4880
J. Phys. Chem. 1993,97, 4880-4881
Rate Constants for the Addition of the Benzyl Radical to Ca in Solution M. Walbiner and H. Fischer' Physikalisch-Chemisches Institut der Universitat Ziirich. Winterthurerstrasse 190, CH-8057 Ziirich, Switzerland Received: February 9, 1993; In Final Form: March 23, 1993
The rate constant for the addition of benzyl radicals to c 6 0 in toluene is determined by time-resolved ESR spectroscopy as k298 = (1.4 f 0.2) X lo7 M-I s-l with log AIM-' s-I = 9.5 f 0.3 and Ea = 13.5 f 0.2 W/mol. This agrees with a previous correlation between log k/M-' s-I and electron affinities for benzyl addition to alkenes and shows that the addition of C 6 0 is strongly facilitated by charge transfer in the transition state.
Introduction
In many reactions c 6 0 behaves as an electron-deficient superalkene' as, for instance, exemplified by the facile addition of nucleophilic carbon-centeredradicals.2 The rate constants for addition of such radicals to simplealkenes CH2=CXY are known to exhibit large polar substituent effect^,^ and for three prototype radicals, tert-butyl, 2-hydroxy-2-propy1, and benzyl, we have previously found a linear increaseof log k with increasing electron affinity of the alkenes4 c60 has an electron affinity of 2.65 eV,S which is much larger than that of the common alkenes (50.5 eV). Hence, if polar effects also dominate additions to (260,one expects larger rate constants. For the benzyl radical reaction with 24 alkenes the rateconstant sobeyed log k*gS/M-I s-I = 3.36 + 1.14 EA/eV ( r = 0.87),4b and deviations were within 1 order of magnitude. Extrapolation to 2.65 eV and multiplication by 30 (60 equivalent sites but one-sided addition only) leads to a predicted kZgs= 7.2 X lo7 M-l s-I for the addition of benzyl to c 6 0 with an uncertainty of a factor of 10. In the following we test this prediction.
12.
9.
6.
3.
I
T
220
260
240
-
300K
280
Figure 1. Rate constants for the addition of C ~ H S & -to I ~ c 6 0 vs temperature.
1
logk/M-'s''
6 .
Results and Discussion c 6 0 was prepared in a Krltschmer-Huffman type apparatus6 (7-mm graphite rods, 100-A dc discharge, 200 mbar of He), by toluene extraction and ~hromatography,~ in yields of 10-1 2% and about 99% purity. Benzyl radicals were generated by photolysis of dibenzyl ketone. The fast decarbonylation of the phenylacetyl intermediates precludes its interference in the addition and termination kinetics. At 353 K CW photolysis of deoxygenated toluene solutions with 0.2 M ketone and a high Cm content (1.5 X M) in an ESR flow system revealed for high flow rates, i.e., low conversion, the formation of the benzyl monoadductZa(a(CH2) = 0.42 G, a(H,) = 0.19 G, AH = 70 mG, g = 2.002 15). For higher conversion the spectrum is dominated bythe tripleadductZb(AH=1.5G,g= 2.002 45). Time-resolving experiments were performed on the ESR signal of benzyl with toluene solutions containing 0.2 M ketone and low Cm concentrations ( I 1 X lo" M). In the absence of c 6 0 the benzyl radical decayed by the second-order self-terminati~n,~ and this was found perturbed with increasing c 6 0 content by the addition to c60. Using described analysis procedure^,^ the pseudo-first-order lifetime T I for addition was extracted, and experiments with different concentrations at 252 K ensured that T--' = k[C60] was obeyed. High flow rates and low photolysis intensities had to be applied to keep the c 6 0 conversion below tolerable limits (540% at 298 K, 520% at lower temperature). The results for k are shown in Figure 1 together with a fit to the Arrhenius equation log k/M-'s-l = (9.5 f 0.3) - (13.5 f 0.2)/8,where 8 = 2.303RT in kJ/mol. In comparison to the alkenes investigated so far,4bJ0 the activation energy is lower and the frequency factor is about 10-fold larger. The room tepprature value k296 = (1.4 f 0.2)
Me,OCOMe
,*
,
At" .
-3
-2
-1
0
1
2
.
3 eV
Figure 2. log k for the addition of C6H& to alkenes CH2=CXY vs alkene electron affinities and log k/30 for the addition to (260. X lo7M-I s-I is much larger than the largest constant for alkenes (a-methylacrylonitrile, 6600 M-I s-l); Le., the electron deficient superalkene behavior is confirmed. Our prediction exceeds the experimental value by a factor of 5. In view of the distribution oE.the data in log k vs EA plot (Figure 2), this result is quite satisfactory, and there are reasons for a negative deviation for Ca. For alkenes the addition rates of benzyl are not only governed by polar effects. They are also enhanced by increasing exothermicities of the reactions and diminished by stericrepulsionthrough groups neighboring the site of attack.1° Judging from the reversibility of alkyl addition to c 6 0 at elevated temperatures and the steric crowding in the adducts,2 bath effects may lower the addition constant. Recently, k293 = (2.7 f 0.5) X lo8 M-' s-l was reported for the addition of eC13 to C a and k293 = 2 X lo6 M-I s-I was given for a reaction of CClsOO*.~~ Little is known about the rate
0022-365419312097-4880$04.00/0 0 1993 American Chemical Society
Letters constants for additions of these radicals to simple alkenes, It seems reasonable, however, that CCls adds faster than benzyl. It is less nucleophilic (IP = 8.3 eV vs 7.2 eV1*)but much less stabilized. The peroxy species should be somewhat electrophilic, Le., less reactive toward electron-deficient compounds. Using other previously given log k vs EA correlations,4 we can predict addition rate constants to Ca for the very nucleophilic rerr-butyl and 2-hydroxy-2-propyl radicals as k298 = 1.6 X 1OloM-l s-l and 9.8 X 1OloM-l s-l, respectively. Probably, the actual values will thus be diffusion controlled and will depend on solvent viscosity. To our ESR methods such large rate constants are not accessible, and, in fact, preliminary experiments with terr-butyl at 298 K failed due to extensive C a conversion; Le., k298 must be much larger than about 5 X lo7 M-l s-l.
Note Added in Proof. After submitting this work, we became aware of a very recent publication13on radical addition rates to C a at room temperature. The direct ESR observation of k = 3 X lo9 M-l s-l for tert-butyl in benzene nicely agrees with our experience and prediction. However, the large value of k = 9.3 X lo8 M-l s-l for benzyl in toluene exceeds our data by nearly 2 orders of magnitude. If true, this would render our correlations (Figure 2) meaningless. Therefore, we repeated our measurements with utmost consideration of errors by substrate depletion using a 5-fold flow rate of 5 mL/min and half of the previous light intensity. Further, the C60 consumption was determined by chromatographic separation of the reacted solutions. From experiments with various c 6 0 concentrations (510" M) we obtained k241 = 4.5 X lo6 M-1 s-l and k263 = 6.7 X lo6 M-l s-l, indistinguishable from the data of Figure 1. The C60 conversion was below 25%. Theauthors13extracted the largevalue for benzyl from the growth and yields of an optical transient assigned to the
The Journal of Physical Chemisrry, Vol. 97, No. 19, 1993 4881 benzyl monoadduct. The 308-nm laser flash was absorbed mainly by Ca and'less by the initiating peroxide. This may have caused substantial depletion of ground-state C6o and, hence, the observation of a different reaction.
Acknowledgment. Financial support by the Swiss National Foundation for Scientific Research is gratefully acknowledged. We thank I. Verhoolen for the preparation of c 6 0 . References and Notes (1) (a) Wudl, F. Acc. Chem. Res. 1992, 25, 157. (b) Birkett, P. R.; Hitchcock, P. B.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Nuiure 1992, 37, 479. (2) (a) Morton, J. R.; Preston, K. F.;Krusic,P. J.; Hill,S. A.; Wasserman, E. J. Phys. Chem. 1992.96.3576. (b) Krusic, P. J.; Wasserman, E.; Keizer, P. N.; Morton, J. R.; Preston, K. F. Science 1991, 254, 1184. (c) Krusic, P. J.; Wasserman, E.; Parkinson, B. A.; Malone, B.;Holler, Jr., E. R.; Keizer, P. N.; Morton, J. R.; Preston, K. F. J. Am. Chem. Soc. 1991, 113,6274. (3) Giese, B. Angew. Chem., Inr. Ed. Engl. 1983, 22, 753. (4) (a) Miinger, K.; Fischer, H.Ini. J. Chem. Kinei. 1985,17,809. (b) HCberger, K.; Walbiner, M.; Fischer, H. Angew. Chem., Ini. Ed. Engl. 1992, 31, 635. (c) HCberger, K.; Fischer, H. Int. J . Chem. Kiner., in press, and
references therein. (5) Wang, L. S.; Conceicao, Jin, C.; Smalley, R. E. Chem. Phys. Leu. 1991, 182, 5. (6) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nuiure 1990, 347, 354. (7) Scrivens, W. A.; Dedworth, P. V.; Tour, J. M. J . Am. Chem. SOC. 1992,114, 7917.
( 8 ) Lunazzi, L.; Ingold, K. U.; Scaiano, J. C. J. Phys. Chem. 1983,87, 529. (9) (10) (11) (12)
Lehni, M.; Schuh, H.; Fischer, H. Inr. J. Chem. Kinei. 1979,11,705. Walbiner, M.;Fischer, H. To be published. DimitrijeviC, N. M. Chem. Phys. Leu. 1992, 194, 457. Lias. S. G.: Bartmess. J. E.: Liebman. J. F.: Holmes. R. D.: Levin. R. D.;Mallard, W. G. Gas-PhaseIon'and Neutral Thermochemistry; i,Phys: Chem. Ref.Duia 1988, 17 (Suppl. 1). (13) DimitrijeviC, N. M.; Kamat, P. V.; Fessenden, R. W. J . Phys. Chem. 1993, 97, 615.
