Abstract:
A semi-empirical study of the effect of substituent on the HOMO-LUMO
energies of the 3,4 disubstituted 5-membered heterocyclic aromatic compounds, on
the equilibrium constants of the Diels-Alder reaction of N, P, 0 and S substituted
heterocyclic 5-membered aromatic rings with acrolein, and on the enthalpy change of the
reactions, gave results in accordance with the familiar Diels-Alder reaction. A linear
correlation was found between E HOMO and ELUMO of the diene and (Jp and (Jm of the
Hammett equation. The log of the equilibrium constant calculated from the computed
enthalpy and entropy of the reactions at 298 K, and the reaction enthalpy itself, also
correlated with (Jp and (Jm, as expected for linear free energy relationships.