Para-substituted Phenols Research Articles

Selective photocatalytic remediation of para-substituted phenol pollutants over iodine-doped Bi2MoO6 via in-situ generated H2O2 and iodine radicals activation

The preferential degradation of specific harmful organic pollutants in photocatalytic processes is critically important, yet remains a significant challenge due to the uncontrolled nature of reactive species, particularly those with high oxidation potentials such as •OH and h+. In this work, we designed an iodine-doped Bi2MoO6 semiconductor to establish a system for photo-induced H2O2 generation. The incorporation of iodine led to a steady enhancement in the selective photo-degradation of para-substituted phenolic pollutants, known precursors to carcinogenic p-benzoquinones, facilitated by the activation of iodine radicals through H2O2. This system exhibited impressive photo-degradation (96 %) and mineralization efficiency for acetaminophen (APAP), along with superior environmental applicability. It also demonstrated selectivity in degrading phenols with para-substitutions. The in-situ generation and activation of H2O2 and •I were identified as the crucial pathways in this photocatalytic system. This work provides a novel modification path to accomplish the simultaneous generation and utilization of H2O2 in the photocatalytic system and ultimately makes a remarkable performance for the targeted remediation of environmental pollutants.

Microwave-Assisted, Copper-Catalyzed Domino O-H/C-H Arylation Reaction toward the Synthesis of Oxygen-Doped Polyaromatic Molecules.

Benzoxanthenes and their analogues are a very important class of compounds mainly due to their wide range of biological and technological applications. The development of a new methodology for their synthesis, involving an Ullmann-type coupling followed by an intramolecular C-H arylation, catalyzed by copper in a domino fashion, is reported. A variety of para-substituted phenols are amenable to this methodology, affording the desired products in moderate to good yields. Our protocol is expedient and practical and is carried out under microwave irradiation in only 3 min under air. A plausible catalytic cycle is proposed based on experimental mechanistic investigations and density functional theory (DFT) calculations.

Electron-transfer-based peroxymonosulfate activation on defect-rich carbon nanotubes: Understanding the substituent effect on the selective oxidation of phenols.

Nanocarbon-based persulfate oxidation technologies are promising for green elimination of phenolic pollutants. Previous studies revealed the electron transfer via defective carbon nanotube (CNTs) for selective oxidation of various phenols. However, an underlying relationship between the molecular structure of phenols and the selectivity of electron transfer-induced oxidation has not been well understood. Herein, we report that defect-rich CNTs could initiate electron-transfer regime from phenols to peroxymonosulfate (PMS), resulting in the efficient degradation of phenols. Further studies uncover a distinctive substituent group-dependent selective oxidation of phenols via the CNT-mediated electron transfer process. Specifically, the degradation rate of para-substituted phenols with electron-donating groups (e.g., -NH2 and -OCH3) is faster than those with electron-withdrawing groups (e.g., -NO2 and -COOH). For a kind of substituted phenols, the substituent position has a great influence on the phenols degradation and their degradation rates follow this sequence: para >ortho >meta -position. Besides, increasing the number of the substituent group can accelerate the degradation of substituted phenols. This study elucidates the substituent effect on the electron transfer-dominated selective oxidation of phenols for the first time, which guides the application of carbon/persulfate system for the targeted remediation of phenols-polluted wastewater.

Oxidation of organic micropollutant surrogate functional groups with peracetic acid activated by aqueous Co(II), Cu(II), or Ag(I) and geopolymer-supported Co(II)

Peracetic acid (PAA) in combination with transition metals has recently gained increasing attention for organic micropollutant abatement. In this study, aqueous Co(II), Cu(II), and Ag(I) were compared for their capacity to activate PAA. Co(II) outperformed Cu(II) or Ag(I) and the optimum conditions were 0.05 mM of Co(II), 0.4 mM of PAA, and pH 3. However, due to a wider applicability in water treatment, pH 7 (i.e., bicarbonate buffer) was selected for detailed investigations. The abatement of different micropollutant surrogates could be described with a second-order rate equation (observed second-order rate constants, kobs were in the range of 42–132 M−1 s−1). For the para-substituted phenols, there was a correlation between the observed second-order rate constants of the corresponding phenolates and the Hammett constants (R2 = 0.949). In all oxidation experiments, the reaction rate decreased significantly after 1–2 min, which coincided with the depletion of PAA but also with the deactivation of the Co(II) catalyst by oxidation to Co(III) and subsequent precipitation. It was demonstrated that Co(II) immobilized on a geopolymer-foam performed approximately similarly as aqueous Co(II) but without deactivation due to Co(III) precipitation. This provides a potential option for the further development of heterogeneous catalytic Co(II)/PAA advanced oxidation processes utilizing geopolymers as a catalyst support material.

Telescoped Continuous Flow Synthesis of 2-Substituted 1,4-Benzoquinones via Oxidative Dearomatisation of para-Substituted Phenols Using Singlet Oxygen in Supercritical CO2

AbstractThis paper describes a continuous multi-step synthesis in supercritical CO2. A continuous flow synthesis of 2-substituted 1,4-benzoquinones is reported, and details of the high-pressure reactors are given. This proceeds via the telescoped dearomatisation of p-substituted phenols using singlet oxygen in supercritical CO2 and an acid-mediated C–C migration. The process has a short residence time of 30 minutes, with overall yields and projected productivities of up to 83% and 9 g/day, respectively. This methodology enables a safe and efficient synthesis of 2-substituted 1,4-benzoquinones from photo-generated singlet oxygen, and cheap and readily available p-substituted phenols. The procedure has high atom efficiency, low photocatalyst loading, and substitutes potentially hazardous and corrosive reagents and solvents for molecular oxygen, CO2, and the less hazardous solid-supported acid Amberlyst-15.

Chlorination of para-substituted phenols: Formation of α, β-unsaturated C4-dialdehydes and C4-dicarboxylic acids

Despite the widespread occurrence of phenols in anthropogenic and natural compounds, their fate in reactions with hypochlorous acid (HOCl), one of the most common water treatment disinfectants, remains incompletely understood. To close this knowledge gap, this study investigated the formation of disinfection by-products (DBPs) in the reaction of free chlorine with seven para-substituted phenols. Based on the chemical structures of the DBPs and the reaction mechanisms leading to their formation, the DBPs were categorized into four groups: chlorophenols, coupling products, substituent reaction products, and ring cleavage products. In contrast to previous studies that investigated the formation of early-stage chlorophenols, the primary focus of this study was on the elucidation of novel ring cleavage products, in particular α, β-unsaturated C4-dialdehydes, and C4-dicarboxylic acids, which, for the first time, were identified and quantified in this study. The molar yields of 2-butene-1,4-dial (BDA), one of the identified α, β-unsaturated C4-dialdehydes, varied among the different phenolic compounds, reaching a maximum value of 10.4% for bisphenol S. Molar yields of 2-chloromaleic acid (Cl-MA), one of the identified C4-dicarboxylic acids, reached a maximum value of 30.5% for 4-hydroxy-phenylacetic acid under given conditions. 2,4,6-trichlorophenol (TCP) was shown to be an important intermediate of the parent phenols and the C4-ring cleavage products. Based on the temporal trends of α, β-unsaturated C4-dialdehydes and C4-dicarboxylic acids, their formation is likely attributable to two separate ring cleavage pathways. Based on the obtained results, an overall transformation pathway for the reaction of para-substituted phenols with free chlorine leading to the formation of novel C4 ring cleavage products was proposed.

A Hückel Model for the Excited-State Dynamics of a Protein Chromophore Developed Using Photoelectron Imaging.

ConspectusChemistry can be described as the movement of nuclei within molecules and the concomitant instantaneous change in electronic structure. This idea underpins the central chemical concepts of potential energy surfaces and reaction coordinates. To experimentally capture such chemical change therefore requires methods that can probe both the nuclear and electronic structure simultaneously and on the time scale of atomic motion. In this Account, we show how time-resolved photoelectron imaging can do exactly this and how it can be used to build a detailed and intuitive understanding of the electronic structure and excited-state dynamics of chromophores. The chromophore of the photoactive yellow protein (PYP) is used as a case study. This chromophore contains a para-substituted phenolate anion, where the substituent, R, can be viewed as an acrolein derivative. It is shown that the measured photoelectron angular distribution can be directly related to the electronic structure of the para-substituted phenolate anion. By incrementally considering differing R groups, it is also shown that these photoelectron angular distributions are exquisitely sensitive to the conformational flexibility of R and that when R contains a π-system the excited states of the chromophore can be viewed as a linear combination of the π* molecular orbitals on the phenolate (πPh*) and the R substituent (πR*). Such Hückel treatment shows that the S1 state of the PYP chromophore has predominantly πR* character and that it is essentially the same as the chromophore of the green fluorescent protein (GFP). The S1 excited-state dynamics of the PYP chromophore probed by time-resolved photoelectron imaging clearly reveals both structural (nuclear) dynamics through the energy spectrum and electronic dynamics through the photoelectron angular distributions. Both motions can be accurately assigned using quantum chemical calculations, and these are consistent with the intuitive Hückel treatment presented. The photoactive protein chromophores considered here are examples of where a chemists’ intuitive Hückel view for ground-state chemistry appears to be transferable to the prediction of photochemical excited-state reactivity. While elegant and insightful, such models have limitations, including nonadiabatic dynamics, which is present in a related PYP chromophore, where a fraction of the S1 state population forms a nonvalence (dipole-bound) state of the anion.

Rotational Barrier and Bond Dissociation Energy and Enthalpy: Computational Study of the Substituent Effects in <i>Para</i>-Substituted Anilines and Phenols

This report presents the N–H and O–H bond dissociation energies (BDEs) and enthalpies (BDEts) of 27 para-substituted anilines and phenols using Density Functional Theory (DFT) with functional wB97X-D and basis set 6-31G**. The computed BDEs/ BDEts show a strong correlation with the calculated rotational barrier (RB) around phenyl–NH2 and phenyl–OH bonds of the parent neutral molecules. Electron-withdrawing (EW) substituents increased RB and BDEs/BDEts, while electron-donating (ED) substituents caused opposite behavior. Geometric, atomic, molecular, and spectroscopic properties of NH2 and OH groups in neutral anilinic and phenolic molecules exhibited excellent correlations with RB and BDEs/BDEts. The geometry around heteroatoms of the radicals displayed constant geometrical changes for all substituents. Spin density maps confirmed that the unpaired electrons in radicals are delocalized in heteroatoms and phenyl rings for all the para-substituents. Spin delocalization in both types of radicals was further enhanced in the presence of para-ED substituents. The increase in electronic density around heteroatoms of radicals with the strength of ED substituents was found proportional to that in neutral molecules. Therefore, the N–H and O–H BDE/BDEt are mainly governed by the stabilization/destabilization of the neutral molecules and, to a significantly lower extent, the stabilization of radicals in the case of strong ED groups.

OXIDATION KINETICS OF PHENOL, ITS DERIVATIVES AND SOME PARA-SUBSTITUTED PHENOL BY PYRAZINIUM CHLOROCHROMATE IN ACIDIC MEDIUM: A NON-LINEAR HAMMETT PLOTS

In the presence of perchloric acid, the kinetics and oxidation of phenol, phenoxy acetic acid, and a few para-substituted phenols and phenoxyacetic acids by pyrazinium chlorochromate (PzCC) were investigated in acetic acid and water medium. The reaction was fractional-order for both the substrates and unit order for oxidant and acid. The products of oxidation are quinone. Comparing the reaction rate for phenols and their derivatives revealed a similar behavior. The oxidation and kinetics of phenol and its derivatives are established using the non-linear Hammett plots plot which reveal that para-methyl groups slow down the reaction rate and para-nitro and para-chloro groups speed up the reaction rate. Based on the kinetic data and product analyses, an appropriate mechanism has been provided

Hydrogen bond assisted anchoring transitions in nematic liquid crystals at the aqueous interface

Homeotropic alignment of liquid crystals (LCs) at the aqueous interface and their fundamental understanding is still limited known, which is a prerequisite for the development of new kinds of LC-based sensors for the detection of biomolecules in the aqueous phase. Herein, we have investigated the orientational ordering of nematic LC at LC-aqueous interface in contact with the different hydrogen bonding strength of para-substituted phenol derivatives (X; functional group attached to phenol at para position = -NO2, -CN, -Cl, -F, -CHO, -H) intending to understand how the intermolecular hydrogen bonding interactions at LC-aqueous interface amplified into supramolecular ordering in LCs. The hydrogen bonding interactions between -CN group of 5CB and -OH group of phenol derivatives lead to a change in tilt angle from planar to homeotropic ordering of 5CB molecules at LC-aqueous interface. Raman measurement of different mixtures between 5CB and para-substituted phenol derivatives have been recorded to correlate the experimental observations at the LC-aqueous interface as well as direct evidence of hydrogen bonding at the LC-aqueous interface. Density functional theory (DFT) has been used to calculate hydrogen bonding strength and charge transfer mechanism between 5CB and phenol derivatives. The orientation of LC at the aqueous interface depends on the strength of the interfacial hydrogen bonding between phenol derivatives and LC, which varies on substitution of phenol and concentration of the phenol molecules.

B(C6F5)3-Catalyzed Sequential Additions of Terminal Alkynes to para-Substituted Phenols: Selective Construction of Congested Phenol-Substituted Quaternary Carbons.

We have developed a borane-catalyzed sequential addition of terminal alkynes to para-substituted phenols, which affords a wide range of ortho-propargylic alkylated phenols bearing congested quaternary carbons. Control experiments combined with DFT calculations suggest that the reaction undergoes a sequential phenol alkenylation/hydroalkynylation process. Further extension of this strategy to the construction of triaryl-substituted quaternary carbons demonstrates the broad utility of this method.

Direct Irradiation of Phenol and Para-Substituted Phenols with a Laser Pulse (266 nm) in Homogeneous and Micro-heterogeneous Media. A Time-Resolved Spectroscopy Study

Direct irradiation of para-substituted phenols under N2 atmosphere in homogeneous (cyclohexane, acetonitrile, and methanol) and micellar (SDS) solution was investigated by means of time-resolved spectroscopy. After a laser pulse (266 nm), two transient species were formed, viz. the para-substituted phenol radical-cations and the corresponding phenoxy radicals. The radical-cations showed a broad absorption band located between 390 and 460 nm, while the phenoxy radicals showed two characteristic bands centered at 320 nm and 400-410 nm. The deprotonation rate constant of radical-cations (kH) of 105 s-1 and the reaction rate constant of the phenoxy radicals (kR) in the order of 109-1010 M-1·s-1 have been derived. The kH rate constants gave good linear Hammett correlation with positive slope indicating that electron-withdrawing substituents enhance the radical-cation acidity. The binding constants (Kb) of the para-substituted phenols with the surfactant were also measured, and NOESY experiments showed that phenols were located in the hydrophobic core of the micelle. Finally, computational calculations provided the predicted absorption spectra of the transients and nice linear correlations were obtained between the theoretical and experimental energy of the lower absorption band of these species.

Benzyl alcohol oxidation mechanisms by one- and two-electron oxidized species of Cu(II)-salen complexes with para-R-substituents, [Cu(R-salen)]n+ (R = MeO, MeS; n = 1, 2)

Benzyl alcohol oxidation by one- and two-electron oxidized copper(II)-salen complexes with the para-substituted phenolate moieties, [Cu(R-salen)]n+ (H2R-salen = N,N’-bis(3-tert-butyl-5-R-salicylidene)ethylenediamine; R = methoxy (MeO), methylthio (MeS) groups; n = 1, 2) was kinetically investigated under the conditions with large excess of the substrate in comparison to the copper complex concentration (CBnOH ≫ CCu). The one- and two-electron oxidized complexes [Cu(R-salen)]n+ (n = 1 and 2) oxidized benzyl alcohol irrespective of R-groups to generate benzaldehyde and the Cu(R-salen) species in stoichiometries of complex : substrate = 2 : 1 and 1 : 1, respectively. Kinetic analyses for benzyl alcohol oxidation by the one-electron oxidized complexes [Cu(R-salen)]+ revealed significantly different kinetics depending on R-groups: one-step pseudo second-order kinetics was assigned for R = MeO, while two-step pseudo first-order kinetics was suitable for R = MeS. The reaction kinetics by two-electron oxidized complexes [Cu(R-salen)]2+ were assigned to two- and three-step first-order processes for R = MeO and MeS, respectively, in which the first step consists of parallel pathways of complex : substrate = 1:1 and 1:2 reactions to generate the one-electron oxidized complex intermediates irrespective of the R-group. However, the following steps were different depending on R-groups. Associating with the results of kinetic isotope effect (KIE) measurements, the reaction mechanisms were discussed and proposed.

Computational studies of the mechanism of Pd-Catalyzed Intramolecular Friedel–Crafts allylic alkylation of phenols

We previously reported a synthetic method of spirocyclohexadienones using an Pd-catalyzed intramolecular ipso-Friedel–Crafts allylic alkylation of para-substituted phenol derivatives. However, the mechanism for the step leading to spirocyclization and driving force behind the remarkably easier formation of the five-membered spirocyclohexadienone as compared with the six-membered spirocyclohexadienone were unclear. Herein, detailed density functional theory (DFT) calculations for the spirocyclization were performed to obtain a plausible reason for the observed behavior. In addition, the mechanistic basis of the characteristic ortho-selectivity in the related Pd-catalyzed intramolecular Friedel–Crafts allylic alkylation of meta-substituted phenol derivatives was elucidated. DFT calculations and experimental studies revealed that the ortho-selective allylation proceeded via an unexpected eleven-membered oxapalladacycle-mediated intramolecular Friedel–Crafts type process.

One‐Pot Generation of Benzynes from Phenols: Formation of Primary Anilines by the Deoxyamination of Phenols

Benzynes were selectively generated in-situ from phenols and trapped regioselectively by potassium hexamethyldisilazide to form primary anilines following acidic workup. The direct conversion of a phenolic hydroxyl group to a free amino group is a useful method for the preparation of primary aryl amines that are hard to synthesize using coupling reactions involving phenol derivatives with ammonia. While reactions of ortho - and meta -substituted phenols produced meta-substituted anilines exclusively, those of para -substituted phenols provided ortho -silylanilines.

Effect of Anodic Pretreatment on the Performance of Glassy Carbon Electrode in Acetonitrile and Electrooxidation of Para-substituted Phenols in Acetonitrile on Platinum and Glassy Carbon Electrode

In the first part of the work electropolymerisation of phenol was studied at glassy carbon electrode. Rapid fouling of its surface indicated the formation of coherent poly(phenyleneoxide) layer which was demonstrated by the repeated cyclic voltammetric scans. Effect of anodic pretreatment potential in acetonitrile solvent was also investigated and the results showed that at potentials higher than 2 V glassy carbon electrode becomes deactivated. Preanodisation of glassy carbon electrode at 3 V in acetonitrile resulted in diminished anodic peak currents by phenols. It was due to the partial deactivation of electrode surface and its extent increased with the pretreatment time. The electrooxidation of para-substituted phenols (p-Cl-phenol, p-NO2-phenol, p-tertbutylphenol, p-methoxyphenol) in acetonitrile resulted in no fouling layer on platinum electrode and the peak currents were significantly higher than in the first scan of unsubstituted phenol in the same concentration. Glassy carbon deactivated continuously by repeating the scans due to the solvent and bonding of products on the surface.

From phenols to quinones: Thermodynamics of radical scavenging activity of para-substituted phenols

Radical scavenging activity and subsequent oxidation resulting in quinone products represent one of the important features of phenols occurring in plants and other biological systems. However, corresponding thermochemistry data can be still considered scarce. For phenol and 25 para-substituted phenols, we investigate the thermodynamics of the individual reaction steps, including three subsequent hydrogen atom transfers, as well as hydroxyl HO radical addition, leading to final ortho-quinone formation. The substituent and solvent effect of water on corresponding reactions enthalpies is elucidated. Solvent enhances substituent induced changes in the investigated reaction enthalpies. The reliability of employed computational methods for the thermodynamics of hydrogen atom donating ability of studied phenols and catechols is assessed, too. Obtained linear equations enable estimation of studied reaction enthalpies from Hammett constants of substituents.

Ecotoxicity of Nitrogen, Sulfur, or Oxygen Heterocycles and Short-Chained Alkyl Phenols Commonly Detected in Contaminated Groundwater.

Nitrogen, sulfur, or oxygen heterocyclic aromatic hydrocarbons (NSO-HETs) and short-chained alkyl phenols (SCAPs) are commonly detected in groundwater at contaminated sites and in the surrounding environment. It is now scientific consensus that these chemicals pose a risk to human and ecosystem health. However, toxicity data are comparably fragmentary, and only few studies have addressed the ecotoxicity of NSO-HETs and SCAPs in a systematic and comparative fashion. To overcome this shortcoming, we tested 18 SCAPs, 16 NSO-HETs, as well as the homocyclic hydrocarbons indane and indene in the Microtox® assay with Aliivibrio fischeri, the growth inhibition test with Desmodesmus subspicatus, the acute immobilization assay with Daphnia magna, as well as the fish embryo toxicity test with embryos of the zebrafish (Danio rerio). Because of the physicochemical properties of the tested chemicals (limited water solubility, volatility, and sorption to test vessels), actual exposure concentrations in test media and their dissipation over time were analytically quantified by means of gas chromatography with mass spectrometry. Analytically corrected effect levels (median effect and lethal concentrations) ranged from 0.017 to 180 mg L-1 , underlining the environmental relevance of some NSO-HETs and SCAPs. Para-substituted phenols showed the overall greatest toxicities in all 4 toxicity tests. We provide, for the first time, a complete high-quality data set in support of better environmental risk assessments of these chemicals. Environ Toxicol Chem 2019;38:1343-1355. © 2019 SETAC.

Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex.

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO2] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols XArOH (X = para substituent NO2, CF3, Cl, H, Me, tBu, OMe, or NMe2) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M-1 s-1) by second-order kinetics to yield TEMPO• and [LCuOOH]- on the basis of electron paramagnetic resonance spectroscopy, the production of H2O2 upon treatment with protic acid, and independent preparation from reaction of [NBu4][LCuOH] with H2O2 ( Keq = 0.022 ± 0.007 for the reverse reaction). The reactions with XArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p Ka, E1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO2, CF3, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe2 (data for X = H, Me, tBu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO2]+ complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.

Multicatalytic dearomatization of phenols into epoxyquinols via a photooxygenation process.

A multicatalytic photooxygenation of substituted phenols in the presence of rose bengal and cesium carbonate under green LED light is reported. This transformation enabled the introduction of both atoms of singlet oxygen and led to the one-pot synthesis of epoxyquinols in a stereoselective way.