J . Phys. Chem. 1991,95,4009-4014
4009
Kinetics of the Reactions of Halogenated Methyl Radicals with Molecular Bromine R. S. Timonen,+J. A. SeetulaJ J. Niiranen, and D. Gutman* Department of Chemistry, Catholic University of America, Washington, D.C. 20064 (Received: September 26, 1990; In Final Form: December 13, 1990)
The kinetics of seven reactions of halogenated methyl radicals (CH2CI,CHCI2,CFCI2, CF2Cl, CF3, CH2Br,and CHJ) with molecular bromine were studied by using a heatable tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature, typically between 296 and 532 K. Arrhenius activation energies were found to be small negative values (typically -2 kJ mol-') for all reactions studied with the exception of that of the CF3 + Br2 reaction (whose activation energy is positive, but which could not be determined accurately). The pattern in reactivity among 11 reactions of substituted methyl radicals with Br2 (which includes the 7 reactions studied here and 4 C(H)x(CH3)+x + Br2 reactions (x = 0-3) studied earlier) has been accounted for by the inductive effect of the substituent atoms or groups. The sum of the Pauling electronegativities of these substituents provides a useful measure of their total inductive effect on the reaction rate constant.
Introduction The reactions of carbon-centered free radicals with molecular bromine are exothermic (typically AIP is in the range -75 to -100 kJ mol-') and generally have very high rate constants.Id For example, Timonen et aI.,I who recently studied the kinetics of the reactions of four alkyl radicals with Br2, report reaction rate constants that are close to the respective collision numbers and have small negative activation energies, indicating that the reactions proceed along attractive reaction coordinates. Other than the alkyl radical Br, reactions mentioned above, few reactions of carbon-centered radicals with molecular bromine have been studied in detail. Rate constants for the reactions of C 3 H 3 and of two perhalogenated radicals (CF3 and C2Fs))" with Br2 have been determined at ambient temperature, and that of HCO with Br2 has been measured as a function of temperat~re.~ Ratios or functions of rate constants involving those of reactions of perhalogenated radicals with Br2 have also been reported,+I0 some as a function of temperature. Finally, the kinetics of two reactions between dihalocarbene radicals (CFCI and CFBr) and Br2 have also been investigated."J2 As part of our continuing investigation of the kinetics and thermochemistry of reactions of polyatomic free radicals with diatomic molecules containing halogen atoms, we have now studied the kinetics of the reactions of seven halogenated methyl radicals with Br2 (reaction enthalpies are in kJ):" CH2CI Br2 CH2CIBr Br AHo = -83 (1)
+
+
----
+ Br, CFC12 + Br2 CF2CI + Br2 CF3 + Br2 CH2Br + Br2
CHCI2
CH21 + Br2
+ CHC12Br + Br CFCI2Br + Br CF2CIBr + Br CF3Br + Br CH2Br2+ Br CH21Br + Br
AHo = -78
(2)
AHo = -62
(3)
AHo = -82
(4)
AHo = -100 AHo = -99
(5) (6)
AHo = -92
(7) This study was conducted in part to attempt to isolate and characterize the influence of the inductive effect of halogen substituents of the radical site on the rate constants of a selected set of metathesis reactions. The reactions selected for study are ones that are sufficiently exothermic that thermochemistry was not expected to be an important factor in determining differences in reactivity. Hence, other influences on reactivity, such as the inductive effect, were anticipated to be more readily observable and susceptible to quantitative characterization from the observed differences in kinetic behavior. Only reaction 5 has been isolated for direct investigation before. Rossi et aL3 obtained the rate constant at 298 K (1.3 X 10-l2) 'Present address: Department of Physical Chemistry, University of Helsinki, 00520 Helsinki, Finland.
0022-3654/9 1/2095-4009$02.50/0
using the very-low-pressure-photolysis technique (all bimolecular rate constants are reported in units of cm3 molecule-' PI). Whittle and co-workers have determined ratios of rate constants involving either k4 or k5 with suitable reference reactions.'** Rate constants for reactions 1-7 were measured as a function of temperature. Details of the experiments, the results obtained, and a discussion of the observed substituent effects are reported here. Experimental Section The experimental facility19as well as its use for kinetic studies of reactions of polyatomic free radicals with Br2 has been described.' Only a summary will be presented here. Pulsed, unfocused, 193- or 248-nm radiation from a Lambda Physik EMG 201MSC excimer laser was collimated and then directed along the axis of a heatable 1.05- (or 2.20-) cm4.d. coated Pyrex tubular reactor. (The 1.05-cm4.d. reactor was used in the studies of reactions 1-5 and the 2.20-cm4.d. reactor was used in the studies of reactions 6 and 7). Gas flowing through the smaller tube a t 5 m s-I (or 2.5 m s-' in the larger one) was completely replaced between laser pulses. The flowing gas contained the free-radical precursor in very low concentration (typically 0.0001%), Br2 in varying amounts, and the carrier gas, He, in large excess (>98%). (1) Timonen, R. S.;Seetula. J. A.; Gutman, D. J . Phys. Chem. 1990,94, 3005. (2) Slagle, I. R.; Yamada, F.; Gutman, D. J . Am. Chem. Soc. 1981, 103, 149. (3) Rossi, M. J.; Barker, J. R.; Golden, D. M. J . Chem. Phys. 1979, 71, 3722. (4) Rossi, M. J.; Barker, J. R.; Golden, D. M. Inr. J . Chem. Kine?. 1982, 14, 499. (5) Timonen, R. S.;Ratajczak, E.; Gutman, D. J . Phys. Chem. 1988,92, 651. (6) Handbook of Bimolecular and TermolecularGas Reactions;Kerr, J. A., Moss, S.J., Eds. CRC Press: Bwa Raton, FL, 1981; Vol. 1. (7) Tuckerman, R. T.; Whittle, E. J . Phorochem. 1985, 31, 7. ( 8 ) Amphlett, J. C.; Whittle, E. Trans. Faraday Soc. 1966, 62, 1662. (9) Evans, B. S.;Whittle, E. Inf. J . Chem. Kiner. 1981. 13, 59. (IO) Weeks,1.; Whittle, E. In?. J . Chem. Kiner. 1983, I S , 1329. ( I 1) Purdy, J. R.; Thrush, B. A. J . Chem. Soc., Faraday Trans. 2 1980, 76, 1304. (12) Purdy, J. R.; Thrush, B. A. Inr. J . Chem. Kiner. 1981, 13, 873. ( I 3) Heats of formation used to determine reaction enthalpies were taken
from the following sources: CH2CI,I4CHCI2,l4CFCI2,I5CF2C1,I6,CF,," CH2Br,I4CHJ," CH2CIBr,I4CHC,I2Br,l6CFCI2Br," CF2CIBr,I6CF,Br,'* CH2Br2,14CH2IBr,I6Br,16 and Brl. (14) Tschuikow-Roux, E.;Faraji, F.; Paddisan, S.;Niedzielski, J.; Miyokawa, K. J . Phys. Chem. 1988, 92, 1488. (15) Luke, B. T.; Loew, G. H.; McLean, A. D. J . Am. Chem. Soc. 1987, 109. 1307. (16) Lias, S.G.;Bartmess, J. E.;Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref. Dura 1988, 17, Supplement No. 1. (17) McMillen, D. F.; Golden, D. M. Annu. Reo. Phys. Chem. 1982,33, 493.
(18) Kudchadker. S. A.; Kudchadker, A. P. J . Phys. Chem. Re/. Data 1978, 7, 1285. (19) Slagle, I. R.; Gutman, D. J . Am. Chem. SOC.1985, 107, 5342.
0 1991 American Chemical Society
4010 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991
Timonen et al.
I
The free radical precursor used for CH2Cl was CH2CIBr (Aldrich, 99%), for CHClz was CHC12Br (Aldrich, >98%), for CCI2F was CC13F (Aldrich, >99%), for CCIF2 was CCI2F2 (Matheson, 99%), for CF3 was CF3Br (SCM Chemicals 99%), for CH2Br was CH2Br2(Aldrich, >99%), and for CH21was CH212 (Aldrich, 99%). He was obtained from Matheson (99.995%), and Brz from Aldrich (>99%). The free-radical precursors and Br2 were degassed by using freeze-pump-thaw cycles. Helium was used as provided. Gas was sampled through a 0.4-"diameter hole (located at the end of a nozzle in the wall of the reactor) and formed into a beam by a conical skimmer before it entered the vacuum chamber containing the photoionization mass spectrometer. As the beam traversed the ion source, a portion was photoionized and then mass selected. Temporal ion signal profiles were recorded from a short time before each laser pulse to as long as 26 ms following the pulse by using a multichannel scaler. Data from 2000 to 30000 repetitions of the experiment were accumulated before the data were analyzed. Experiments were conducted under conditions where only two significant reactions consumed the labile reactant CR3: CR3 + Br2 CR3
-
-
CRJBr + Br
(A)
heterogeneous loss
(B)
In all sets of experiments conducted to determine a reaction rate constant, tests were also conducted to ensure that radicalradical and radicalatom reactions had negligible rates compared to either reaction A or B. Initial concentrations of CR3 were reduced until the measured atom or radical decay constants in the presence or absence of the stable reactant no longer depended on the initial radical precursor concentration or on the laser fluence. The laser fluence was attenuated by using quartz plates to reduce the photolysis of Br2 to a negligible amount (
