Kinetics of the bromine catalyzed elimination of HCl from 1,1,1‐trichloroethane

Rodgers, A. S.; Jerus, P.

doi:10.1002/kin.550200706

Cited by 3 publications

(2 citation statements)

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“…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•.…”

Section: Experimental Data Basementioning

confidence: 99%

“…However, for thermal bromination, if concentrations are adjusted such that k propagation is the desired k Br for hydrogen atom abstraction, then k exp = k Br [K(Br 2 )] 1/2 where K(Br 2 ) is the known dissociation constant for Br 2 . In this way, absolute measurements of k Br have been reported for ethane, 1,1,1-trichloroethane, acetone, methanol, 1,1,1,2,3,3,3-heptafloropropane, pentafluoroethane, fluoroform, chloroform, and bromoform . The VLPR technique, in which the Knudsen cell reactor operates as a “continuously stirred tank reactor” and the concentrations of Br• and RH are determined directly by mass spectrometry on the effusing beam, has been applied to diethyl ether and i -butane. , However, the A value from a temperature variation study is often modified from the experimental value by statistical mechanics considerations and hence the extrapolation to 298 K is affected.…”

Section: Experimental Data Basementioning

confidence: 99%

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

Poutsma

2016

J. Phys. Chem. A

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Recently we presented structure-reactivity correlations for the gas-phase ambient-temperature rate constants for hydrogen abstraction from sp(3)-hybridized carbon by chlorine atom and hydroxyl radical (Cl•/HO• + HCR3 → HCl/HOH + •CR3); the reaction enthalpy effect was represented by the independent variable ΔrH and the "polar effect" by the independent variables F and R, the Hammett constants for field/inductive and resonance effects. Both these reactions are predominantly exothermic and have early transition states. Here, we present a parallel treatment for Br• whose reaction is significantly endothermic with a correspondingly late transition state. Despite lower expectations because the available database is less extensive and much more scattered and because long temperature extrapolations are often required, the resulting least-squares fit (log k298,Br = -0.147 ΔrH - 4.32 ΣF - 4.28 ΣR - 12.38 with r(2) = 0.92) was modestly successful and useful for initial predictions. The coefficient of ΔrH was ∼4-fold greater, indicative of the change from an early to a late transition state; meanwhile the sizable coefficients of ΣF and ΣR indicate the persistence of the "polar effect". Although the mean unsigned deviation of 0.79 log k298 units is rather large, it must be considered in the context of a total span of over 15 log units in the data set. The major outliers are briefly discussed.

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