Ab initio calculations and RRKM/Master Equation modeling of chloroalkanes → alkenes + HCl reactions for use in comparative rate studies

Abstract

This paper reports computations that were done in conjunction with an experimental study, reported in an accompanying paper, involving single pulse shock tube measurements of the thermal decomposition of 2-chloropropane, chlorocyclopentane, and chorocyclohexane. The overall aim of the combined work is to provide a well-defined, self-consistent, and reliable set of rate constants for those species over an extended temperature range for use as reference reactions in comparative rate studies. To provide additional validation of the results for the compounds of direct interest, the dehydrochlorination reactions of the related compounds chloroethane, 1-chloropropane, and 2-chlorobutane are also considered. The present work reviews and summarizes the literature information regarding the molecular properties, thermochemistry, and chemical kinetic data for the above six alkyl chlorides. Quantum chemical methods are used to compute the structure and energies of reactants, products, and transition states and the fundamental nature of these types of reaction (four-centered "semi-ion pair" transition states) is discussed. The experimental and theoretical results are compared in detail, and uncertainties are assessed. The computations are used, in conjunction with experimentally determined rate constants, to develop Rice-Ramsperger-Kassel-Marcus (RRKM)/Master Equation models and thereby allow extrapolation of the experimental data over an extended range of temperatures. C 2011 Wiley Periodicals, Inc. * Int J Chem

Understanding the thermal dehydrochlorination reaction of 1-chlorohexane. Revealing the driving bonding pattern at the planar catalytic reaction center

“…[3][4][5][6][7][8] Extensive work, both experimental 2, 4, 9-10 and theoretically oriented 3,[9][10][11] have pointed out to the polarization of the ‫ܥ‬ ఋା ••• ܺ ఋି bond as the key chemical event associated to the rate determining step of the dehydrohalogenation process (See Scheme 1). 3,[8][9][10][11][12][13][14][15][16][17][18] The intimate nature of the electronic rearrangement driving this type of transformations remains certainly unknown. Thermal decomposition of alkyl chlorides in the gas phase via a four-membered planar transition structure (TS), including polar (1), concerted (2), ion pair (3) or semi ion pair alternatives (4).…”

“…The position of selected points are shown on the top Cl-C α bond variation 13. The seven regions (a-g) associated to the key chemical events (e.g., bond breaking/formation) are indicated.…”

Data Formats for Elementary Gas Phase Kinetics, Part 1: Unique Representations of Species at the Molecular Level

“…Another set of examples are a ‐chlorocyclohexane and e ‐chlorocyclohexane where the chlorine atoms are in axial and equatorial positions, respectively. Specification is desirable because the reactivities are different: The axial conformer can readily eliminate HCl, whereas the equatorial conformer cannot . In e ‐chlorocyclohexane, the C–Cl chlorine bond is quasi in the plane and hence is quasi‐anti (a dihedral angle of 180° or “trans”) with respect to the next‐nearest neighbor C–C bond whereas in a ‐chlorocyclohexane the C–Cl bond is quasi out of the plane in a “gauche” position relative to the next nearest neighbor C–C bond.…”

Thermochemistry and Kinetics of the Thermal Decomposition of 1‐Chlorohexane

“…The kinetics of the thermal decomposition of halogenated compounds has been the subject of different investigations for several decades . Experimental and theoretical studies have been motivated not only by the interest in the environmental impact of these species but also by a desire to understand the reaction mechanisms involved.…”

“…Chlorinated compound have received special attention because they could produce Cl atoms or reactive chlorinated species which could be involved in atmospheric reactions. There are numerous related investigations of the thermal decomposition of chloroethane , 1‐chloropropane , 1‐chlorobutane , and 1‐chloropentane , but for 1‐chlorohexane, which has extended use in industry as a solvent; there exists limited kinetic information . Studies and reviews of the thermal decomposition of 1‐chloroalkanes, from chloroethane to 1‐chloropentane, show that the corresponding activation energies are about 54–58 kcal mol −1 , and the preexponential factors are (0.2–4) × 10 14 s −1 .…”