Phenoxonium Ion Research Articles

Probing of silver oxide nanoblades for 4-hydroxybenzoic acid quantification: a tool for food and water safety assessment

From various emerging harmful contaminants, 4-hydroxybenzoic acid (HBA) has gained considerable recent attention due to its penetration into certain consumer products. In our research, for the first time, we report the synthesis and utilization of novel-shaped silver oxide nanoblades (Ag2O NBs) towards direct electrochemical sensing of HBA. The synthesis of Ag2O NBs was accomplished using a chemical method to produce nanoblades with a thickness of 15–20 nm, high homogeneity (band gap 2 eV), crystallinity, and a cubic unit cell. The electrocatalytic competence of the synthesized Ag2O NBs was established through their admittance behavior and capability towards oxidation of HBA. The fabricated Ag2O NBs/Nafion/Au electrode was evaluated electrochemically for its conduction behavior and electrocatalytic activity towards oxidation of 4-hydroxybenzoic acid (HBA) into phenoxonium ion. A cyclic voltammetry (CV) technique was standardized and optimized for the determination of HBA content in authentic samples, and the results were found to be in close agreement with that of spiked samples. The selectivity of the fabricated electrode and standardized technique was also evaluated in the presence of potential interfering species. This fabricated sensor demonstrated a detection limit of 4.58 ppm (linear range: 5–50 ppm) and sensitivity of 83.87 μA cm−2 ppm−1. Additionally, the practicability of the fabricated sensor was realized through analysis of various types of field samples.

Intramolecular oxygen transfer in the gas-phase dissociation of protonated sulfonamides

In this study, an intriguing intramolecular oxygen transfer reaction in the gas-phase dissociation of protonated sulfonamides was investigated by electrospray ionization tandem mass spectrometry (ESI-MSn). Sulfonamides with various substituent groups (R) were chosen for collision induced dissociation (CID) experiments, and a peak of a characteristic phenoxonium ion (RC6H4O+) was observed for the sulfonamides with substituent groups that favor the formation of conjugated phenoxonium ions. A mechanism involving a three-membered ring transition state (TS1) was proposed for the generation of this fragment ion, which was supported by DFT calculations. And also, the ratio of intensity of IRC6H4O+ and I(RC6H4++RC6H4O+) correlates well with the Hammett para-substituent constants of substituent groups. Our work deepens the understanding of the intramolecular oxygen transfer reactions of sulfonamides in the gas phase.

Short‐Step Anodic Access to Emissive RNA Homonucleosides (Eur. J. Org. Chem. 7/2014)

The cover picture shows a schematic impression of our synthetic approach toward new emissive nucleoside analogues. Our methodology employs an anodic oxidation of methoxyphenols (right) in lithium perchlorate/nitromethane electrolyte solution, generating reactive phenoxonium ions, which were then trapped by nucleophilic allyl ribose (left) to construct emissive base analogues in one step from non-emissive starting materials. The straight one-way road represents a short-step synthesis, and allyl ribose should “yield” the right-of-way to methoxyphenols to “yield” the desired products. Details are discussed in the Short Communication by K. Chiba et al. on p. 1371 ff.

Synthesis of Pterocarpan-Type Heterocycles via Oxidative Cycloadditions of Phenols and Electron-Rich Arenes

Oxidation of 4-alkoxyphenols or 4-methoxynaphthol with phenyl iodonium(bis)trifluoroacetate (PIFA) in the presence of electron rich 2H-chromenes or dihydronaphthalenes affords pterocarpans or 5-carbapterocarpans via a formal [3 + 2] cycloaddition process. Acid-catalyzed ionization of quinone dimethylmonoacetal in the presence of 7-methoxy-2H-chromene gave similar results, suggesting a phenoxonium ion intermediate in the oxidative process.

Voltammetric Behavior of Trolox in Methanol and Ethanol Solutions

AbstractElectrochemical oxidation of trolox on carbon electrodes was investigated by means of some voltammetric techniques in methanol and ethanol solutions in the presence of an excess of acid or base. The oxidation occurs according to EEC mechanism, i.e. in two quasireversible one‐electron steps yielding the corresponding phenoxyl radical and, subsequently, phenoxonium cation. Phenoxonium ion decays because of a pseudo‐first order follow‐up chemical reaction with nucleophiles. The electrochemical behavior of trolox is strongly influenced by the surrounding environmental conditions. In acidic solutions, a single two‐electron anodic wave was observed. The presence of a strong base drastically improves the stability of the intermediate, phenoxyl radical. Under these conditions, the two‐electron wave was found to split into its one‐electron components over a range of scan rates. The resolution is much distinct in ethanol. Simulations with DigiSim software were used to fit the cyclic voltammograms in order to estimate thermodynamic and kinetic data for the reaction mechanism studied.

Investigation into Phenoxonium Cations Produced during the Electrochemical Oxidation of Chroman-6-ol and Dihydrobenzofuran-5-ol Substituted Compounds

A series of chroman-6-ol and dihydrobenzofuran-5-ol based compounds with structures similar to vitamin E were examined by cyclic voltammetry and controlled potential electrolysis. The compounds displayed characteristic voltammetric features that enabled their electrochemical behavior to be interpreted in relation to the oxidation mechanism for vitamin E. The electrochemical experiments indicated the presence of several oxidized species: cation radicals, phenoxyl radicals, phenoxonium ions, hemiketals, and p-quinones, whose lifetimes varied depending on the extent of methylation of the aromatic ring (R(1), R(2), R(3)) and the nature of substituents R(4) and R(5).

Pulse radiolysis and theoretical investigation on the initial mechanism of the e-beam polymerization of epoxy resins. The results obtained on (phenoxymethyl)oxirane

The radical cation of (phenoxymethyl)oxirane ( PGE ⋅ + ) is generated by pulse radiolysis in dichloromethane solution and by direct action of radiation on the title compound. In the pure system its UV–vis spectrum is characterized by two bands at 340 and 430 nm, such that the electronic structure corresponds to a dipole bearing the positive charge on the phenoxy side. At the same time, the phenoxy-oxirane bridge is weakened. Then, the radical cation fragments into a phenoxonium ion and an oxyranylmethyl radical ( k PGE - fragm = 1.16 × 10 7 s - 1 ) . The latter immediately rearranges to an allyloxyl radical by ring opening. It is then conceived that the action of onium salts, as radiation curing initiators of phenoxy-oxirane derivatives, occurs after epoxy ring opening.

Electrochemical oxidation of trolox and α-tocopherol in acetic acid: A comparative study

A comparative study on the anodic oxidation of trolox and their parent compound, α-tocopherol, was carried out on GC and Pt electrodes in glacial acetic acid containing sodium perchlorate as the supporting electrolyte. The effect of nucleophilic base, acetate ions, was also investigated. Different electrochemical techniques were used to characterize the electrode mechanism. RDE experiments and chronoamperometry at microelectrodes revealed that the processes investigated occur with loss of two electrons. The overall reaction path involves the loss of one electron and proton (1e−, 1H+) to form a phenoxyl radical. This is further oxidized at the electrode surface by the loss of the second one to generate a carbocation (phenoxonium ion) which is susceptible to nucleophilic attack and undergoes chemical conversion into chromanone. The electrochemical behaviour is strongly influenced by the presence of acetate in the background electrolyte. Quantitative fitting of cyclic voltammograms via comparison with theory, based on digital simulation, was used for the determination of the thermodynamic and kinetic parameters of respective steps of the electrode processes.

Transformation of α-Tocopherol (Vitamin E) and Related Chromanol Model Compounds into Their Phenoxonium Ions by Chemical Oxidation with the Nitrosonium Cation

[reaction: see text] Alpha-tocopherol (alpha-TOH), the main oil component making up vitamin E, and its nonnatural solid 6-hydroxy-2,2,5,7,8-pentamethylchroman and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid structurally related analogues were oxidized quantitatively with 2 mol equiv of NO+ SbF6(-) in CH3CN at 233 K to form phenoxonium cations (alpha-TO+ SbF6(-)) in a chemically reversible two-electron/one-proton process. Solution-phase infrared spectroscopy, 1H and 13C NMR spectroscopy, and corresponding theoretical calculations of the spectroscopic data using density-based and wave-function-based models support the identity of the remarkably stable phenoxonium cations. The presence of an oxygen atom in the para position to the hydroxyl group and the chromanol ring structure appear to be important factors in stabilization of the phenoxonium ions, which raises the interesting possibility that the cations play a crucial role in the mode of action of vitamin E in biological systems. Although the phenoxonium cations are reactive toward nucleophiles such as water, they may be moderately stable in the hydrophobic (lipophilic) environment where vitamin E is known to occur naturally.

Intramolecular Anodic Carbon−Carbon Bond Formation from Oxidized Phenol Intermediates. Effect of Oxygenated Substituents on the Yields of Spiro Dienones in Electrochemical and Iodobenzene Diacetate Oxidations

The single-cell, constant-current anodic oxidation of a series of 4-(2-alkenylaryl)phenols was studied in which the aryl substituents were 2-vinyl-4,5-dimethoxy, 6a; 2-propenyl-4,5-dimethoxy, 9a; 2-vinyl-4,5-methylenedioxy, 6b; 2-propenyl-4,5-methylenedioxy, 9b; 2-vinyl-4-methoxy, 6c; 2-propenyl-4-methoxy, 9c; 2-vinyl-5-methoxy, 6d; and 2-propenyl-5-methoxy, 9d. Two compounds having methoxyl groups on the phenolic ring were also studied: 4-(2‘-propenylphenyl)-2-methoxyphenol, 9e, and 4-(2‘-propenylphenyl)-3-methoxyphenol, 9f. A novel synthetic route to the methoxy compounds 6c, 9c, 6d, and 9d was developed. This involved as a key step the MAD, bis(2,6-di-tert-butyl-4-methylphenoxide)methylaluminum, mediated addition of organolithium and Grignard reagents to functionalize quinol ether derivatives. Anodic oxidation of compounds 6a, 9a, 9b, 6c, 9c, and 9d gave good yields of spiro dienones arising from trapping of the phenoxonium ion by the alkenyl side chain followed by reaction of the resulting cation with methanol. The products from the electrochemical oxidation of these 4-(2‘-alkenylaryl)phenols were compared with those obtained from the iodobenzene diacetate oxidations. In general, the compounds that gave good yields of spiro dienones from electrochemical oxidation gave good yields from iodobenzene diacetate oxidations. However, there were two cases in which the results of the two oxidations did not parallel each other. Whereas anodic oxidation of 9f gave a low yield of spiro dienone, the iodobenzene diacetate route gave a 74% yield of this product. In the case of 9d, the opposite effect was noted: the electrochemical route gave a higher yield than did the iodobenzene diacetate oxidation.

One‐step multi‐electron transfer mechanism of polynuclear copper complexes for molecular conversions

AbstractThe oxidative coupling reaction of 2,6‐dimethylphenol may result in either a desired polymeric substance (i.e. the polyphenylene ether, PPE) or the undesired “dimeric” species diphenoquinone, DPQ. The relative amounts of each product depend on the experimental conditions and the used catalytic system.Usually copper amine compounds are used as a catalyst for the oxidative coupling reactions. They have the advantage of easy access and produce high yields of high molecular PPE; however, other metal coordination compounds, like those of Mn, may also be used as catalysts.The present paper focuses on mechanistic studies with various copper (aliphatic and aromatic) amine compounds as catalysts. Owing to the steric constraints of the amine ligands, dinuclear Cu(II) compounds, with small bridging anionic ligands, are easily formed. Such species are believed to be the catalyst precursors. Upon addition of a base (1:1 on copper) and excess phenol, phenolate ligands coordinate as bridging ligands to copper. After a two‐electron transfer reaction, the resulting phenoxonium ligand, which is a rather poor ligand, remains attached to the Cu(I), probably coordinating via its aromatic ring. Nucleophilic attack by a phenol to the phenoxonium ion at the 4‐position is likey to be most important to the coupling reaction. In the beginning of the reaction the undesired side product DPQ is also formed via a C–C coupling reaction. With copper(II) compounds containing imidazole‐type chelating ligands, good activity was obtained; in the case of pyrazole‐based and bridging S‐donor chelating ligands, that no or very weak activity was found.In a study of the mechanism of the propagation reaction the rate‐determining reaction was thought to be probably a one‐step, two‐electron transfer, during which the two Cu(II) ions in the dinuclear complex oxidize the phenolate to phenoxonium. After the phenoxium ion is formed the bonding with the (then) Cu(I) species is weakened and the reactions with phenolic end groups can take place. The effect of the amine ligands appears to be both steric and electronic. With certain ligands the reoxidationof the reduced catalyst is not possible.

Dihydrobenzofuran derivatives via the anodic cycloaddition reaction of p-methoxyphenols and vinyl sulfides

The anodic oxidation of p-methoxyphenols in the presence of vinyl sulfides gives high yields of dihydrobenzofurans arising via a formal 1,3-cycloaddition reaction of the vinyl sulfide and the anodically generated phenoxonium ion.

Intramolecular anodic carbon-carbon bond-forming reactions of oxidized phenol intermediates leading to spirodienones. Structural effects on reactivity and evidence for a phenoxonium ion intermediate

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIntramolecular anodic carbon-carbon bond-forming reactions of oxidized phenol intermediates leading to spirodienones. Structural effects on reactivity and evidence for a phenoxonium ion intermediateJohn S. Swenton, Kenneth Carpenter, Ying Chen, Michael L. Kerns, and Gary W. MorrowCite this: J. Org. Chem. 1993, 58, 12, 3308–3316Publication Date (Print):June 1, 1993Publication History Published online1 May 2002Published inissue 1 June 1993https://doi.org/10.1021/jo00064a017RIGHTS & PERMISSIONSArticle Views745Altmetric-Citations51LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Adsorption of alkyl-substituted phenols onto montmorillonite: investigation of adsorbed intermediates via visible absorption spectroscopy and product analysis

The mechanism of oxidative adsorption of substituted phenols on H + -Cu 2+ -exchanged montmorillonite has been investigated via visible absorption spectroscopy. A characteristic red absorption maximum at approximately 520 nm appears to derive from a phenoxonium ion. The presence of water increased phenol reactivity by increasing the pH of the clay and thus facilitating phenol oxidation

Reactions of some substituted phenols with chromyl and vanadyl chlorides

The oxidation of 2,4,6-tritertbutylphenol and several other alkyl and halophenols by CrO2Cl2 and VOCl3 was studied. The products of CrO2Cl2 oxidation are mostly quinones and diphenoquinones, whilst those of VOCL3 oxidation also include major amounts of dealkylated phenols and C—C coupled dimers. The product distributions are interpreted in terms of a mechanism involving phenoxyl radicals, ligand transfer from metal to radical, and either phenoxonium ions or metallate esters where there is sufficient electron withdrawal from the organic group for it to exhibit carbenium ion properties. The differences in behaviour between CrO2Cl2, VOCl3, and CuCl2 are attributed to different balances between the oxidation potential and Lewis acidity of the metal complexes. It is concluded that CrO2Cl2 is not a good model for proposed ferryl intermediate in heme oxidase systems since it induces 1 → 3 rather than 1 → 2 halogen shifts and an NIH shift that is best explained by carbenium ion-like intermediates.

Anodic pyridination of 2-hydroxy-3-methoxy-5-methylbenzaldehyde and its Schiff's base derivatives

Anodic oxidation of substituted phenols, 2-hydroxy-3-methoxy-5-methylbenzaldehyde (1) and its Schiff's base derivatives (2) in acetonitrile containing an excess of pyridine gave pyridinated phenols. Pyridination occurred at the methyl group of the hydroxybenzaldehyde (1) and the Schiff's base (2g) derived from p-nitroaniline, while for the other Schiff's base derivatives of (1) studied the pyridinium group was on the ring meta to the hydroxy and ortho to the imino group. The pyridination process was investigated by cyclic voltammetry and controlled potential and constant current electrolysis of (1), (2), and related phenols including 2-hydroxy-3-methoxybenzaldehyde Schiff's bases (4). Two different routes are suggested for pyridination, a process involving a phenoxonium ion and one involving a quinone methide. Under the conditions of controlled potential electrolysis, ring pyridination proceeds through the phenoxonium ion route and side-chain pyridination through the quinone methide route. The role of the imino group in the anodic oxidation of the phenolic Schiff's bases (2) and (4) is discussed. The principal mode is suggested to be catalysis of the dimerisation of the corresponding cation radicals by accepting the proton of the hydroxy-group intramolecularly to remove the positive charge from the reaction centre.

Electrochemical oxidation of 2,6-di-tert-butyl-4-isopropylphenol

The anodic oxidation of 2,6-di-tert-butyl-4-isopropylphenol, Ia, as well as the 4-ethyl (Ib) and 4-methyl (Ic) derivatives, has been studied in acetonitrile using carbon electrodes. Depending upon solution conditions and electrode potential, products derived from intermediate phenoxy radicals or phenoxonium ions are found. Oxidation of the phenoxides, IIa–c, leads to phenoxy radicals. Disproportionation of phenoxy radicals to form quinone methides is important for all three phenols, and in the case of IIa the quinone methide so formed, 2,6-di-tert-butyl-4-isopropylidene-2,5-cyclohexadienone, is susceptible to tautomerization followed by oxidation to a bis-quinone methide. The phenoxy radicals formed by oxidation of IIb are known to take part in a reversible dimerization to form a para-quinol ether and this reaction affects the cyclic voltammetry in a particularly interesting manner. Oxidation of Ia (or oxidation of its phenoxy radical) gives principally 2,6-di-tert-butyl-4-hydroxy-4-isopropyl-2,5-cyclohexadienone, a product formally derived from a phenoxonium ion intermediate. 2,6-Di-tert-butyl-4(2-hydroxy-2-propyl)phenol can also be formed on oxidation of Ia. The electrochemical behavior of these phenols is discussed in relation to chemical and electrochemical data of other phenols.

Electrochemical oxidation of 2,6-di-tert-butyl-4-isopropylphenol

The anodic oxidation of 2,6-di-tert-butyl-4-isopropylphenol, Ia, as well as the 4-ethyl (Ib) and 4-methyl (Ic) derivatives, has been studied in acetonitrile using carbon electrodes. Depending upon solution conditions and electrode potential, products derived from intermediate phenoxy radicals or phenoxonium ions are found. Oxidation of the phenoxides, IIa–c, leads to phenoxy radicals. Disproportionation of phenoxy radicals to form quinone methides is important for all three phenols, and in the case of IIa the quinone methide so formed, 2,6-di-tert-butyl-4-isopropylidene-2,5-cyclohexadienone, is susceptible to tautomerization followed by oxidation to a bis-quinone methide. The phenoxy radicals formed by oxidation of IIb are known to take part in a reversible dimerization to form a para-quinol ether and this reaction affects the cyclic voltammetry in a particularly interesting manner. Oxidation of Ia (or oxidation of its phenoxy radical) gives principally 2,6-di-tert-butyl-4-hydroxy-4-isopropyl-2,5-cyclohexadienone, a product formally derived from a phenoxonium ion intermediate. 2,6-Di-tert-butyl-4(2-hydroxy-2-propyl)phenol can also be formed on oxidation of Ia. The electrochemical behavior of these phenols is discussed in relation to chemical and electrochemical data of other phenols.

Electrochemical oxidation of 2,4,6-tri-tert-butylphenol

The electrochemical oxidation of 2,4,6-tri-tert-butylphenol as well as the phenoxide and phenoxy radical derived from it has been investigated in acetonitrile and ethanol+water. The ease of oxidation decreases in the order phenoxide, phenoxy radical, phenol with the separation between potentials for phenoxide oxidation and phenoxy radical oxidation being 1.2 V in acetonitrile. The phenoxide is oxidized to the stable phenoxy radical in a highly reversible reaction in acetonitrile and alkaline ethanol+water. Oxidation of the radical produces a phenoxonium ion which is attacked by water giving 2,4,6-tri-tert-butyl-4-hydroxy-2,5-cyclohexadienone. This two electron product is also formed upon oxidation of the phenol in either solvent. However, in acidic media the hydroxydienone dealkylates give 2,6-di-tert-butylhydroquinone which is oxidized to the final product 2,6-di-tert-butyl-1,4-benzoquinone. The dealkylation is quite rapid in anhydrous acetonitrile but the rate is depressed by the addition of water. A novel double potential step experiment was used to characterize the oxidation of the phenoxy radical. A step to a potential where the phenoxide is oxidized to the phenoxy radical is followed by a step to a potential where the phenoxy radical is oxidized. The current during the second step is unusually small because protons produced by the oxidation of the radical deactivate the phenoxide. The current-time curve was found to agree with that predicted by digital simulation.

Electrochemical oxidation of 2,4,6-tri- tert-butylphenol

The electrochemical oxidation of 2,4,6-tri- tert-butylphenol as well as the phenoxide and phenoxy radical derived from it has been investigated in acetonitrile and ethanol+water. The ease of oxidation decreases in the order phenoxide, phenoxy radical, phenol with the separation between potentials for phenoxide oxidation and phenoxy radical oxidation being 1.2 V in acetonitrile. The phenoxide is oxidized to the stable phenoxy radical in a highly reversible reaction in acetonitrile and alkaline ethanol+water. Oxidation of the radical produces a phenoxonium ion which is attacked by water giving 2,4,6-tri- tert-butyl-4-hydroxy-2,5-cyclohexadienone. This two electron product is also formed upon oxidation of the phenol in either solvent. However, in acidic media the hydroxydienone dealkylates give 2,6-di- tert-butylhydroquinone which is oxidized to the final product 2,6-di- tert-butyl-1,4-benzoquinone. The dealkylation is quite rapid in anhydrous acetonitrile but the rate is depressed by the addition of water. A novel double potential step experiment was used to characterize the oxidation of the phenoxy radical. A step to a potential where the phenoxide is oxidized to the phenoxy radical is followed by a step to a potential where the phenoxy radical is oxidized. The current during the second step is unusually small because protons produced by the oxidation of the radical deactivate the phenoxide. The current-time curve was found to agree with that predicted by digital simulation.