The study of Friedel-Crafts type reaction mechanisms

Abstract

In recent years the role of pi and delta complexes in electrophilic aromatic substitutions has aroused much controversy. Olah has argued that the transition state in electrophilic aromatic substitution is not a fixed one. With strong electrophiles it can occur early along the reaction coordinate resembling a pi complex, with weak electrophiles the tranistion state occures late, resembling a pi complex. This argument is based on a number of benzoylations and benzylations on benzene and toluene in which the electrophilicity of the electrophile has been varied. These conclusions have come under strong criticism and in order to shed light on the exact mechanism the following research was undertaken. This study is centered around the Friedel-Crafts acid catalysed cyclodehydration of 2-benzylbenzoic acids although the cyclodehydration of 2-naphthoylbenzoic acid and 2-benzoylbenzoic acids were also studied. The cyclodehydrations were effected mainly with methanesulphonic acid, but sulphuric acid, polyphosphoric acid and trifluoromethanesulphonic acid were also employed. Rate constants were measured and products of cyclodehydration were identified for all model compounds. The cyclodehydration was investigated by varying the electrophilicity on both the substrate and the electrophile. Consequently many new substituted 2-benzylbenzoic acids were synthesised. Hammett plots were also made and p values calculated. In addition to this the deuterium isotope effect was also investigated, but none was observed. Acidity function work (HR) was also carried out on methanesulphonic acid to try to extend the range to 100% acidity. Analysis of the rate constants and product distributions shows that the transition state resembles a delta complex rather than a pi complex. Hammett and acidity function studies do, however, indicate that the rate limiting step in the reaction is variable. This is dependent on whether there are electron withdrawing or electron donating groups present in the substrate. With electron donating groups the loss of water in the formation of the electrophile is the rate limiting step whereas with electron withdrawing groups the formation of the delta complex is rate limiting.