Effect of substitution on dissociation kinetics of C2H5X, (X = F, Cl, Br and I): A theoretical study

Abstract

The decomposition of alkyl halide is controversial concerning C–X bond dissociation reaction and molecular (HX) elimination reaction. In order to identify the feasibility of dissociation reaction, electronic structure calculations were performed at DFT level of theory in conjunction with BMK, B3LYP, and MPW1PW91 functional, and 6-311G** basis set. In addition to this, we have carried out the detail theoretical kinetic analysis for the unimolecular dissociation of ethyl halide (C2H5X, X = F, Cl, Br and I), in the temperature range from 500 to 1700 K and pressure range from 10−9 to 103 kPa. Our kinetic data for decomposition reaction reveals that, the order of reaction is independent on temperature, but it alters with applied pressure. Also, an analysis of the branching ratio of C–X bond dissociation and HX elimination reaction, as a function of temperature, shows that the molecular (HX) is eliminated predominantly from fluoro-, chloro- and bromo-derivative of ethane. On the other hand, the reverse trend was observed upon pyrolysis of iodoethane. Above discrepancy in dissociation behaviour has been attributed towards larger C–X bond length and higher ring strain in cyclic transition state structures involving heavier atom.