Elimination reactions – Calculation of rate constants from equilibrium constants and distortion energies by means of no barrier theory

Abstract

No barrier theory has been applied to the E2 reactions of five alkyl bromides with ethanolic ethoxide. The model used for these reactions is that the reaction proceeds from the encounter complex of base and alkyl halide to the product encounter complex of halide ion and alkene (and alcohol), and requires five simple processes, which combine to give the concerted elimination: transfer of a proton from carbon to base; a change in geometry at the carbon which loses a proton from sp3 to sp2; breaking the C-leaving group bond; a change in geometry at the carbon which loses the leaving group from sp3 to sp2; and a change in the length of the carbon–carbon bond. The free energy of activation can be calculated with an rms error of 2.58 kcal mol–1 (1 cal = 4.184 J).Key words: Elimination, no barrier theory, rate constant, equilibrium constant.