Radical Reaction Mechanism Research Articles

Decomposition of dimethyl polysulfides under solar irradiation in oxic aqueous solutions

Environmental contextThe quality of drinking water can be greatly compromised by the presence of dimethyl polysulfides. We studied the rate and mechanism of decomposition of dimethyl polysulfides in aqueous solution under solar irradiation, and found that they decompose photochemically in seconds to minutes, i.e. much faster than under dark conditions. These results suggest that photochemical pathways of dimethyl polysulfide decomposition may prevail in euphotic zones of natural aquatic systems. AbstractThe presence of malodorous dimethyl polysulfides (DMPSs) has been documented in marine and limnic systems as well as in tap water distribution systems. These compounds compromise the quality of drinking water. Under oxic conditions and in the absence of radiation, DMPSs with n ≥ 3 sulfur atoms disproportionate into DMPSs with n+1 and n−1 sulfur atoms, and, finally, to dimethyl disulfide (DMDS) and S8. DMDS, in turn, decomposes to methyl mercaptan (MT) and methanesulfinic acid. Under these conditions, the half-lives of DMPSs vary from months for dimethyl pentasulfide (DM5S) to hundreds of thousands of years for DMDS. In this work, we studied the kinetics and mechanisms of the decomposition reactions of DMPSs with 2–5 sulfur atoms in aqueous solutions in the presence of oxygen and under exposure to solar radiation. The quantum yields of decomposition of DMPSs with 2, 3, 4 and 5 sulfur atoms do not depend on either the concentration of DMPSs or pH, and are 40±10, 2.0±0.2, 35±10 and 10±4 respectively. The quantum yields, which are higher than unity, suggest that under exposure to solar radiation the photochemical decomposition of DMPSs proceeds by a radical chain reaction mechanism. Half-lives of DMPSs in oxic aquatic solutions exposed to solar radiation under a very clear atmosphere and a solar elevation angle of 90° were calculated from the quantum yields and were found to be as low as 43±13s for DMDS, 40±4s for dimethyl trisulfide (DMTS), 2.1±0.6s for dimethyl tetrasulfide (DM4S) and 4.2±1.7s for DM5S.

An ingenious characterization of reaction network using sub-network reconstruction

Reaction network model is essential for reactor simulation and kinetic profiling of a complex process. Comprehensive and valid proofs of network mechanism are usually lacking among experimental and simulation studies of complicated reaction process, such as ethylene furnace co-cracking. In this paper, a self-developed Ethylene cracker Simulation and Optimization System (EcSOS) software is applied for co-cracking process numerical simulation and an ingenious characterization method of reaction network is proposed for visualization and deep profiling of the radical reaction network. An effective sub-network reconstruction process is implemented by connecting the substance nodes using actual mass flow and refining key interactions. The product yield evolving trend in simulation and its reason was successfully explained by the reaction network analysis results. The radical reaction mechanism changing obtained by reaction backtracking was systematically sorted out and summarized.

Characterization of Radical SAM Adenosylhopane Synthase, HpnH, which Catalyzes the 5′‐Deoxyadenosyl Radical Addition to Diploptene in the Biosynthesis of C35 Bacteriohopanepolyols

AbstractAdenosylhopane is a crucial intermediate in the biosynthesis of bacteriohopanepolyols, which are widespread prokaryotic membrane lipids. Herein, it is demonstrated that reconstituted HpnH, a putative radical S‐adenosyl‐l‐methionine (SAM) enzyme, commonly encoded in the hopanoid biosynthetic gene cluster, converts diploptene into adenosylhopane in the presence of SAM, flavodoxin, flavodoxin reductase, and NADPH. NMR spectra of the enzymatic reaction product were identical to those of synthetic (22R)‐adenosylhopane, indicating that HpnH catalyzes stereoselective C−C formation between C29 of diploptene and C5′ of 5′‐deoxyadenosine. Further, the HpnH reaction in D2O‐containing buffer revealed that a D atom was incorporated at the C22 position of adenosylhopane. Based on these results, we propose a radical addition reaction mechanism catalyzed by HpnH for the formation of the C35 bacteriohopane skeleton.

Characterization of Radical SAM Adenosylhopane Synthase, HpnH, which Catalyzes the 5'-Deoxyadenosyl Radical Addition to Diploptene in the Biosynthesis of C35 Bacteriohopanepolyols.

Adenosylhopane is a crucial intermediate in the biosynthesis of bacteriohopanepolyols, which are widespread prokaryotic membrane lipids. Herein, it is demonstrated that reconstituted HpnH, a putative radical S-adenosyl-l-methionine (SAM) enzyme, commonly encoded in the hopanoid biosynthetic gene cluster, converts diploptene into adenosylhopane in the presence of SAM, flavodoxin, flavodoxin reductase, and NADPH. NMR spectra of the enzymatic reaction product were identical to those of synthetic (22R)-adenosylhopane, indicating that HpnH catalyzes stereoselective C-C formation between C29 of diploptene and C5' of 5'-deoxyadenosine. Further, the HpnH reaction in D2 O-containing buffer revealed that a D atom was incorporated at the C22 position of adenosylhopane. Based on these results, we propose a radical addition reaction mechanism catalyzed by HpnH for the formation of the C35 bacteriohopane skeleton.

Bismuth-catalyzed methylation and alkylation of quinone derivatives with tert-butyl peroxybenzoate as an oxidant

A bismuth-catalyzed methylation of quinones in the presence of tert-butyl peroxybenzoate (TBPB) was developed via a radical reaction mechanism. Particularly, TBPB was used not only as an efficient oxidant, but also as a green methyl source in such transformation. Moreover, this method could also be efficiently extended to the alkylation of quinones. This reaction tolerated a series of functional groups and prepared a series of derivatives of vitamin K3 and benzoquinone. Notably, antimalarial parvaquone was synthesized by the reaction.

Single Unit Monomer Insertion: A Versatile Platform for Molecular Engineering through Radical Addition Reactions and Polymerization

Single unit monomer insertion (SUMI) is an emerging technology for precise polymer synthesis through a radical addition reaction mechanism with monomer additions occurring one at a time. It possess...

Radical Reactivity, Catalysis, and Reaction Mechanism of Arylcopper(II) Compounds: The Missing Link in Organocopper Chemistry.

Organocopper(I) compounds are recognized as carbon nucleophiles, while organocopper(III) complexes are involved in copper catalysis as intermediates to undergo a cross-coupling reaction with various anionic nucleophiles. In contrast to the chemistry of organocopper(I) and (III) compounds, organocopper(II) chemistry is virtually a missing link in integral organocopper chemistry because structurally well-defined organocopper(II) compounds have barely been isolated or studied. We report in this Article an investigation of the radical reactions of stable and structurally well-defined arylcopper(II) compounds, obtained readily from the arene C-H bond reaction of macrocyclic azacalix[1]arene[3]pyridines and Cu(ClO4)2. We have found that arylcopper(II) compounds acted as essentially radical species to undergo an efficient three-component reaction with radical initiators 2,2'-azobis(isobutyronitrile) (AIBN) or 2,2'-azobis(2,4-dimethylvaleronitrile) (ABVN) and α,β-unsaturated compounds CH2═CHX (X = CO2CH3, CN, CONH2, COCH3, and SO2Ph) to afford polyfunctionalized products. Combined experimental and theoretical studies revealed that radicals couple directly with the Caryl atom of arylcopper(II) compounds to form Calkyl-Caryl bonds through a Cu(II)/Cu(I) mechanism. Comprehension of the formation and radical reactivity of arylcopper(II) compounds has allowed the development of a copper-catalyzed three-component radical reaction for arene C-H bond functionalization.

Polymerization and oxidation of phenols in supercritical water.

The treatment of toxic and difficult-to-degrade phenolic compounds has become a key issue in the coking, pharmaceutical, and chemical industries. Considering the polymerization and oxidation of phenolic compounds in supercritical water partial oxidation/supercritical water oxidation (SCWPO/SCWO), the present study reviewed the removal efficiency and reaction pathway of phenolic compounds and phenolic waste/wastewater under different reaction conditions. Temperature is the dominant factor affecting the SCWO reaction. When the oxidizing ability is insufficient, the organics polymerize to form phenolic compounds. The gradual increase of oxidant equivalent causes the intermediate product to gradually oxidize to CO2 and H2O completely. Finally, the free radical reaction mechanism is considered to be a typical SCWO reaction mechanism.

A Comparative Study of the Catalytic Behaviour of Alkoxy-1,3,5-Triazapentadiene Copper(II) Complexes in Cyclohexane Oxidation

The mononuclear copper complexes [Cu{NH=C(OR)NC(OR)=NH}2] with alkoxy-1,3,5-triazapentadiene ligands that have different substituents (R = Me (1), Et (2), nPr (3), iPr (4), CH2CH2OCH3 (5)) were prepared, characterized (including the single crystal X-ray analysis of 3) and studied as catalysts in the mild oxidation of alkanes with H2O2 as an oxidant, pyridine as a promoting agent and cyclohexane as a main model substrate. The complex 4 showed the highest activity with a yield of products up to 18.5% and turnover frequency (TOF) up to 41 h−1. Cyclohexyl hydroperoxide was the main reaction product in all cases. Selectivity parameters in the oxidation of substituted cyclohexanes and adamantane disclosed a dominant free radical reaction mechanism with hydroxyl radicals as C–H-attacking species. The main overoxidation product was 6-hydroxyhexanoic acid, suggesting the presence of a secondary reaction mechanism of a different type. All complexes undergo gradual alteration of their structures in acetonitrile solutions to produce catalytically-active intermediates, as evidenced by UV/Vis spectroscopy and kinetic studies. Complex 4, having tertiary C–H bonds in its iPr substituents, showed the fastest alteration rate, which can be significantly suppressed by using the CD3CN solvent instead of CH3CN one. The observed process was associated to an autocatalytic oxidation of the alkoxy-1,3,5-triazapentadiene ligand. The deuterated complex 4-d32 was prepared and showed higher stability under the same conditions. The complexes 1 and 4 showed different reactivity in the formation of H218O from 18O2 in acetonitrile solutions.

New model phthalonitrile resin system based on self‐promoted curing reaction for exploring the mechanism of radical promoted‐polymerization effect

ABSTRACTThe reaction mechanism and product properties of radical promoted‐polymerization effect of phthalonitrile (PN) were further studied in this article. Different from the previous blending model system, this work provided a structurally symmetrical self‐promoted PN model compound [di‐imide PN model monomer (DAIM‐PN)], which was more conductive to study the polymerization mechanism. Combination of “in situ” infrared, proton nuclear magnetic resonance, and matrix‐assisted laser desorption/ionization time of flight mass spectrometry offered a clearer picture of polymerization mechanism. The results indicated that DAIM‐PN formed isoindoline and its cyclized tetramer of phthalocyanine selectively during the curing process. This work also demonstrated that the aliphatic chain participated in the reaction of nitrile group and provided more evidence for the reaction site (secondary carbon). Besides, high performance polymers with good processing properties, excellent thermal stability (overall char yield >74% at 800 °C, N2), and heat resistance (Tg > 470 °C) were obtained by blending and copolymerizing DAIM‐PN with 1,3‐bis(3,4‐dicyanophenoxy)benzene. The effectiveness and application potential of the reaction mechanism were confirmed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48134.

Spectral and Kinetic Study of 3-Styrylquinoxalin-2(1 H)-ones Photoreduced by N-Phenylglycine and Amines.

The photoreduction by amines and N-phenylglycine, NPG, of six styrylquinoxalin-2(1 H)-ones derivatives substituted in the styryl moiety, R-SQ, was studied by using flash photolysis. The photoreaction is initiated via a single electron transfer from the electron donor (amines or NPG) to R-SQ excited triplet state, 3R-SQ*, with the formation of a triplet state radical ion pair or a charge transfer exciplex, 3[CRIP/CTE]. These species live longer than the respective 3R-SQ* and have very similar transient spectra. In the presence of NPG, these 3[CRIP/CTE] evolve on μs time scale to the respective hydrogenated radicals, R-SQH•, whose transient spectra and reaction rate constants with NPG are reported. The identity of these hydrogenated radicals was supported by the spectra obtained with the α-H donor triethylamine and previous pulse radiolysis studies in 2-propanol. Our findings allow proposing a radical chain reaction mechanism that explains the observed spectral behavior and rationalizes formation of the main product formed by binding of four PhNHCH2• derived from NPG decarboxylation.

KINETIC BEHAVIOR OF THE INTERACTION BETWEEN TRICALCIUM SILICATE AND LIQUID PHASE

This paper presents a 2D-simulation of the main stages of the tricalcium silicate hardening which reflects a possible mechanism of radical reactions with regard to the spin state of the interacting particles. The principles of the universal static model are used for 2D-simulation. It is shown that the particle charge determines the ionic mechanism. In this case, the electron spin direction does not affect the reaction process. It is demonstrated that the spin direction is important for the interacting particles.

A naked-eye colorimetric sensor for chloroform

A phenoxazine based molecule termed SP has been synthesized and used as selective sensor for halogenated solvents. This molecule shows selective fast response towards halogenated solvent via naked-eye detectable chromism. SP shows colorless solution when dissolved in most solvents initially but changes to blue color in chloroform under UV irradiation (λ = 365 nm) within 5 s. The luminescence spectra of SP in halogenated solvent show a large bathochromic shift (> 100 nm) with 60-fold enhanced emission intensity compared to that in halogen-free solvents. It is also worth mentioning that the photoinduced reaction between SP molecule and the halogenated solvents occurred. Based on the detailed NMR, fluorescence and mass spectra, the possible radical reaction mechanism was proposed. Different from the majority of solvatochromic sensors that based on the polarity of solvents for detection of halogenated solvents, our sensor system worked in a special fashion

Biocatalytic oxidation of flavone analogues mediated by general biocatalysts: horseradish peroxidase and laccase.

Horseradish peroxidase (HRP) and laccase are well known oxidases, which have been widely applied for the biosynthesis of organic compounds. In the present work, flavone analogues as an important type of bioactive natural product could be oxidized by HRP or laccase, which afforded dimeric and oxidative flavones. All of the flavone analogues usually possessing phenolic groups could be transformed using HRP. However, only flavonols, isoflavones and chalcones with phenolic groups and dihydroxylflavones were effective substrates of laccase. The radical reaction mechanism with the B-ring of flavone analogues as the radical reaction trigger was proposed for the oxidation of flavones. In silico molecular docking analyses for assaying the interaction between flavone analogues and oxidases indicated that the phenolic groups at the B rings of flavones docked into the HEME active pocket of HRP well. Kinetic behaviors of the oxidation for various flavone analogues mediated by HRP or laccase displayed Hill and substrate inhibition kinetic models. Therefore, in the present work, the oxidation of various flavone analogues mediated by HRP or laccase has been successfully characterized, which would be helpful for the preparation of flavone derivatives.

Mechanistic investigation of Sono–Phosphotungstic acid/phase transfer agent assisted oxidative desulfurization of liquid fuel

AbstractThe aim of this paper is to identify the physical and chemical mechanism of ultrasound in presence of phase transfer agent (PTA) and phosphotungstic acid on desulfurization process. With different experimental procedures, we made an effort to identify the individual effect of phase transfer agent and ultrasound on oxidative desulfurization. Ultrasound‐assisted process is the modern technique to enhance the kinetic of the liquid–liquid heterogeneous system. Physical and intrinsic properties of liquid fuels such as diffusivity of PTA‐cation complex and also through effect on cavitation have a strong influence on Desulfurization of liquid fuels. Due to low vapor pressure of solvents, the beneficial effect of higher static pressure (that eliminates cavitation effect) is not seen in present reaction system. Phase transfer agent or phosphotungustic acid‐assisted ultrasound desulfurization has radical and ionic reaction mechanism, which is revealed in terms of lower activation energy (energetic nature of radicals) as well as collision factor (due to instability of radicals) than magnetically stirred system. This study has revealed crucial mechanistic aspects of ultrasound assisted oxidative desulfurization in presence of phase transfer agent and phosphotungustic acid, which will give more insight for further research.

Electrochemical Behavior of an Anti-Viral Drug Valacyclovir at Carbon Paste Electrode and Its Analytical Application

Valacyclovir (VCH) is an antiviral drug, used in the management of viral infections such as herpes simplex and varicella-zoster in humans. It is rapidly converted to acyclovir which has antiviral activity against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) and Varicella-zoster virus (VZV) both in vitro and in vivo. Electrochemical behavior was studied using cyclic voltammetric method, and the analytical application was studied using differential pulse voltammetric technique. The process on the surface of electrode was found to be irreversible and diffusion controlled. The charge transfer coefficient, heterogeneous rate constant, the number of electron transferred and activation parameters were calculated. Possible free radical reaction mechanism taking place on the surface of electrode was proposed. Calibration plot constructed using differential pulse voltammetric technique and applied for quantitative analysis in pharmaceutical and human urine sample. Limit of detection (LOD) and limit of quantification (LOQ) were calculated and found to be 0.028 and 0.09 μM, respectively. The present work describes the electrochemical behavior of an antiviral drug, VCH and its determination in pharmaceutical samples. The method shows the development of a sensor for selective and sensitive determination of VCH.

Green Transforming Metallurgical Residue into Alkali-Activated Silicomanganese Slag-Based Cementitious Material as Photocatalyst

Silicomanganese slag is a solid waste in metallurgical industry and can be transformed into an alkali-activated silicomanganese slag-based cementitious-material (ASSC) for the first time. The ASSC shows quite low electro-conductivity and can be raised dramatically by incorporated carbon black (CB) in the matrix of ASSC to create an electro-conductive alkali-activated silicomanganese slag-based cementitious-composite (EASSC), served as a low cost and environmentally-friendly photocatalyst for the removal of dye pollutant in the paper. The interrelationships of mechanical, optical, electroconductive, microstructural, and photocatalytic properties are evaluated. The network of CB plays a critical role in the electron transfers. The electrical conductivity of EASSC doped 4.5% CB drastically increases by 594.2 times compared to that of ASSC. The FESEM, XRD, and XPS results indicated that the EASSC with mean grain size about 50 nm is composed of amorphous calcium silicate hydrate (CSH), alabandite (α-MnS) and CB. The UV–vis DRS and PL exhibit that the absorption edges of electro-conductive alkali-activated silicomanganese slag-based cementitious-composite EASSC samples are gradually blue-shifted and the photoluminescence intensities progressively decrease with increasing CB content. The activities of photocatalytic degradation of basic violet 5BN dye are positive correlated to the electro-conductivities. The separation efficiency of photo-generated electron-hole pairs is enhanced due to the electron transfers from α-MnS to the network of CB. The photocatalytic degradation of dye pollutant belongs to the second order kinetics via a reaction mechanism of superoxide radical (•O2−) intermediate.

Mechanisms of triacylglycerides oxidation in artificial lipid droplets

Lipid droplets (LDs) are inducible cellular organelles that have a pivotal role in physiological processes. Depending on their size and host cell type they serve functions ranging from lipid storage (e.g. in adipocytes), cytoprotection (in non-adipose tissue) and inflammation (in macrophages and leukocytes).[1] LD are micelles consisting of a neutral lipid core (triglycerides - TAG, cholesteryl esters - CE and fatty acids - FA) surrounded by a monolayer of phospholipids (PL). Under conditions of oxidative stress (e.g. cancer) lipid droplets have been found to contain increased levels of oxidized TAG (oxTAG). [2] Oxidized lipid species have been shown to be involved in the disturbance of cellular processes by various mechanisms but are also known to serve regulatory functions [3]. Non-enzymatic lipid oxidation is based on an oxygen dependent radical-propagation mechanism. Due to the highly unpolar milieu in the LDs core, polar oxygen-centered radicals (e.g. hydroxyl radical) cannot penetrate through the outer LDs phospholipid monolayer indicating that other mechanisms of radical reactions might govern generation of oxidized lipids in LDs, either involving longer lived, hydrophobic organic radicals or radical propagation from PL monolayer into the LD core. LDs of specific composition (molar ratio: PC/PE 3:1; PL/TAG 1:20) and defined size (140 nm) were prepared by differential ultracentrifugation steps. Size and composition of generated micellar structures was elucidated by nanotracking light scattering microscopy and fluorescence spectrophotometry. Generated micelles were oxidized by either Fenton chemistry (hydroxyl radicals) or by AAPH (organic radicals) and further analyzed by LC-MS using RPC-ESI-QTOF or ESI-LTQ-Orbitrap.

Temperature dependent photocatalysis of g-C3N4, TiO2 and ZnO: Differences in photoactive mechanism

Photocatalysis has been believed as one of the green and sustainable avenues to address energy and environmental crises by converting solar energy to chemical energy via reactions. Temperature is usually a vital factor controlling kinetics and thermodynamics of a reaction, but it has been less investigated in photocatalysis. In this work, the effect of reaction temperature on photocatalysis was investigated in a simple process, photocatalytic degradation of Congo Red (CR) on three typical catalysts, g-C3N4, TiO2 and ZnO, to differentiate the interfacial radical generation and reaction mechanism. The results showed that the temperature has a positive effect on the photocatalytic activity of the three catalysts. The scavenger experiments at various temperatures indicated that the generation of reactive species from the three photocatalysts is different and that the free radicals can be produced more quickly at higher temperatures, causing improved activities in photocatalysis. However, photocurrent analysis and EIS at various temperatures showed that the temperature had a different effect on recombination rate and transfer barriers of the charge carriers from each catalyst. Therefore, the dramatic enhancement in photodegradation activities probably originated from a novel mechanism of the photothermocatalytic oxidation. The interfacial reaction and mechanism from the influence of reaction temperature on the photocatalytic process was proposed.

AgSbF6-Mediated Selective Thiolation and Selenylation at C-4 Position of Isoquinolin-1(2 H)-ones.

A new and facile AgSbF6-mediated protocol for the construction of C-4 thiolated or selenylated isoquinolin-1(2 H)-ones via a radical pathway was established. This reaction proceeded efficiently with excellent regioselectivity, a broad substrate scope, and good functional group tolerance. A radical reaction mechanism involving thiyl radicals as key intermediates is proposed for the present transformation.