ABSOLUTE RATE CONSTANTS FOR METATHESIS REACTIONS OF ALLYL AND BENZYL RADICALS WITH HI (DI) - HEAT OF FORMATION OF ALLYL AND BENZYL RADICALS

The metathesis reaction C3H5 + HI(DI)→ C3H6 (C3H5D) + I.(k3) has been studied in the gas phase using the VLPP technique. The result with DI, using diallyl oxalate as a radical source, is log (k3/M-1 s-1) = (10.11 ± 0.30) - (5.7 ± 1.5)/ɵ at T = 1000 K. The result using 3, 3ʹ-azopropene as a radical source is log (k3/M-1 s-1) = 8.93 ± 0.18 at T = 1000 K, where ɵ = 2.303RT in kcal mol-1. For the metathesis reaction C6H5CH2-+ DI → C6H5CH2D + I-(k3), log(k3/ M-1S-1) = (10.46 ± 0.30)-(6.3 ± 1.5)/ɵ at T = 1000 K. These rate expressions were extrapolated to lower temperatures using a transition-state model in order to compute the equilibrium constants for the above metathesis reactions using the rate constants for the reverse metathesis from iodination studies. The equilibrium constants yield ΔH°f(allyl) = 39.4 ± 1.5 kcal/mol and ΔH°f(benzyl) = 47.80 ± 1.5 kcal/mol at T = 300 K. These values correspond to stabilization energies of 11.4 ± 1.5 and 10.1 ± 2.0 kcal/mol, respectively (i.e., DH(allyl-H) = 86.6 ± 1.5 kcal/mol and DH(C6H5CH2--H) = 87.9 ± 1.5 kcal/mol). © 1979, American Chemical Society. All rights reserved.