Mechanisms of Oxidation of 1,2,5-Trimethylpyrrole:  Kinetic, Spectroscopic, and Electrochemical Studies

Abstract

The oxidation of 1,2,5-trimethylpyrrole (TMP) in aqueous and organic solvents is studied by various techniques. Heating oxygenated chlorobenzene solutions of TMP results in autoxidation that is initiated via reaction of TMP with O2 and partly propagated via oxidation of TMP by a TMP-derived peroxyl radical. In radiolytic experiments, TMP is oxidized rapidly by Br2 •- (k = 2.3 × 109 L mol-1 s-1), I2 •- (k = 5.1 × 108 L mol-1 s-1), CCl3O2 • (k = 5 × 108 L mol-1 s-1), and N3 • radicals in aqueous solutions and by peroxyl radicals in organic solvents. One-electron oxidation forms the radical cation, which exhibits significant absorption in the UV (λmax ∼ 270 nm, ε ∼ 1300 L mol-1 cm-1) and weaker absorptions in the visible range. This species undergoes rapid dimerization (2k ∼ 5 × 108 L mol-1 s-1), and the dimer is very easily oxidized to a stable product absorbing around 460 nm. NMR analysis of the product formed in irradiated CH2Cl2 solutions is in accord with a dication of dimeric TMP. Other products are also formed under different conditions, probably resulting from addition of peroxyl radicals to the pyrrole ring. In cyclic voltammetry experiments at low scan rates, an irreversible peak at a potential of 0.72 V vs SCE is found for oxidation of TMP in acetonitrile solutions, and a stable product absorbing at 460 nm is formed. The formation of this product involves the transfer of more than one electron per TMP monomer. At very high scan rates, a reversible oxidation step is observed, from which a redox potential of 0.87 V vs SCE is derived for the couple TMP/TMP•+. Several mechanisms are suggested for the consumption of O2 by TMP in organic solvents, including electron transfer and σ-bonding via peroxyl radical addition.

Insight into the Mechanisms of Middle Distillate Fuel Oxidative Degradation. Part 2: On the Relationship between Jet Fuel Thermal Oxidative Deposit, Soluble Macromolecular Oxidatively Reactive Species, and Smoke Point

“…We have previously proposed that electron-rich aromatic molecules such as pyrroles, 11,12 indoles, 13 and carbazoles can be thermally promoted to react directly with molecular oxygen. [14][15][16] It is conceivable that such chemistry could ultimately lead to production of a high concentration of peroxyl radicals.…”

Radiation‐Induced Synthesis of Nanostructured Conjugated Polymers in Aqueous Solution: Fundamental Effect of Oxidizing Species

“…The (EDOT-OH)C hydroxyl adduct as well as the EDOT + C radical cation subsequently undergo a second-order radical-radical reaction with themselves to produce EDOT dimers, as observed experimentally in this work and as reported in the case of both thiophene systems [32] and pyrrole derivatives. [35] In the field of conducting polymers, experiments concerning the photodegradation of polythiophene-based polymers demonstrated in the same way that HOC radicals lead to hydroxyl adducts, whereas N 3 C species give cation radicals of the polymers. [47] Given that radiation-induced polymerization proceeds through a recurrent step-by-step oxidation mechanism, both HOC and N 3 C radicals should react successively on EDOT dimers and then on oligomers to produce continuously hydroxyl adducts in the case of HOC and radical cations in the case of N 3 C. Then, one can expect the exclusive presence of hydroxyl groups in the HOC-induced radiosynthesized PEDOT polymers.…”

“…[32] In agreement with all of the previous results, it was found that HOC radicals add preferentially to the a position in thiophene (Th) to give (ThÀ OH)C adducts. [35] In our case and according to the literature, EDOT monomers should then be oxidized by HOC and N 3 C radicals according to the two following pathways [Eqs. [35] In our case and according to the literature, EDOT monomers should then be oxidized by HOC and N 3 C radicals according to the two following pathways [Eqs.…”

“…[32] Differently, N 3 C radicals abstract electrons of pyrrole derivatives, which leads to radical cations. [35] In our case and according to the literature, EDOT monomers should then be oxidized by HOC and N 3 C radicals according to the two following pathways [Eqs. (10) and (11)]:…”

“…N 3 C is known as an alternative oxidizing system that is more selective than HOC. It is a soft, oneelectron oxidant [35] with a redox potential E8' NHE (N 3 C/N 3 À ) = 1.33 V NHe at pH 7, [35][36][37][38] and it is used in this work for oxidizing EDOT.…”