Rate constant for the reaction of CH₃C(O)CH₂ radical with HBr and its thermochemical implication

Abstract

The fast flow method with laser induced fluorescence detection of CH 3 C(O)CH 2 was employed to obtain the rate constant of k 1 (298 K) = (1.83 ± 0.12 (1σ)) × 10 10 cm 3 mol −1 s −1 for the reaction CH 3 C(O)CH 2 + HBr ↔ CH 3 C(O)CH 3 + Br (1, −1). The observed reduced reactivity compared with n-alkyl or alkoxyl radicals can be attributed to the partial resonance stabilization of the acetonyl radical. An application of k 1 in a third law estimation provides

Extension of Structure–Reactivity Correlations for the Hydrogen Abstraction Reaction by Bromine Atom and Comparison to Chlorine Atom and Hydroxyl Radical

“…The database of experimental gas-phase rate constants k Br is less extensive and less internally consistent than those for k Cl or k OH , nor do we have the advantage of a compilation of evaluated and recommended values as we had for k Cl and k OH . In addition, because of the lower reactivity of Br•, Arrhenius parameters have typically been obtained at higher temperature and therefore require longer, and questionable, extrapolations to 298 K. A compilation is given in Table . − All formulas have the carbon from which hydrogen abstraction occurs at the beginning followed by the substituents attached, that is, CH 3 X, CH 2 XY, or CHXYZ. For compounds with multiple potential abstraction sites, the experimental rate constant is typically assumed to be dominantly that of the most reactive site, a relatively safe assumption (see below) given the known high selectivity of Br•, compared with, for example, Cl•.…”

Thermochemistry of Acetonyl and Related Radicals

“…Interestingly, almost two decades earlier, photoionization mass spectrometric work by Orlov and co-workers, not cited in any of the previous articles, ,− had already flagged a lower value of −41 kJ mol -1 . They reasoned that since replacement of H by methyl generally stabilizes a C-centered radical, this meant that the then-known enthalpies for the reaction could not be reconciled and hence cast doubt on the heat of formation of acetonyl.…”

“…Farkas and co-workers used laser-induced fluorescence (LIF) detection of acetonyl in a flow tube to measure the rate at which it H-abstracts from HBr, and from it, they estimate Δ (298 K) = either −24.3 ± 7.5 or −28.1 ± 3.1 kJ mol -1 depending upon much earlier measurements of the reverse reaction 30 or from their own determination of the photobromination of acetone . However, as the authors themselves acknowledge, their results cannot be used to resolve the question because of the fourfold uncertainty in the rate of the reverse reaction.…”

Thermochemistry and Bond Dissociation Energies of Ketones

“…Detailed chemical kinetic models have been developed and are readily available for hydrocarbon combustion with some, but far fewer, studies for ketones. Some chemical kinetic models have been developed involving smaller ketones such as for the oxidation of acetone − and several reactions involving acetonyl radicals. − Chemical kinetic models for 2-butanone and 3-pentanone oxidation have recently been reported , where properties for radicals formed from hydrogen abstraction had to be estimated using group additivity and comparisons to acetone. The value used for the secondary carbon–hydrogen bonds was several kcal mol –1 higher than the actual value, and this could affect both unimolecular dissociation and abstraction kinetics in the model.…”