MODELING OF THE HOMOGENEOUSLY CATALYZED AND UNCATALYZED PYROLYSIS OF NEOPENTANE - THERMOCHEMISTRY OF THE NEOPENTYL RADICAL

The mechanism for neopentane (NpH) pyrolysis in the absence and presen ce of additives isobutene, HCI and HBr, in the temperature range 750-8 00 K, has been reinvestigated with the aid of computer simulation and sensitivity analysis techniques. With best values assigned to all rate constants in the kinetic chain, a basic mechanism comprising 18 rever sible reactions involving 19 atomic, radical, and molecular species ha s been used to simulate pure neopentane pyrolysis data. Predictions of major and minor product yields provided quantitative agreement with e xperimental data against which the model was tested. The mechanism was supplemented by additional species and reactions in order to simulate experimental neopentane pyrolysis data in the presence of HCl and HBr additives. An apparent discrepancy between a recent direct measuremen t of k5, the rate constant for thermal decomposition of the neopentyl radical [1], and that reported from studies of neopentane pyrolysis in the presence and absence of HCI [2], has been identified as being due to the use of an incomplete mechanism in the latter determination. Si mulations of hydrogen halide catalyzed pyrolyses exhibit a high sensit ivity to the thermochemical parameters associated with the neopentyl r adical (Np). The influence of uncertainties in DELTAH(f)-degrees(Np) a nd S298-degrees(Np) are evaluated and lead to suggested values DELTAH( f)-degrees(Np) = 8.7 +/- 0.8 kcal mol-1 and S298-degrees(Np) 78.8 +/- 1.0 cal mol-1 K-1. (C) 1993 John Wiley & Sons, Inc.