Ozonation of pyridine in aqueous solution: Mechanistic and kinetic aspects

Abstract

Direct and radicalic ozonation of pyridine in aqueous solution is investigated. Experiments are performed in a semibatch apparatus with continuous admission of ozone to the reacting solution. Reaction processes are followed by means of HPLC and chemical analyses at different ozonation times. Direct ozonation is performed by addition of t-butanol to the reacting system to prevent radicalic development. In these conditions the ozonation of pyridine mainly leads to production of pyridine N-oxide according to first-order kinetics with respect to both ozone and pyridine. Ozonation experiments at different pHs (3.0–6.0) indicate that pyridinium ion has no appreciable reactivity towards ozone. A direct ozonation process thus occurs through the electrophilic attack of ozone on the nitrogen atom of pyridine with a rate constant k = 2.0 ± 0.3 mol −1 ls −1 at 298 K. Ozonation experiments which allow radicalic development of the reaction process show that pyridine oxidation occurs with a strong cleavage of the heterocyclic ring even in the initial ozonation stage. Ammonia, nitrate and amidic compounds, among which is N-formyl oxamic acid, are produced. Formation also of nitroso and/or nitro derivatives can be inferred by the experimental results. Radicalic ozonation of pyridine exhibits an autocatalytic character as shown by the rapid increase of the reaction rate in the initial ozonation stage. Autocatalysis is likely due to reaction intermediates coming from slow direct attack of ozone on pyridine and is capable of promoting the ozone decomposition in aqueous solution. A reaction scheme based on the addition of hydroxyl radicals to the aromatic ring of pyridine is proposed to explain the observed products distribution.