Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

74 EXPERIMENTAL METHODS FOR SLOW REACTIONSatom are present, the trapping efficiency will depend on the equilibrium constants,Ka,b and Kc,d, in reactions such as (35) and (36)Br. + MeCH = CHz % MeCHCH,Br (35)Bra + MeCH = CH,HBr + CHzCH = CHz(36)In flow systems, phenolgJ, toluene, benzene, aniline and cyclopentene' 30 havebeen used both as radical traps and as carrier gases. The major reaction in eachcase seems to be hydrogen abstraction but some radical addition almost certainlyoccurs. These compounds tend to be less efficient than NO and the aliphaticstraight chain olefins.Another class of compounds that have been used are the halogens and halogenacids, C12231, 12232, HClZ33, HBrz34 and HIzo9. These react with radicals togive molecular products that are relatively stable (with X,), or very stable (withHX), where X stands for halogen. Clearly, the trapping reactions with methylradicals will be (37) and (38)Me+X2 + MeX+XMe+HX + CH4+X(37)(38)TABLE 15ARRHENIUS PARAMETERS AND ENTHALPY CHANGES FORTHE REACTIONS (37) AND (38)X, or HX log A E AH*(A, 1. mole-' sec-l) (kcal. mole-l) (kcal. mole-l)- 4 - 74C12* 9.54 2.5 -23BraC 8.75 0.9 -21.5I2d 9.70 01 -17.5HFe 9.2 34 33HCIa 8.14 4.9 1.1HBrc 8.95 2.9 -14.4HId 9.47 1' -30.5* L. BATT AND F. R. CRUICKSHANK, J. Phys. Chem., 70 (1966) 723.P. D. MERCER AND H. 0. PRITCHARD, J. Phys. Chem., 63 (1959) 1468.* Estimated from k37/k88. A. F. TROTMAN-DICKENSON, Gas Kinetics, Butterworths, London, 1955.L. BAIT AND F. R. CRUICKSHANK, J. Phys. Chem., 71 (1967) 1836.Taken or estimated from H. E. O'NEAL AND S. W. BENSON, J. Chem. Phys., 34 (1961) 514.a Estimated from the values for the reverse process by G. C. FETTIS AND J. H. KNOX in Progressin Reaction Kinetics, Vol. 2, G. Porter (Ed.), Pergamon, Oxford, 1964, p.1.1 E,,-E,, is estimated from d as 1 kcal. mole-l; with the choice of E,, = 0, E,, is 1 kcal. mole-'.