Abstract
The chemical oxidation of pyrrole by iron(III) chloride in aqueous solutions of methylcellulose has been investigated. The kinetics of the reaction were studied by following the absorption of the soluble (colloidal), oxidised poly(pyrrole) product at 800 nm. In the pH interval 0–0.5 the reaction was first-order in FeIII and pyrrole concetrations and zero-order in proton concentration. For pH > 1.3, the kinetic curves exhibited behaviour characteristic of autocatalytic reactions. A mechanism is proposed in which the rate-limiting step is the oxidation of pyrrole to a radical cation by an outer-sphere activated-complex mechanism. The observed pH dependence on the rate is attributed to the hydrolysis equilibrium for FeIII in solution, in which lower pH decreases the concentration of OH–, which can hinder pyrrole from approaching the oxidant. The observed autocatalytic behaviour at higher pH is caused by the fact that the polymerisation produces protons. A brief comparison with oxidation by ammonium persulphate is given.
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