Free Radical Reaction Pathway Research Articles

Free Radical Reaction Pathway, Thermochemistry of Peracetic Acid Homolysis, and Its Application for Phenol Degradation: Spectroscopic Study and Quantum Chemistry Calculations

The homolysis of peracetic acid (PAA) as a relevant source of free radicals (e.g., *OH) was studied in detail. Radicals formed as a result of chain radical reactions were detected with electron spin resonance and nuclear magnetic resonance spin trapping techniques and subsequently identified by means of the simulation-based fitting approach. The reaction mechanism, where a hydroxyl radical was a primary product of O-O bond rupture of PAA, was established with a complete assessment of relevant reaction thermochemistry. Total energy analysis of the reaction pathway was performed by electronic structure calculations (ab initio and semiempirical methods) at different levels and basis sets [e.g., HF/6-311G(d), B3LYP/6-31G(d)]. Furthermore, the heterogeneous MnO2/PAA system was tested for the elimination of a model aromatic compound, phenol from aqueous solution. An artificial neural network (ANN) was designed to associate the removal efficiency of phenol with relevant process parameters such as concentrations of both the catalyst and PAA and the reaction time. Results were used to train and test ANN to identify an optimized network structure, which represented the correlations between the operational parameters and removal efficiency of phenol.

Titanium superoxide: a heterogeneous catalyst for anti-Markovnikov aminobromination of olefins

A new facile procedure for the aminobromination of olefins in high yields has been described using p-toluene sulfonamide ( p-TsNH 2) and N-bromosuccinimide (NBS) as nitrogen and bromine sources, respectively, and titanium superoxide as a truly heterogeneous catalyst. The formation of anti-Markovnikov product exclusively in all the cases studied possibly proceeding through a free radical reaction pathway is remarkable.

A radical pathway for organic phosphorylation during schreibersite corrosion with implications for the origin of life

Phosphorylated compounds (e.g., DNA, RNA, phospholipids, and many coenzymes) are critical to biochemistry. Thus, their origin is of prime interest to origin of life studies. The corrosion of the meteoritic mineral schreibersite ((Fe, Ni) 3P) may have significantly contributed to the origin of phosphorylated biomolecules. Corrosion of synthetic schreibersite in a variety of solutions was analyzed by nuclear magnetic resonance spectroscopy, mass spectrometry, and electron paramagnetic resonance spectroscopy. These methods suggest a free-radical reaction pathway for the corrosion of schreibersite to form phosphite radicals ( PO 3 2 - ) in aqueous solution. These radicals can form activated polyphosphates and can phosphorylate organic compounds such as acetate to form phosphonates and organophosphates (3% total yield). Phosphonates (O 3P–C) are found in the organic P inventory of the carbonaceous meteorite Murchison. While phosphonates are rare in biochemistry, the ubiquity of corroding iron meteorites on the early Earth could have provided a source of organic phosphorous compounds for the origin of life, and may have led to the role of organophosphates as a product of early evolution.

Pyrolysis of Benzyl Phenyl Ether Confined in Mesoporous Silica

Benzyl phenyl ether (PhOCH2Ph, BPE), a model for related structural features in lignin and low-rank coal, has been immobilized in MCM-41 hexagonal mesoporous silicas having pore diameters of 2.7, 2.2, and 1.7 nm. Pyrolysis studies conducted at 275 °C have revealed that the free-radical reaction pathways previously observed on nonporous Cabosil silica remain operative in the MCM-41 silicas. The BPE pyrolysis rate is slightly slower (ca. 2- fold) in these nanoporous solids than on Cabosil, which is correlated with a corresponding decrease in the rate of radical formation by homolysis of the weak O−C bond. Two rearrangement pathways dominate the chemistry and account for ca. 67 mol % of the products at all pore sizes studied, which is markedly increased compared with the corresponding Cabosil case (ca. 50 mol %). The first pathway is recombination of the incipient benzyl and phenoxyl radicals to form surface-immobilized benzylphenol isomers, 1. The second pathway involves an O−C phenyl shift in surface-attac...

Methanol as a Cosolvent and Rate-Enhancer for the Oxidation Kinetics of 3,3‘,4,4‘-Tetrachlorobiphenyl Decomposition in Supercritical Water

Experimental results and kinetic analyses are presented for the dechlorination/oxidation of the 3,3‘,4,4‘-tetrachlorobiphenyl (T4CB) congener in supercritical water (SCW) at 25.3 MPa and 673−773 K. A plug-flow reactor is employed with nominal T4CB feed concentrations of 2.97−5.48 μmol/L in methanol solutions (1.71 mmol/L) using H2O2 as the oxidant. Conversions vary from 20.2% (5.76 s at 673 K) to 99.5% (20.7 s at 773 K). The overall conversion is independent of the concentration of O2 and follows apparent second order with Arrhenius parameters of A = 1020.7±0.1 s-1 (mol/L)-1 and Ea = 219 ± 2 kJ/mol. Methanol functions as a rate enhancer by inducing a free-radical reaction pathway of consecutive dechlorinations leading to biphenyl, which then oxidizes to mineral products. Positively identified reaction intermediate products are lower chlorinated PCB congeners (3,3‘,4- and 3,4,4‘-TriCB; 3,3‘-, 3,4-, 3,4‘-, and 4,4‘-DiCB; and 3- and 4-CB), biphenyl, CO, and CO2. Successive global reaction networks with second-order kinetics using both individual and lumped pseudo intermediates and final products are shown to represent well the main features of the reaction pathway. Conversions of the individual T4CB in methanol are compared with those of T4CB in Aroclor 1248/methanol solutions under similar conditions. The kinetic parameters for the two cases have very similar values, which are also close to those of Aroclor 1248, suggesting the possibility of treating this complex PCB mixture as a pseudo T4CB. Significant thermal decomposition of T4CB/methanol in SCW occurs at temperatures higher than 773 K (12.1% at 10.4 s).

NMR spin trapping: detection of free radical reactions using a phosphorus-containing nitrone spin trap.

This study employs (31)P-nuclear magnetic resonance (NMR) to probe for changes in molecular structure arising from reactions between free radicals and a phosphorus-containing nitrone spin trap, 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). A number of biologically relevant free radical reactions were detected: a) reactions of DEPMPO with ( small middle dot)OH resulted in a new (31)P-NMR resonance at 27.05 ppm (shifted from the parent compound at 23.67 ppm); evidence suggests that this species is a diamagnetic hydroxy-pyrrolidone reduction product; b) (31)P-NMR spectra of DEPMPO/( small middle dot)CH(3) reactions resulted in peaks at 24.54, 30.83, and 32.31 ppm, while DEPMPO/( small middle dot)CH(2)OH produced peaks at 24.05, 30.80 and 32.52 ppm; in the presence of excess ascorbate, only resonances between 30 and 32 ppm were evident, which we have tentatively assigned to the hydroxylamine isomers of their respective adducts; and c) reaction of DEPMPO with O(2)( small middle dot-), produced by xanthine/xanthine oxidase or stimulated neutrophils, resulted in a single line, indistinguishable from DEPMPO/( small middle dot)OH reaction products. We conclude that NMR spin trapping is a useful approach for detecting free radical reaction pathways. It may have future applications for human free radical biology and imaging. Magn Reson Med 42:228-234, 1999.

Pyrolysis mechanisms of lignin: surface-immobilized model compound investigation of acid-catalyzed and free-radical reaction pathways

The pyrolysis of surface-immobilized β-alkyl aryl ethers, m ≈ PhOCH 2 CH 2 Ph, ≈ PhCH 2 CH 2 OPh, and ≈ PhCH 2 CH 2 OPh- o- OCH 3, has been studied with a dispersed acid catalyst at 300–450 °C in order to gain an insight into the relative importance of ionic reactions vs. free-radical reactions in the thermal depolymerization of lignin. In the absence of acid catalyst, surface-immobilization does not hinder the free-radical reaction pathway to produce phenols, alkenes, alkanes, and aldehydes as the major products. Pyrolysis in the presence of small particle sized (15 nm) SiO 2-1% Al 2O 3 at 300 °C, where free-radical reactions are suppressed, produces an acceleration in the rate of decomposition and a substantial alteration in the product distribution as a consequence of the solid-solid interactions between the catalyst and substrate. Products from the silica-alumina catalyzed reactions are characteristic of acid-catalyzed cracking reactions which involve carbocation intermediates to produce products from ether cleavage and aromatic alkylation and dealkylation reactions resulting most notably in the near absence of alkenes. The surface-immobilized primary products continue to undergo acid-catalyzed reactions to form larger aromatics and coke. At temperatures in which the free-radical and acid-catalyzed reactions can compete (> 375 °C), products from the acid-catalyzed reaction dominate. A comparison of the products from the pyrolysis of lignin, biomass, and the surface-immobilized model compounds provides new evidence that the thermal degradation of lignin occurs principally by a free-radical reaction pathway.

The influence of sonic energy on the hydrolysis of procaine hydrochloride in aqueous buffer at pH 9.15

The hydrolysis of procaine hydrochloride in borate buffer solution pH 9.15 under sonication is described. Both sonicated and non-sonicated solutions follow first-order kinetics. Activation energy calculations indicate that sonication facilitates a free radical reaction pathway. Experiments are described which show that the rate of reaction is reduced when the hydroxyl radical scavengers glucose and n-propyl gallate are added to the procaine hydrochloride solution. Examination of the effects of temperature indicates that below 65° C the influence of hydroxyl radicals on the rate of reaction is important, since their formation is facilitated by low vapour pressure.

Quantitative PMO Analysis of a Free Radical Reaction Pathway

Quantitative PMO Analysis of a Free Radical Reaction Pathway