691 3
Clearly there are still problems in this area despite the "simplicity" of the electronic spectra of copper(I1). Some 10 of the 40 or more complexes which have been investigated in these studies are badly behaved and are therefore signalling structural or electronic abnormalities which are at present unclear. Complexes of 1,3-diaminopropane are all badly behaved; compare, for example, Tables I and 11. Possibly this reflects the greater conformational freedom of the six-membered rings in these complexes. Once these abnormalities
have been explained and understood, and some enthalpy data become available for compounds with a substituent other than water, then the correlations discussed here lay a secure foundation for the evaluation of bond strengths from solid state electronic data. Acknowledgments. We are indebted to the National Research Council (Canada) for financial support and to Professor Paoletti of the University of Florence for samples of certain tetraamine complexes and for permission to quote some of his data prior to publication.
The Kinetics of the Positional Isomerization of 2,3-Dimethyl-2-butene. The Heat of Formation of the 2,3-Dimethylbutenyl Radical and the Effect of Methyl Substituents on the Allyl Radical Stabilization Energy'" A. S. Rodgers" and M. C. R. Wulb Contribution f r o m the Thermodynamics Research Center, Department of Chemistry, Texas A&M Uniuersity, College Station, Texas Received M a y 2, 1973
77843.
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2,3-dimethy1-2-butene I 2,3-dimethyl-l-butene Abstract: The iodine atom catalyzed isomerization, I (kl, kz),has been studied from 410 to 530°K. The reaction was found to be surface catalyzed in glass reaction vessels but homogeneous from 477 to 530°K in Teflon coated glass reaction vessels. The rate constant kl in this range was given by: log (kl/l. mol-' sec-1) = (7.36 f 0.12) - (6.28 =t0.28)jO. From these data the enthalpy of formation of the 2,3-dimethylbutenyl radical (DM-BR) was calculated as: AH*"(DM-BR,g, 298) = 9.6 f 1 kcal mol-'. The bond dissociation energies of the primary C-H bond in 2,3-dimethyl-2-butene is DH"(C,-H) = 78.0 k 1, and the tertiary C-H bond in 2,3-dimethyl-l-butene is DH"(Ct-H) = 76.3 + 1 kcal mol-'. Consequently, the stabilization energy of the dimethylbutenyl radical is 16.7 to 18.5 kcal mol-' depending on the localized model used. When this is compared with the stabilization energies of the allyl and butenyl radicals one is led to the conclusion that the methyl group increases the stabilization energy of the allyl radical by approximately 3 kcal mol-' per group. pproximately 10 yr ago Benson and Bose2 proposed a free radical mechanism for the iodine catalyzed positional isomerization of olefins. This was closely followed by a study of the positional isomerization of 2-butene by Egger, Golden, and Benson3 from which the stabilization energy4 of the butenyl radical (methSuballyl) was determined as 12.4 i. 1.4 kcal mol-'. sequently, Golden, Rodgers, and B e n ~ o n ~ showed ,~ that the stabilization energy of the allyl radical was 9.5 i: 1.4 kcal mol-', a value recently confirmed by equilibrium studies.' In addition t o establishing the low value for the allyl radical stabilization energy (i.e., 10-12 rather than 16 to 25 kcal mol-'),7 these data also
A
(1) (a) This research was supported by the Robert A. Welch foundation. (b) Robert A. Welch predoctoral fellow, 1969-1970. (2) S. W. Benson, A. N. Bose, and P. Nangia, J . Amer. Chem. Soc., 85,1388 (1963). (3) I 2 kcal mol-‘ even if Eb and E, should prove greater than the estimated 1 kcal mol-’. As a result then, kb/(kb k,) and kc/(kb k,) will still be temperature independent and E, = El and Ed = Ez as before. However, the bond dissociation energies would now be given by
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DHon98(DM-2B,C,-H) = 78.0 - (Ec - 1.0) kcal mol-’ and DHoZg8(DM-1B,C,-H)= 76.3 - (Eb - 1.0) kcal mol-’ They would become weaker! Thus, while our quantitative results depend upon the estimation of Eb and E,, the general conclusion that the allyl radical is stabilized by methyl substituents (and by about 3 kcal mol-’ per group) does not.
Kinetics of Competing Free Radical Reactions with Ni troaromatic Compounds C. L. Greenstock* and I. Dunlop
Contribution from the Medical Biophysics Branch, Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba, Canada R O E ILO. Received M a y 3, 1973 Abstract: Rate constants for the competing reactions between alcohol donor free radicals and nitroaromatic compounds, and their relative efficiencies, have been measured by pulse radiolysis. The kinetics of formation of the nitroaromatic anion species are dependent upon the sum of the rate constants for both competing reactions and not upon the rate constant for radical oxidation alone. The rate constants for donor radical oxidation and adduct formation increase with increasing redox potential of the electron acceptor. Although a- and 0-hydroxy radicals have similar reactivities, a-hydroxy radicals are oxidized preferentially whereas &hydroxy radicals predominantly form adducts.
T
he high reactivity of free radicals with organic nitro compounds and other electron acceptors is well recognized. Electron-transfer oxidation by nitro compounds of .C02H or .COz-, a-hydroxy, and a-alkoxy radicals has been demonstrated by electron spin resonance studies. 1-4 Alkyl and @-hydroxy radicals and the radicals ‘OH and “H2, on the other hand, react readily by addition to the electron The rates of alkyl radical adduct f o r m a t i ~ n , ~like .~ those of a-hydroxy radical oxidation, 3 , 8 increase with increasing redox potential of the electron acceptor. (1) D. J. Edge and R. 0. C. Norman, J . Chem. SOC.B, 1083 (1970). (2) W. E. Griffiths, G . F. Longster, J. Myatt, and P. F. Todd, J . Chem. SOC.B, 533 (1967). (3) M. McMillanandR. 0 .C. Norman,J. Chem. SOC.B, 590(1968). (4) K. Eiben and R. W. Fessenden, J . Phys. Chem., 75,1186 (1971). (5) A. Rembaum and M. Szwarc, J . Amer. Chem. Soc., 77, 4468 (1955). (6) F. J. Lopez Aparicio and W. A. Waters, J . Chem. SOC.B, 4666 (1952). (7) W. J. Heilman, A. Rembaum, and M. Szwarc, J . Chem. SOC.B, 1127 (1957). (8) C. E. A d a m , C. L. Greenstock, J. I. van Hemmen, and R. L. Willson, Radiat. Res., 49,85 (1972).
We have studied the kinetics of these competing donor free radical reactions with nitroaromatic compounds by direct observation of the reactive species, using the technique of pulse radiolysis.9 The extent of donor radical oxidation is obtained from the yield of the transient radical anion of the nitro compound, and the rate constants .for radical oxidation and adduct formation are determined from the rates of build-up of the nitroaromatic radical anion for different concentrations of nitro compound, according to the kinetic analysis described. These data are particularly important in the study of chemical and biological radiosensitization.8~’0~1’Electron-affinic compounds,11 particularly nitroaromatic and nitroheterocyclic compounds, are believed to radiosensitive at low concentrations by (9) J. W. Hunt, C. L. Greenstock, and M. J. Bronskill, I n t . J . Radiat. Phys. Chem., 4,87(1972). (10) C. L. Greenstock and I. Dunlop, Int. J . Radiat. Biol.,23, 197 (1973). (1 1) R. L. Willson and P. T. Emmerson, “Radiation Protection and Sensitization,” H . L. Moroson and M. Quintiliani, Ed., Taylor and Francis, London, 1970, p 73.
Greenstock, Dunlop 1 Competing Free Radical Reactions with Nitroaromatics
