On the correlation between the carbonyl vibrational frequency of substituted thenoyl chlorides and the rate of their reaction with aniline

Abstract

An interpretation is proposed for the observed linear correlation between the carbonyl i.r. of 3- and 5-substituted thenoyl chlorides and the logarithm of the rate constant of their reaction with aniline. Simple Hückel MO theory, both in its complete form and in its perturbation approximation, shows that the substituent effect on the π-electron population of the carbon atom of the CO group is linearly related to the substituent effect on the reaction rate. The activation energy of the reaction has been evaluated from the HMOs of the separate molecules by fitting them into a supermolecule scheme. The bond formed in the activated state was treated as a charge transfer from the HOMO of aniline to the LEMO of the substituted thenoyl chlorides. This model accounts for the observed correlation between the substituent effect on the reaction rate and that on the ν(CO) absorption band. The complete HMO treatment gives a better agreement than the perturbation approach. The following suggestions are made. (i) A similar correlation is expected for the substituent effect in furan- and pyrrole-2-carbonyl chlorides. (ii) A search should be made for a charge-transfer absorption band in the spectra of the reacting molecules.