Thermal reversible isomerisation of 2-ethyl-cis-penta-1,3-diene to 4-methyl-trans-hexa-1,3-diene

Abstract

The reversible isomerisation of 2-ethyl-cis-penta-1,3-diene to 4-methyl-trans-hexa-1,3-diene is an example of the now well-authenticated 1,5-hydrogen migration reaction which occurs in a number of diene systems. The reaction is almost certainly unimolecular and the first-order rate constants, obtained in the temperature range 190–250° fit the Arrhenius equations, k₁= 10^10·77 exp (–31,640/RT) sec.^–1; k_–1= 10^11·03 exp (-34,530/RT) sec.^–1. Equilibrium constants have been determined by direct analysis in the range 230–340° and hence enthalpy and entropy differences between the isomers have been evaluated. The A factors of the isomerisations are discussed and compared with estimated values.