Inter- and intra-molecular catalysis in the iodination of o-hydroxyacetophenone

Abstract

The kinetics of the iodination of o-hydroxyacetophenone have been investigated at 25° in aqueous buffer solutions prepared from 1,1,1,3,3,3-hexafluoro-2-methylpropan-2-ol or from boric acid. The kinetic order with respect to iodine is between zero and unity, and the rates of ionisation or enolisation of the ketone can be derived by extrapolating to infinite concentration of iodine. These rates increase with increasing concentration of the basic constituent of the buffer, but the rate of intramolecular ionisation or enolisation of o-hydroxyacetophenone can be obtained by extrapolating to zero buffer concentration. In interpreting these results kinetic measurements were also made on the cyclisation of ω-chloro- and ω-bromo-o-hydroxyacetophenone to give coumaran-3-one, the iodination of ω-chloro-o-hydroxyacetophenone and of coumaran-3-one, and the condensation of p-hydroxyacetophenone with 1-bromobutan-2-one. The pK values of o-hydroxyacetophenone and of 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol have also been measured.

The rates of intramolecular processes are compared with those of similar intermolecular reactions. The intramolecular cyclisation of the anion of ω-bromo-o-hydroxyacetophenone is much more efficient than the analogous intramolecular proton transfer in the anion of o-hydroxyacetophenone. Explanations are suggested for the fact that intramolecular reactions involving nucleophilic displacements are in general much more effective than those involving proton transfers.