 ) 1013.6 exp(-43.9/RT); k-8 ) 1010.8 exp(-4.7/RT); k9 ) 108.1 exp(-26.1/RT); and k10 ) 1010.48 exp(-8.2/RT) (all data in M, s, kcal/mol units, [An] ) [AnH2] ) 2.5 M; [1-Cl-N] ) 0.8 M). The parameters for k8 are estimated from Savage;5 k-8 and k10 are taken as for H addition to hexadeuterobenzene and for hydrodechlorination of chlorobenzene, respectively.23 With these data v9/v10 was calculated to range from 500 at 350 °C to 140 at 400 °C. So, once generated, H atoms are taken up very fast by anthracene (reaction -8) and in this way the [H•] concentration remains low. Therefore, the influence of free H• will be negligible in the conversion of chloro- and other substituted naphthalenes. Hydrogenation of the Aromatic Ring. In the RHT reaction, the fate of the intermediate radical depends on the strength of the C-X bond (see above). With an increasing strength the desubstitution process is slowed down due to reversibility. Under those conditions, several other pathways become visible: ring hydrogenation to yield tetralin from naphthalene, 1,2,3,4tetrahydroanthracene from anthracene, or bond rupture as in the case of methoxynaphthalene; see Figure 3. Now the second step involves the β-cleavage of a weak O-C bond, and hence RHT with methoxynaphthalene remains irreversible.24,25 As to hydrogenation of the naphthalene ring, e.g., 1-CH3-naphthalene is converted into 1- or 8-CH3-(1,2,3,4)tetralin. In general, yields are very low, ≈0.5% at 380 °C. A special type of hydrogenation is encountered for (22) (a) Ru¨chardt, C.; Gerst, M.; No¨lke, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 1523. (b) Gerst, M.; Ru¨chardt, C. Chem. Ber. 1993, 126, 1039. (23) Manion, J. A.; Louw, R. J. Phys. Chem. 1990, 94, 4127. (24) Direct homolysis CH3O-naphthalene f CH3• + •O-naphthalene is not conceivable, with k ) 1015 exp(-62 kcal/mol/RT) in 1 h, a conversion of 0.3% is reached at 370 °C. (25) Arends, I. W. C. E.; Louw, R.; Mulder, P. J. Phys. Chem. 1993, 97, 7914.

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Energy & Fuels, Vol. 10, No. 1, 1996 241

Dedeuteration of Naphthalene-d8. Apparently, desubstitution of deuterium is a well-defined but slow reaction in this temperature range and the following equilibrium will take place.

ND8 + 9-AnH• a (ND8H)• + An a ND7H + 9-AnD• (11) ND8 + AnH2 a (ND8H)• + 9-AnH• a ND7H + AnHD (12) Figure 3. Hydrogen transfer via RHT to methoxynaphthalene leading to naphthol.

Figure 4. Two routes (besides RHT or RRD in Figure 1) for hydrogenation of 1-chloronaphthalene.

X-naphthalene (X ) Cl, Br), where besides chloro- and bromotetralin, also unsubstituted tetralin was observed (chlorotetralin:tetralin ) 2:1, bromotetralin:tetralin ) 1.3:1). The yield of tetralin in experiments with 1-chloronaphthalene is about 8% on formed naphthalene. Since hydrogenation of the naphthyl moiety is not observed in the case of 1-fluoronaphthalene under the same conditions (see Table 1), another route of formation needs to be advanced. The mechanism for tetralin formation is thought to occur via addition of hydrogen to the 4-position (transfer to R-positions is faster), followed by H abstraction from 9,10-AnH2 and HX elimination (as shown for X ) Cl, route b in Figure 4). The resulting dihydronaphthalene is hydrogenated rapidly into tetralin through RRD (which under these conditions is fast (see Table 1, entry vinylnaphthalene). Attack of hydrogen at other positions leads to either X-tetralin or naphthalene (route a and RHT/RRD in Figure 1, respectively). Note that instead of RHT, as depicted in Figure 4, also RRD could be the initial hydrogen transfer pathway. The formation of 1,2,3,4-tetrahydroanthracene in the hydrogen donor mixture, about 20% on initial [An] at 350 °C, is also the result of a similar mechanism, as has been previously noticed by Autrey.4 The fact that tetrahydroanthracene formation is much faster than tetralin production is due to the difference in thermokinetics for the first step: The enthalpy for H addition to the 1-position is 5 kcal/mol more exothermic in case of anthracene (see Table 2) and hence the Ea for RHT or RRD will be lower as well. Overall, in the X-naphthalene case the rate for hydrogenation of the aromatic ring is in the same range as desubstitution and underscores the presence of similar hydrogen transfer pathways. This was further studied, using naphthalene-d8 in a 1:1 An/AnH2 mixture.

Irrespective of the mechanism RHT or RRD, according to eqs 11 or 12, respectively, the reaction enthalpies for H or D transfer are expected to be similar and hence the ratio of H (backwards) or D (forward) transfer from (ND8H)• will be governed by statistics only. The hydrogenation of ND8 to give tetralin is about 2 times slower than dedeuteration; 1.1-1.4% additional tetralin is formed from naphthalene and 3% naphthalened7, emphasizing that direct hydrogenation of naphthalene is slow. The tetralin, whether already present or newly formed, exchanges H/D quite rapidly. The absolute amount of tetralin-d12 decreases with 88% at 377 °C, and with26 f(d12) ) (1 - z),12 with z the dedeuteration percentage, an overall dedeuteration of 16% is calculated (at 398 °C: 23%). Deuterium incorporation is indeed observed in all anthracene derivatives as expected from reaction 11 or 12. Starting from perdeuterotetralin (denoted TD12), reaction 12, a hydrogen abstraction, seems most plausible:

TD12 + 9-AnH• f TD11• + AnHD

(13)

TD11• + AnH2 f TD11H + 9-AnH•

(14)

Once the TD11• is formed, it will revert to TD11H (reaction 14) under the experimental conditions. From the data, the activation energy for reaction 13, hydrogen abstraction by 9,10-dihydroanthracenyl from tetralin, can be determined when accepting the equilibrium concentration for 9-AnH• and A13 as 108.5 M-1 s-1. With [9-AnH•]eq ) 1.4 and 2.1 × 10-5 M at 377 and 398 °C, this leads to Ea ) 20-21 kcal/mol. This reaction is another example of H transfer to and from resonance stabilized radicals just as in RHT. With an intrinsic barrier of 16.5 kcal/mol and ∆H13 ) 10.7 kcal/mol, an activation energy of 23 kcal/mol would have been predicted. Concluding Remarks Experiments have demonstrated that with Cl and Br the hydrodesubstitution rates are faster than expected based on current thermochemical parameters for RHT and RRD. The mechanisms for desubstitution of arenes involving the 9-An•/An system appear to involve four pathways: radical hydrogen transfer, reverse radical displacement, radical dispacement, and condensation with the anthracene moiety. The actual reaction mode will depend on the energetics of the intermediate species. Overall rates can therefore be related to the C-X bond in the starting material, the intrinsic activa(26) From probability theory, it can be calculated that if some fraction z of total deuterium has been removed from a sample of C10D12, the fraction of each deuterotetralin is given by f(C10D12H12-n) ) {12!/ n![(12 - n)!]}(1 - z)12(z)12-z; n ) 0-12.

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tion barriers, and entropy changes. From this work it can be concluded that the faster RD route is favoured for X-naphthyl compounds with C-X bonds below ca. 95 kcal/mol. A similar selectivity was found for gas phase systems with hydrogen over beds of activated carbons.27,28 Complete dehalogenation of chloro- and bromobenzene is already achieved at 500 °C within 4 s, whereas compounds like methyl- and hydroxybenzene show (27) Arends, I. W. C. E. Thesis, Leiden University, The Netherlands, 1993. (28) Arends, I. W. C. E.; Ophorst, W. R.; Louw, R.; Mulder, P. Carbon, submitted for publication. (29) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic Press: New York, 1970. (30) Shaw, R.; Golden, D. M.; Benson, S. W. J. Phys. Chem. 1977, 81, 1716. (31) Stein, S. E.; Brown, R. L. J. Am. Chem. Soc. 1991, 113, 787. (32) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds; Chapman and Hall: London, 1986. (33) With H affinity of 30 kcal/mol which is taken from Malhotra1b (H affinity at 2-position is 26 kcal/mol). This seems to us a more realistic value then the 32 and 24 kcal/mol for H affinity at the 1- and 2-position in naphthalene as calculated by Stein.31

Arends and Mulder

moderate conversion on activated carbon around 500 °C and amino- and fluorobenzene are inert. Imagining activated carbon as sheets of polycyclic (hydro)aromatics, a similar ruling mechanism as found here in anthracene/9,10-dihydroanthracene systems might account for the large difference between the behavior of chloro- and bromoarenes on the one side compared to other substituted arenes on the other side. The presence of RD in the case of halogenated aromatics desubstitution results, when applied to e.g. hazardous waste, to an increase of the molecular weight of the reaction mixture. Hydrochloric acid is released (i.e., the feed is detoxified) at the expense of the formation of polycyclic compounds. EF950128F (34) The difference of 4 kcal/mol between 1- and 2-chloronaphthalene as in Pedley32 is not considered to be reliable, because in general ∆Hf’s for 1- and 2-substituted naphthalenes differ by less than 1 kcal (compare 1 vs 2-iodonaphthalene in Pedley of 0.3 kcal/mol). The value of 29 kcal/mol is closest to the value calculated with group additivities17 of 28.9 kcal/mol.