Meta Compounds Research Articles

Electrochemical and QSPR studies of several hydroxy- and amino-polysubstituted benzenes constituents of useful compounds

The environmental impact of hydroxy- and amino-polysubstituted benzenes—constituents of natural and synthetic molecules of interest in food, pharmaceutical, and textile industries—demands new procedures for degradation of such pollutants, electrochemical oxidation being an important tool. Electrochemical studies have been performed to characterize these benzene derivatives by cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), and rotating disk electrode voltammetry (RDE). Their oxidation potential (EOX) has been measured by CV, DPV, and SWV. Linear correlations EOX vs. pH were established for the nondissociated forms of these compounds and good fitting was noticed for EOX vs. Hammett constant for the meta and para-substituted compounds. Diffusion coefficients were also estimated. According to the reversibility of their electrochemical oxidation, studied by CV, the compounds were classified into two main groups: reversible-type I (benzene-1,2-diol, benzene-1,4-diol, and benzene-1,2,4-triol) and irreversible-type II (benzene-1,3-diol, benzene-1,2,3-triol, benzene-1,3,5-triol, 4-aminophenol, benzene-1,4-diamine, and 3,4,5-trihydroxybenzoic acid). A quantitative structure–property relationship (QSPR) study has revealed the characteristics that influence their EOX values. A very good agreement resulted between the calculated and experimental EOX values. The proposed descriptors may be used in the prediction of EOX value for other related compounds. Thus, the optimal conditions for the degradation by electrochemical procedures of environmental pollutants containing such structures can be easily estimated.

Methoxy-derivatization of alkyl chains increases the in vivo efficacy of cationic Mn porphyrins. Synthesis, characterization, SOD-like activity, and SOD-deficient E. coli study of meta Mn(iii) N-methoxyalkylpyridylporphyrins

Cationic Mn(III) N-alkylpyridylporphyrins (MnPs) are potent SOD mimics and peroxynitrite scavengers and diminish oxidative stress in a variety of animal models of central nervous system (CNS) injuries, cancer, radiation, diabetes, etc. Recently, properties other than antioxidant potency, such as lipophilicity, size, shape, and bulkiness, which influence the bioavailability and the toxicity of MnPs, have been addressed as they affect their in vivo efficacy and therapeutic utility. Porphyrin bearing longer alkyl substituents at pyridyl ring, MnTnHex-2-PyP(5+), is more lipophilic, thus more efficacious in vivo, particularly in CNS injuries, than the shorter alkyl-chained analog, MnTE-2-PyP(5+). Its enhanced lipophilicity allows it to accumulate in mitochondria (relative to cytosol) and to cross the blood-brain barrier to a much higher extent than MnTE-2-PyP(5+). Mn(III) N-alkylpyridylporphyrins of longer alkyl chains, however, bear micellar character, and when used at higher levels, become toxic. Recently we showed that meta isomers are ∼10-fold more lipophilic than ortho species, which enhances their cellular accumulation, and thus reportedly compensates for their somewhat inferior SOD-like activity. Herein, we modified the alkyl chains of the lipophilic meta compound, MnTnHex-3-PyP(5+) via introduction of a methoxy group, to diminish its toxicity (and/or enhance its efficacy), while maintaining high SOD-like activity and lipophilicity. We compared the lipophilic Mn(III) meso-tetrakis(N-(6'-methoxyhexyl)pyridinium-3-yl)porphyrin, MnTMOHex-3-PyP(5+), to a hydrophilic Mn(III) meso-tetrakis(N-(2'-methoxyethyl)pyridinium-3-yl)porphyrin, MnTMOE-3-PyP(5+). The compounds were characterized by uv-vis spectroscopy, mass spectrometry, elemental analysis, electrochemistry, and ability to dismute O(2)˙(-). Also, the lipophilicity was characterized by thin-layer chromatographic retention factor, R(f). The SOD-like activities and metal-centered reduction potentials for the Mn(III)P/Mn(II)P redox couple were similar-to-identical to those of N-alkylpyridyl analogs: log k(cat) = 6.78, and E(1/2) = +68 mV vs. NHE (MnTMOHex-3-PyP(5+)), and log k(cat) = 6.72, and E(1/2) = +64 mV vs. NHE (MnTMOE-3-PyP(5+)). The compounds were tested in a superoxide-specific in vivo model: aerobic growth of SOD-deficient E. coli, JI132. Both MnTMOHex-3-PyP(5+) and MnTMOE-3-PyP(5+) were more efficacious than their alkyl analogs. MnTMOE-3-PyP(5+) is further significantly more efficacious than the most explored compound in vivo, MnTE-2-PyP(5+). Such a beneficial effect of MnTMOE-3-PyP(5+) on diminished toxicity, improved efficacy and transport across the cell wall may originate from the favorable interplay of the size, length of pyridyl substituents, rotational flexibility (the ortho isomer, MnTE-2-PyP(5+), is more rigid, while MnTMOE-3-PyP(5+) is a more flexible meta isomer), bulkiness and presence of oxygen.

Correlations between Carbene and Carbenium Stability: Ab Initio Calculations on Substituted Phenylcarbenes, Nonbenzenoid Arylcarbenes, Heteroatom-Substituted Carbenes, and the Corresponding Carbocations and Hydrogenation Products

Calculations were completed at the G3MP2 level on a large group of carbon- and heteroatom-substituted carbenes (X-CH, singlets and triplets), carbenium ions (X-CH2(+)), and their hydrogen addition products (X-CH3). One series includes 11 meta- and 12 para-substituted phenylcarbenes, X = Ar. Gas-phase enthalpies of reaction were calculated for four processes: singlet-triplet enthalpy gaps of the carbenes, DeltaH(ST); enthalpies for deprotonation of the cations yielding singlet carbenes, DeltaH(ACID); hydride ion affinities of the carbenium ions, HIA; and enthalpies of hydrogenation of the singlet carbenes, DeltaH(HYDROG). A plot of HIA vs DeltaH(HYDROG) values provides a direct comparison of substituent effects on the stabilities of the singlet carbenes and the corresponding benzylic cations. These effects are larger for the cations but are remarkably consistent over a wide range of reactivity: 166 kcal/mol in HIA. All four processes were analyzed according to the relative importance of polarizability, polar, and resonance effects. Polar and resonance effects are large and of similar magnitude for meta compounds. For the para compounds resonance effects are more dominant. Calculations were made on three nonbenzenoid arylcarbenes: Ar = cycloheptatrienyl(+), cyclopentadienyl(-), and cyclopropenyl(+). The cyclopentadienyl(-)-substituted system fits the HIA vs DeltaH(HYDROG) correlation, but the other two fall well off the line, suggesting markedly different interactions are at play. A set of heteroatom-substituted carbenes and carbocations was also examined. Points for these groups lie well above the correlation line for the HIA vs DeltaH(HYDROG) plot defined by the aryl compounds, confirming that heteroatoms stabilize the singlet carbene proportionally more than the carbocation.

Meta and Para Effects in the Ultrafast Excited-State Dynamics of the Green Fluorescent Protein Chromophores

Femtosecond transient absorption and fluorescence upconversion experiments have been performed to investigate the photoinduced dynamics of the meta isomer of the green fluorescent protein chromophore, m-HBDI, and its O-methylated derivative, m-MeOBDI, in various solvent mixtures at neutral, acidic, and basic pH. The para isomer, p-HBDI, and its O- and N-methylated derivatives, p-MeOBDI and p-HBDIMe(+), were also studied for comparison. In all cases, fast quenching of the excited S1 state by internal conversion (IC) to the ground state was observed. In the para compounds, IC, presumably promoted by the internal twisting, arises in <1 ps. A similar process takes place in the meta compounds in nonaqueous solvents but with notably slower kinetics. In aqueous solutions, the meta compounds undergo ultrafast intermolecular excited-state proton transfer that competes with isomerization.

Benzene‐1,2‐, 1,3‐, and 1,4‐di‐N‐substituted carbamates as conformationally constrained inhibitors of acetylcholinesterase

Benzene-1,2-, 1,3-, and 1,4-di-N-substituted carbamates (1-15) are synthesized as the conformationally constrained inhibitors of acetylcholinesterase and mimic gauche, eclipsed, and anti-conformations of acetylcholine, respectively. All carbamates 1-15 are characterized as the pseudo substrate inhibitors of acetylcholinesterase. For a series of geometric isomers, the inhibitory potencies are as follows: benzene-1,4-di-N-substituted carbamate (para compound) > benzene-1,3-di-N-substituted carbamate (meta compound) > benzene-1,2-di-N-substituted carbamate (ortho compound). Therefore, benzene-1,4-di-N-substituted carbamates (para compounds), with the angle of 180 degrees between two C(benzene)-O bonds, mimic the preferable anti C-O/C-N conformers of acetylcholine for the choline ethylene backbone in the acetylcholinesterase catalysis.

Benzene‐di‐N‐substituted carbamates as conformationally constrained analogs of Pseudomonas lipase substrates

AbstractBenzene‐1,2‐, ‐1,3‐, and ‐1,4‐di‐N‐substituted carbamates (1–15) are synthesized as the constrained analogs of gauche, eclipsed, and anti conformations of diesters of ethylene glycol, respectively. Carbamates 1–15 are characterized as the pseudo‐substrate inhibitors of Pseudomonas species lipase. Long‐chain carbamates are more potent inhibitors than short‐chain ones. Different geometries of benzene‐di‐substituted carbamates, such as benzene‐1,2‐di‐N‐octylcarbamate (3) (ortho compound), benzene‐1,3‐di‐N‐octylcarbamate (8) (meta compound), and benzene‐1,4‐di‐N‐octylcarbamate (13) (para compound), show similar inhibitory potencies for the enzyme. In other words, kinetic data suggest that the enzyme does not discriminate ortho, meta, and para geometries of these constrained analogs.

TICT Formation in Para- and Meta-Derivatives of N-Phenylpyrrole

The photophysical properties of m- and p-cyano N-phenylpyrrole (m- and p-PBN) are compared. Both compounds show highly red-shifted and strongly forbidden emission in polar solvents, assigned to a charge transfer state. The forbidden nature is indicative of very weak coupling between the two pi-systems, and a twisted emissive structure is suggested (TICT state). Comparison to quantum chemical calculations indicates that the twisted structure possesses an antiquinoid distortion of the benzonitrile group, i.e., the central bonds in the ring are lengthened instead of shortened. m-PBN is the first meta compound which shows a CT emission assignable to a TICT state. It differs from p-PBN by a less exergonic formation of the CT state from the LE/ICT quinoid state. Consequently, it shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence in this solvent (LE/ICT and TICT).

Development of Structure−Taste Relationships for Monosubstituted Phenylsulfamate Sweeteners Using Classification and Regression Tree (CART) Analysis

Twenty monosubstituted phenylsulfamates (cyclamates) have been synthesized and have had their taste portfolios determined. These have been combined with 63 compounds already in the literature to give a database of 83 ortho, meta, and para compounds. A training set of 75 compounds was randomly selected leaving eight compounds as a test set. A series of nine predictors determined with Corey-Pauling-Koltun models, calculated from the PC SPARTAN PRO program and Hammett sigma values taken mainly from the literature, have been used to establish structure-taste relationships for these types of sweeteners. The taste panel data for all compounds were categorized into three classes, namely, sweet (S), nonsweet (N), and sweet/nonsweet (N/S), and a novel "sweetness value" or weighting was also calculated for each compound. Linear and quadratic discriminant analysis were first used with the S, N, and N/S data, but the results were somewhat disappointing. Classification and regression tree analysis using the sweetness values for all 75 compounds was more successful, and only 14 were misclassified and six of the eight test set compounds were correctly classified. For the 29 meta compounds, one subset using just two parameters classified 83% of these compounds. Finally, using various methods, predictions were made on the likely tastes of a number of meta compounds and a striking agreement was found between the tree prediction and those given by earlier models. This appears to offer a strong vindication of the tree approach.

Structural and Magnetic Evidence Concerning Spin Crossover in Formamidinate Compounds with Ru25+ Cores

The magnetic and structural properties of two Ru2(DArF)4Cl compounds, where DArF is the anion of a diaryl formamidine, are presented here. The compounds with Ar = p-anisyl and m-anisyl both show temperature dependence of chiT (chi = molar magnetic susceptibility), but for different reasons. For the para compound, there is a Boltzmann distribution between a pi*3 ground state and a delta*pi*2 upper state, and this is confirmed by a temperature dependence of the Ru-Ru bond length: 2.4471(5) A at 23 K and 2.3968(5) A at 300 K. For the meta compound, a delta*pi*2 configuration persists over the range of 23-300 K as shown by an invariant Ru-Ru bond length, but the chiT drops with decreasing temperature owing to zero-field splitting of a 4B2u ground state.

Protonation effect on the excited state behaviour of EE-1-(n-pyridyl)-4-phenylbutadienes (n = 2, 3 and 4).

The acid-base equilibria of three aza-derivatives of EE-1,4-diphenylbutadiene and the excited state properties of their neutral and protonated forms have been studied in aqueous solutions. The prevalent relaxation of the neutral molecules is photoisomerization by a diabatic singlet mechanism accompanied by substantial internal conversion while the quantum yield of the radiative pathway is very small, particularly for the ortho- and para-substituted pyridines. Protonation affects only slightly the isomerization yield of these two compounds while drastically decreasing that of the meta-substituted analogue. Molecular orbital calculations and comparison with the corresponding azastilbenes indicate that the first excited singlet state of the protonated meta compound has a net charge transfer character, which leads to an increase of the torsional barrier in S1. Application of the Forster cycle to the three compounds showed that they become much stronger bases in S1. The 3-pyridyl derivative proved to be peculiar also in this respect since its equilibration with the proton occurs during the S1 lifetime thus allowing the acid-base equilibration in the excited state to be directly monitored by fluorimetric titration.

Time-dependent density-functional theory investigation of the formation of the charge transfer excited state for a series of aromatic donor–acceptor systems. Part II

TDDFT singlet excitation energies calculations have been performed for a series of donor–acceptor para-substituted N,N-dimethyl-anilines (4DMAB-CHO, 4DMAB-COMe, 3DMAB-CHO, 3DMAB-COMe, 4DMAB-COOH, 4DMAB-COOMe, 4DMAB-CONH2, 4DMAB-CON(CH3)2, and DMAPY) using both B3LYP and MPW1PW91 functionals with a 6-311*(2p,d) basis set. The aim of this study is to investigate the influence of the variation of the acceptor group on the fluorescence behavior of these molecules. This work is a complementary investigation to part I [J. Chem. Phys. 117, 4146 (2002), preceding paper], where variation of the donor group has been studied. Ground-state geometries were optimized using density-functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For all molecules, the potential energy surface has been investigated following the twisting intramolecular charge transfer (TICT) model proposed in the literature as a possible mechanism to explain the fluorescence behavior. Computed vertical absorption energies using the MWP1PW91 and B3LYP functionals compare within 0.02 and −0.09 eV in average with gas-phase experimental data (4DMAB-COMe and 4DMAB-COOMe). Almost all para compounds are predicted to be dual fluorescent within the TICT model, in agreement with experiments. The present calculations predict 4DMAB-CONH2 to be dual fluorescent in nonpolar solvent in disagreement with experimental results. For meta compounds, the literature is sparse and no fluorescence spectra have been reported for these systems. Our results indicate that these molecules should exhibit only one band in the fluorescence spectra. Our calculations reinforce the validity of the TICT model as a possible mechanism to explain the fluorescence activity of these donor–acceptor systems.

Ortho- and meta-(Diethynylcyclopentadienyl)tricarbonylmanganese: Building Blocks toward the Construction of Metal Fragment Supported Fullerenynes?

Starting from [(trimethylsilyl)ethynyl]cymantrene (1), the two isomeric 1,2- and 1,3-[bis-(trimethylsilyl)ethynyl] cymantrenes (3a, 4a ) were prepared by a deprotonation/iodination/coupling sequence. The coupling reaction was performed on the mixed ortho-/meta-iodo[(trimethylsilyl)ethynyl]cymantrenes (2) by using Stille methodology (tin alkynes and PdCl{sub 2}(CH{sub 3}CN){sub 2} as catalyst in ethyl ether/DMF). Separation of the isomers (3a, 4a) is achieved by chromatography over flash silica gel. The ortho-diethynylcymantrene 3a is isolated in 42%, while the corresponding meta compound 4a is formed in 26%. If the deprotonation reaction of 1 in pentane/TMEDA is performed under carefully controlled conditions (-78{degree}C), the formation of only ortho 2 is observed in 67% yield. Coupling with trimethylstannyl(trimethylsilyl)acetylene furnished 3a in 57%. Crystal structures have been carried out for complexes 3a. Removal of the alkyne-bound trimethylsilyl groups is achieved in yields of >80% by treatment of 3a and 4a with potassium carbonate in methanol. The parent diethynylcymantrenes (3b, 4b) are stable compounds. Treatment of 1,2-diethynylcymantrene 3b under Hay conditions gives coupled material, a polymer poly[(ortho-cymantrene)butadiynylene] (6) in 68% yield (M{sub n} = 9300). 15 refs., 4 figs., 1 tab.

Heterobifunctional trityl derivatives as linking reagents for the recovery of nucleic acids after labeling and immobilization

The synthesis of two heterobifunctional 4,4′-dimethoxytrityl derivatives carrying a N-hydroxysuccinic acid group as active ester in para5a or meta position 5b is described. While 5a has been synthesized previously via six steps a novel and more efficient , four step synthesis for both compounds is introduced here. The meta compound 5b provides for a more acid labile tntyl ether linkage compared to the para derivative 5a. Using these compounds as linking agents biopolymers in particular nucleic acids can easily be recovered after labeling or immobilization by treatment with a mild acid such as 80% aqueous acetic acid.

Methyl tunnelling and rotational potentials in solid xylenes and fluorotoluenes

CH3 rotational excitations have been investigated in solid xylenes and fluorotoluenes using inelastic neutron scattering (INS) and nuclear resonance (NMR) techniques. For two of the six materials the temperature dependence of the tunnel line (shift and broadening) has been compared with the density of states and the slopes of the NMR-T, data. The rotational potentials have been derived from tunnel splittings, librational excitations and activation energies. The tunnel energies have been found to occur between 0.026 and 26 mu eV. In one case nonequivalent methyl groups have been identified. As compared with results from isolated molecules, the potentials of ortho isomers are distinguished by a considerable intramolecular contribution, while the rotational potential of the para and meta compounds are governed by intermolecular interactions. The hindering potential calculated for p-xylene on the basis of pair potentials is inconsistent with the experimental results; reasons are discussed.

Studies on the potential mutagenicity of p-phenylenediamine in oxidative hair dye mixtures

An aqueous mixture of p-phenylenediamine dihydrochloride, resorcinol and hydrogen peroxide, incubated for 30 min at 37°C, was not mutagenic in the Ames test and in the mouse lymphoma assay and did not produce chromosome aberrations in human lymphocytes, whereas the same mixture without resorcinol was mutagenic in the Ames test and the chromosome aberration assay, probably due to the formation of Bandrowski's base. The formation of mutagenic Bandrowski's base and other reactive products could be demonstrated by thin-layer chromatography and subsequent Ames testing. Preincubation for 0–7 h of 5 different oxidative mixtures of p-phenylenediamine dihydrochloride and various couplers at 37°C resulted in mutagenicities in the Ames test ranging from ‘very strong effects’ at 0 or 0.5 h for 2,4-diaminoanisole dihydrochloride to ‘not mutagenic’ at up to 7 h preincubation time for a recently developed alternative coupler. A comparable ranking of mutagenic of the same mixtures could be obtained by Ames tests with dyed buffalo hair strands, either tested directly on the agar plates or by testing the solvent extracts of the colored strands. The optimal combination without any sign of mutagenic tendency was finally tested in a commercially available hair dye formulation. This complex mixture was also not mutagenic in the Ames test. Our main conclusions are as follows. (I) Oxidative hair dye mixtures of p-phenylenediamine dihydrochloride and non-mutagenic couplers are not mutagenic if tested under normal conditions of use; the formation of mutagenic reaction products (such as Bandrowski's base) can be prevented if the oxidative reaction time is limited to 30 min and if about equimolar proportions of para to meta compounds are used. (II) Optimal non-mutagenic combinations of primary intermediates (such as p-phenylenediamine dihydrochloride) and couplers can be evaluated in the Ames test by either time-dependent preincubation of the mixtures or the described test protocol with hair strands.

Infrared studies of the interaction between dihydroxybenzenes and their ethers with the surfaces of γ‐Al2O3, MoS2 and a CoMo/γ‐Al2O3 catalyst

AbstractHydroquinone and resorcinol are strongly chemisorbed on alumina by their oxygen atoms and form a flat adsorption mode. Catechol may be chemisorbed, but very weakly in a flat mode on molybdenum sulfide. On alumina it is adsorbed in an inclined mode through the oxygen atoms. For the CoMo/Al2O3 catalyst an inclined mode is preferred at lower temperatures although a flat mode may be competitive at higher temperatures.The monomethyl ethers are chemisorbed through the phenyl oxygen. The mode of adsorption for the ring is probably flat for the meta and para compounds and inclined for the ortho compound. The ortho compound shows demethoxylation at 250°C followed with the para compound 300°C. Steric constraints evidently hinder to a part adsorption of the ortho compound on the catalyst surface.The dimethyl ethers are chemisorbed in a flat mode on the supports. The adsorbed compounds react by demethylation on the alumina type supports with the para compound reacting at the lowest temperature, around 200°C, and the meta compound only at the highest temperature used, 300°C. The ortho compound has an intermediate reactivity.Adsorption of all ethers on molybdenum sulfide is very weak.

Oxygen-17 NMR Spectroscopy: Effect of Substituents on Chemical Shifts and Width Line in Ortho Substituted Nitro Benzenes

Abstract 17O-NMR spectra, of ortho, meta and para substituted nitrobenzene are reported. The 17O nucleus of NO2 group in ortho derivatives absorb at lower field that meta and para compounds. Moreover, width line of 17O resonance appears to be strongly dependent by substituent position. The 17O chemical shifts are correlated with U. V. spectra and X-ray diffraction data.

Gas phase methylation of the dihydroxybenzenes

Tandem mass spectrometry has been used to determine the site of gas-phase methylation of o-, m-, and p-dihydroxybenzene under chemical ionization conditions. The reagent ions were CH 3ClCH 3 + generated from methyl chloride, and CH 3FCH 3 + generated from methyl fluoride. Ring methylation is favored over substituent methylation by approximately 3:1 with both reagent gases. However, the behavior of the meta compound with methyl chloride is exceptional, yielding almost exclusive methylation in the ring. The less reactive dimethylchloronium ion apparently reacts more selectively than its fluorine analog and therefore preferentially generates the thermodynamically more stable adduct. The enhanced stability of the ring bonded form in the meta case follows from simple resonance considerations. The pressure of the reagent gas generally had a significant effect on the ratio of ring-to-substituent methylation. This is accounted for in terms of the approach to equilibrium achieved at elevated pressure. In addition to the ring- and substituent-bonded structures, the presence of a loosely bound ion is also indicated from the collision-induced dissociation spectra of the ion/molecule products. This is particularly evident in the products of reaction of catechol and hydroquinone with methyl fluoride at low pressure. In methylation, as well as in the accompanying halomethylation reactions, isomer identity is retained in the adduct ions.

Conformatonal analysis of n-cyclophanes by molecular mechanics. II. 5-, 6-, and 7-metacyclophane

The optimal geometries, gas phase thermodynamic functions, heats of formation, and strain energies of 5-, 6-, and 7-metacyclophane have been calculated using a molecular mechanical method developed by Boyd et al., and the results compared with available experimental data and with similar results previously obtained for the n-paracyclophane isomers. In general, strain energies in the meta series are some 12 kcal/mol less than in the para series, the strain energy of an n-metacyclophane being roughly that of an (n+1)-paracyclophane. The meta compounds may thus be said to be one degree more stable than the para compounds.

Hydrogenolysis and hydrocracking of the carbonoxygen bond. 3. Thermolysis in tetralin of substituted anisoles

Mild pyrolysis and hydrogenolysis products of coal contain substantial amounts of pyrocatechol and resorcinol and their homologues whereas hydroquinone and its homologues are absent or present in only low amounts. In the present work the model compounds anisole and methyl-, methoxy- or hydroxy-substituted anisoles were studied to elucidate substituent effects on the carbon—oxygen bond cleavage in the presence of tetralin. The experiments were carried out at 618 K and 6 MPa (H 2). The major reaction is demethylation to the corresponding phenols. A steric effect can be seen in the ortho compounds and an electronic effect when the substituent is a strongly electron-releasing group. In compounds with oxygen substituents para to each other little or no hydroquinone can be isolated whereas the ortho and meta compounds, respectively, give pyrocatechol and resorcinol. It is suggested that the low yield or absence of hydroquinone in this work and in coal pyrolysis is due to the high reactivity of the intermediate p-hydroxyphenoxy radical, which gives rise to adducts and other compounds of high molar mass. The ortho radical is sterically hindered and the meta radical has a lower reactivity and are hence abstracting hydrogen from the hydrogen donor or coal.