Kinetics of aromatic iodination reactions using iodine, diiodine pentoxide and sulfuric acid in acetic acid

Kinetic studies of an aromatic iodination reaction using benzene or acetanilide, iodine, diiodine pentoxide and sulfuric acid in glacial acetic acid have been carried out, The reaction, followed using FT-Raman spectroscopy to monitor the disappearance of the aromatic over time, exhibits fractional order dependence in benzene but first order dependence in acetanilide. The difference in order between the two aromatics indicates that benzene participates in an equilibrium reaction before the rate limiting step of the reaction. This reversible step is proposed to be the reversible formation of a pi-complex between adsorbed I-2 and benzene as it adsorbs onto I2O5. Based on the calculated order, acetanilide most likely does not form a pi-complex with adsorbed I-2, rather it reacts from solution to form a sigma-complex with activated I-2 on the I2O5 surface. In both reactions, it is proposed that the rate limiting step is the formation of the sigma-complex.