The Woodward–Hoffmann rules and molecular orbitals
Fleming Ian · Oxford University Press eBooks · 2015
This chapter highlights the characteristic feature of all pericyclic reactions, which is the concertedness of bond-making and bond-breaking, and the absence of any intermediates. The chapter shows how organic chemists have proven that pericyclic reactions are concerted, particularly in the case of the Diels–Alder reaction. It also cites a number of powerful rules related to pericyclic reactions, including their basis in the conservation of symmetry of the molecular orbitals. The chapter explains the Arrhenius parameters for Diels–Alder reactions, which show that there is an exceptionally high negative entropy of activation with a low enthalpy of activation reflecting the exothermic nature of the reaction. It describes the compact transition structure that is in agreement with the negative volumes of activation which can be measured by carrying out the reaction under pressure.