J . Phys. Chem. 1986, 90, 517-523 CHART I
CHiCl CHC13 CF$I CFC13
CHZC12 CCl4 CF2CI2 CHFCl2
0.0 (0.65) 0.42 (1.35) 0.0 (CO) 0.83 (1.70)
0.26 0.74 0.49 0.54
(1.20) (1.90)
(0.0) (0.0)
measurements. Using their method we amve at the valuesI9shown in Chart I (in eV, values derived from the present experiments are also shown in parentheses). Wentworth's method is based on an empirical correlation and should be used with caution. Furthermore, it involves the calculation of the difference between rather large numbers, so that errors on the order of f 0 . 2 eV are expected. It is noted that the method leads to results in the range of 0-1 .O eV, while ours spans a range that is twice as large. The difference, which appears to (19) Bond energies are from D. F. McMillen and D. M. Golden, Annu. Rev. Phys. Chem., 33, 493 (1982).
517
be beyond the estimated error for both methods, may stem from the fact that different processes relate to different values of EA (between the adiabatic and vertical values, see ref 1). In conclusion, we find that the electron-jump model is in qualitative agreement with the experimental results for molecules whose lowest-lying electronic excited states are higher in energy than the A state of NO. For such molecules, a rough estimate for the electron affinity may be derived and is given in the last column of Table 11. Acknowfedgment. This work was partially supported by the Robert Szold Institute for Applied Science. Experimental help by S. Ruhman and E. Zylberstein is gratefully acknowledged. Registry No. NO, 10102-43-9; Hf, 1333-74-0; N2, 7727-37-9; CH,, 74-82-8; SiF4,7783-61-1; CF,, 75-73-0; CF3C1,75-72-9; CF2C12,75-71-8; CFC13, 75-69-4; CC14, 56-23-5; CHSCCl,, 7 1-55-6; CHCIzF, 75-43-4; CHCIF2,75-45-6; CHClzBr, 75-27-4; CH3Br,74-83-9; SF6,255 1-62-4; CF31, 2314-97-8; CHJ, 74-88-4; benzene, 71-43-2; toluene, 108-88-3; chlorobenzene, 108-90-7; bromobenzene, 108-86-1.
Hydrogen Transfer between Anthracene Structures R. Billmers, L. L. Griffith, and S. E. Stein* Chemical Kinetics Division, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: July 26, 1985)
This work reports results of kinetic studies of hydrogen migration between 9,lO-dihydro positions in anthracene structures. The transfer of,two H atoms from 9,lO-dihydroanthracene (AnH2) to 2-ethylanthracene (EAn) follows simple bimolecular kinetics with k/M-'s-I = 109.w.'4 exd-18540 f 180/5') (250-375 "C). At 300-350 O C , H transfer to 9,lO-dimethylanthracene led to nearly equimolar mixtures of cis- and tranr-9,1O-dimethyl-9,1O-dihydroanthracene, consistent with a free radical mechanism. The rate-limiting step appears to be transfer of a single benzylic H atom from a donor molecule to an acceptor molecule, resulting in the formation of two highly stabilized free radicals. Reactions of this nature are likely to serve as major sources of free radicals in condensed-phasethermolysis reactions. Measurements of their rate constants offer a new, relatively direct means of determining bond strengths. From the above rate expression, we derive an AnH-H bond strength of 78.4 f 1.8 kcal mol-'. From literature data for a similar reaction (Halpern et al.) we obtain an H-Mn(CO)5 bond strength of 63 kcal mol-'. Based on an observed lowering of the reaction rate with added anthracene, a rate constant was derived for 0-H transfer from a 2-ethyl-9-hydroanthryl radical to anthracene. At 350 'C, this value was 120 M-I s-', indicating a high activation barrier ( 18 kcal mol-I) for this seldom reported process. N
Introduction Termination of organic free radicals can occur through recombination or, if one radical has a labile hydrogen atom, through disproportionation
R
+ QH 5R-QH h i p
-RH+Q Many examples of both reactions are known, and there exists a large body of relevant rate data.' Radical formation can occur through the reverse of each of the above two processes. Unimolecular bond homolysis, the reverse of radical recombination, has been widely studied as a source of free radicals and as a means of obtaining free radical heats of formation.2 The reverse of radical disproportionation, on the other hand, is a bimolecular process which has hardly been studied at all. This is perhaps most evident from the absence of any widely (1) (a) Kerr, J. A.; Moss, S. J. 'Bimolecular and Termolecular Gas Reactions"; CRC Press: Boca Raton, FL, 1981; Vol. 11. (b) Gibian, M. J.; Corley, R. C. Chem. Rev. 1973, 73, 441. (2) (a) McMillan, D. F.;Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493. (b) Benson, S. W. Natl. Stand. ReJ Data Ser. (US., Natl. Bur. Stand.) 1970, NSRDS-NBS 21.
used name for this reaction. We will call it molecular disproportionation (MD). M D reactions occasionally appear in reaction mechanisms of high-temperature, gas-phase decomposition reactions. Due to mechanistic complexity, however, rates of these reactions cannot in general be reliably derived from these studies. An exception is the reaction of ethylene with ~yclopentene,~ for which Benson has proposed an M D reaction as the rate-limiting step.4 M D reactions may be even more important in condensed-phase thermal reactions where the high concentrations encourage bimolecular proce~ses.~These reactions are, in fact, a class of "molecule-assisted homolysis" (MAH) processes which Pryor6a has long proposed as common radical initiation steps in liquidphase reactions. One of the few examples of such a reaction for which rate constants were measured was reported by Gajewski (3) Benson, S . W. Int. J . Chem. Kinet. 1980, 12, 755. (4) Lalonde, P. J.; Back, M. fnr. J. Chem. Kinet. 1980, 12, 301. (5) Stein, S. E. In 'New Approaches in Coal Chemistry"; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 169, p 97. (6) (a) Pryor, W. A. 'Free Radicals"; McGraw-Hill: New York, 1957. Graham, W. D.; Green, J. G.; Pryor, W. A. J . Org. Chem. 1979, 44, 907. Pryor, W. A. In 'Free Radicals in Biology"; Pryor, W. A., Ed.; Academic Press: New York, 1976; Vol. I, pp 1-43. (b) Gajewski, J. J.; Gortva, A. M. J . Am. Chem. SOC.1982, 104, 334.
This article not subject to US.Copyright. Published 1986 by the American Chemical Society
518
Billmers et al.
The Journal of Physical Chemistry, Vol. 90,No. 3, 1986
-3.01/
anthracene, and 2-ethylanthracene.
T/"C
375 325
275
225
Results and Discussion 9,10-Dihydroanthracene/bEthylanthracene Reaction. Results of our kinetic studies of reaction 1 are given in Table I. For
\
-4.04
AnHz
EAn
An
-
7
. 0 1 1.75 2.0 11~(103~) Figure 1. Apparent second-order rate constant for AnHz EAn An + EAnH,, koM,vs. Ti based on EAnH2 formation. Rate constants were derived from Table I by using mean values between 0.1 I [AnH2]/[EAn] I 10. 1.5
+
-
and Gortva for H transfer between two p-isotoluene molecules (3-methylene- 1,4-~yclohexadiene).~~ Their measured activation energy of 21.8 kcal mol-I was close to the (very approximate) reaction enthalpy. However, largely because of the high reactivity of the reactant molecules, the precise fate of the radicals could not be deduced from these studies. In our studies of hydrogen transfer mechanisms in high-temperature liquids, we found, and report here, convincing evidence for the dominance of M D in the transfer of hydrogen between anthracene units. The high resonance stability of radicals formed in this process apparently allows this reaction to occur at temperatures low enough to limit alternative reactions. From these results we are also able to derive rates for P-H transfer from a radical to a molecule, a process for which even less data are available.
Experimental Section Chemicals. Most chemicals were purchased from Aldrich Chemical C O . ~A nearly equimolar mixture of cis- and trans9,10-dihydro-9,10-dimethylanthracene (DMAnH,) was prepared by reduction of 9,lO-dimethylanthracene (DMAn) by sodium in ethanoLaa Perdeuterio-9,lO-dihydroanthracenewas prepared in a similar manner using deuterated reagents. cis-DMAnH, was obtained by catalytic hydrogenation (Pt/C) of DMAn in a Parr hydrogenation apparatus (25% yield),8b followed by separation of most of the major, but less soluble, product, 1,2,3,4-tetrahydro-9,10-dimethylanthracene,by recrystallization. No transDMAnH, was formed. Reactants were generally purified by recrystallization from methanol and found to be 99+% pure by capillary gas chromatography (GC). Kinetic Studies. All kinetic studies were done in evacuated, sealed Pyrex tubes using conventional technique^.^ Product mixtures were dissolved in tetrahydrofuran, separated by capillary column gas chromatography (25-m SE-30), and analyzed by flame ionization and mass spectrometric detectors. All response factors were assumed to be proportional to carbon number. This was verified to f5% for a mixture of anthracene, 9,lO-dihydro(7) ,Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply the material or equipment identified is necessarily the best available for the purpose. ( 8 ) (a) Dalling, D. K.; Zilm, K. W.; Grant, D. M.; Heeschen, W. A.; Horton, W. J.; Pugmire, R. J. J . Am. Chem. SOC.1981, 103, 4817. (b) Fu, P. P.; Lee, H. M.; Harvey, R. G. J . Org. Chem. 1980, 45, 2797. (9) Stein, S.E.; Robaugh, D. A.; Alfieri, A. D.; Miller, R. E. J . Am. Chem. SOC.1982, 104, 6567.
EAnH2
[AnH,]/[EAn] greater than -0.1 and with dilution by biphenyl up to a factor of 100, rates followed simple, second-order kinetics (rate CY [AnH,] [EAn]) described by the following Arrhenius expression over the range 250-375 O C (see Figure 1). kobsd/M-l
S-]
= 109,64*0.'4 exp(-18540 f 180/TJ
These results are consistent with the nonchain, free radical mechanism shown in the following scheme with reaction 2 as the rate-limiting step. AnHz EAn -+ AnH + EAnH (2, -2)
+
+ -
+ An EAnH AnH EAnHz + An 2EAnH EAnH, + EAn 2AnH
AnH,
(3) (4) (5)
Here AnH is the 9-hydroanthryl radical (I) and EAnH denotes a
1
the 9 (and lO-)-hydr0-2-ethylanthrylradical (11,111). This is the simplest free radical mechanism; other steps, including direct H2 transfer, will be considered later. In general, more An than EAnHz was formed. This imbalance was erratic and most severe under extreme conditions ([AnH,] > 10[An],
