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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Ethyl halide
• rate constant
• DFT study
• kinetic modelling
• ring strain

Acknowledgements

The authors acknowledge Science and Engineering Research Board (SERB), New Delhi for providing financial support in the form of Young Scientist Scheme (Project no. YSS/2015/001106).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Science and Engineering Research Board [grant number YSS/2015/001106].