Stability Of Free Radicals Research Articles

Two-stage liquefaction of a subbituminous coal

The two-stage conversion of a subbituminous coal has been investigated using an autoclave reactor system. The overall performance of the reaction is found to be determined by the effectiveness of the first-stage operation and by the method of sequencing of the stages. The initial thermal products can undergo condensation reactions which render them unresponsive to subsequent catalytic conversion and which increase the light gas yield. This can occur during the first stage reaction, if there is limited capacity for free-radical stabilization, and upon storage and/or thermal cycling between stages. The latter effects are circumvented by operating the two stages in immediate sequence. Condensation is also reduced by increasing the solvent quality and the solvent:coal ratio. The presence of a catalyst during thermal decomposition of coal can greatly improve conversion and product stability even at short reaction time and can reduce constraints on solvent quality. The more feasible approaches to improving first-stage operation appear to be in controlling the solvent composition and in employing hydrogenation catalysts.

Stability of chemical species

AbstractIn the framework of our theoretical approach of the structure and reactivity of chemical intermediates we have been led to reexamine the concept of stability which is widely used by the experimentalists often without specifying its true meaning. In this work we propose a more general definition of the concept of stabilization energy, namely, where ΔHa is the heat of atomization of the species under consideration and the EAB's are standard bond energy terms derived from the heats of atomization of reference compounds. Using experimental heats of formation or semiempirical ones deduced from theoretical heats of reaction of appropriate isodesmic processes, we have calculated the stabilization energies of various types of chemical species: saturated, unsaturated and conjugated molecules, free radicals, carbocations, and carbanions. The results obtained can be rationalized in terms of steric hindrance, angular strain, polar interactions, electron delocalization, and substituent effects. Moreover, we have shown that heats of hydrogenation and bond dissociation energies do not provide accurate information on the thermodynamic stabilization of unsaturated compounds and free radicals, respectively. Among other applications the concept of stabilization energy allowed us to propose a detailed classification of free radicals and to rationalize their reactivity. Considering the particular case of radical recombination reactions we have been able to deduce interesting equations showing the relations between the concept of stabilization energy and other quantities commonly used in chemical physics, namely, the bond cleavage enthalpy [BDE(CC) if one considers alkane thermolysis], the thermodynamic stability measured by the free enthalpy change of a given reaction, and the kinetic stabilization related to the activation energy of a chosen chemical process This analysis allowed us to give a new interpretation of the adjectives transient, persistent, and stable introduced by Griller and Ingold and to show that the persistence of a radical may be due to other factors than steric ones. In conclusion, the concept of stabilization energy appears to be a good tool for rationalizing the static and dynamic properties of chemical species.

Factors affecting the stability and equilibria of free radicals. Part 11. 1-Benzoyl-2,2-bis-(3,5-di-t-butylphenyl)hydrazyl and its temperature-dependent electron spin resonance spectrum

The title free radical was prepared by oxidizing the corresponding hydrazine with lead tetra-acetate in benzene or toluene. Its e.s.r. spectrum undergoes reversible changes in the temperature range –90 to +60 °C. Simulated e.s.r. spectra indicate that the two di-t-butylphenyl groups are non-equivalent owing to restricted rotation around the N–N bond at low temperatures, but at 60 °C these two aryl groups become equivalent. The ΔG‡ value of the rotation barrier is about 31 kJ mol–1 in the temperature range –30 to +50 °C.

Formation and stabilization of free radicals in the co-crystallization of peroxydisuccinic and fumaric acids

Formation and stabilization of free radicals in the co-crystallization of peroxydisuccinic and fumaric acids

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

The half-wave reduction potentials, measured by cyclic voltammetry, and n,π* triplet energies, measured by phosphorescence spectroscopy, were determined for a series of eighteen symmetrically and unsymmetrically substituted benzophenones. Attempts are made to correlate the results with Hammett substituent constants. Better correlations are observed when the data are correlated with a two-parameter function consisting of Hammett substituent constants and a set of substituent parameters describing variations in free radical stability. Significant deviations from "normal" behaviour are observed for benzophenones substituted by both electron-donating and electron-withdrawing substituents. These deviations are attributed to merostabilization of the radical-like species, and an empirical approach designed to evaluate the importance of this effect is developed. Abinitio calculations of molecular orbital energies in meta- and para-substituted benzaldehydes are used to evaluate the substituent effects on E1/2red and ETn,π* in terms of the effect on the energies of the n- and π*-orbitals.

The effect of free radical stability on the rate of bromination of hydrocarbons

The effect of alkyl free radical stability on the rate of free radical halogenation of hydrocarbons can be convincingly demonstrated by the comparative photobromination of the arenes toluene, ethylbenzene, and cumene.

Etude du caractere nucleophile des radicaux lors de la reaction de transfert sur la liaison O-O des peracides

Peracids RC0 3H yield free radicals R' which react either with the peracid or with solvent giving the alcohol ROH and the hydrocarbon RH. The nucleophilic character of the free radicals was modified either by substitution of the carbon bearing the odd electron by inductive groups or by changing the free radical hybridation by the means of blocked structures such as cyclic or bicyclic free radicals. For each R, the measurement of the ratio ROH/RH establishes a reactivity scale for R with the peracid O-O bond. This reactivity does not depend on free radical stability but depends strongly on nucleophilic character. A primary free radical is less reactive than a secondary one, and is much less reactive than a tertiary one. A bridgehead free radical as the bicyclo[2.2.1]heptyle-1 does not react with the peracid. These results are interpreted to indicate a transition state with charge transfer (polar effect), the peracid being electrophilic and the free radical nucleophilic; PMO theory is discussed.

The anomalous nature of the stabilization of free radicals on the surface of adsorbents

The anomalous nature of the stabilization of free radicals on the surface of adsorbents

Stabilization of free radicals through ? irradiation of molecules adsorbed by zeolites

1. A study has been made of the stability and ESR spectra of free radicals resulting from γ irradiation at −196°C of triethylbenzene, the diethyl ester of dimethylmalonic acid, and tert-butylbenzene adsorbed on NaX and CaX zeolites at room temperature and above. Only the radicals formed from irradiation of molecules with diameters commensurate with the zeolite aperture diameters show anomalously high stability.

The stabilization and spectra of free radicals and molecular ions in rare gas matrices

The stabilization and spectra of free radicals and molecular ions in rare gas matrices

Copolymerization of styrene with vinyl acetate in presence of tributyl amine (TBA)

AbstractThis work presents the results of solution and emulsion copolymerization of vinyl acetate and styrene in the presence of a tertiary amine such as tributyl amine. This amine increases the stability of the vinyl acetate free radicals and favors copolymerization reaction. Emulsion polymerizations in the presence of TBA resulted in a terpolymer of styrene‐vinyl acetate‐vinyl alcohol. Vinyl alcohol is due to hydrolysis in the basis aqueous medium.The homo‐and copolymers obtained by precipitation from the reacting mixtures were fractionated according to procedures devised from solubility trials and from the work of Soen et al.13Fractions were characterized by nuclear magnetic resonance (NMR), infrared (IR), gel permeation chromatography (GPC), and thermal analysis.Heteropolymers were obtained with both techniques, and a terpolymer was obtained by emulsion polymerization, due to ester hydrolysis. The monomer conversion was more than 26%; 15% of the motives in the polymer were alcohols and 4%, acetates. With solution polymerization the conversion was more than 11%; 14% of the motives, as indicated by NMR, were acetates and the rest styrene.

Factors affecting stability and equilibria of free radicals—IX: Non-equivalence of aryl groups in 1,1-diaryl-2-benzenesulphonyl-hydrazyls and related compounds

1,1-Bis-(3',5'-di-t-butylphenyl)-2-benzenesulphonyl-hydrazine 11 and the corresponding 2-picryl-hydrazine 13 were synthesized. Oxidation of 11 afforded the stable hydrazyl 12 whose ESR spectrum could be simulated in terms of three hyperfine coupling constants. These experimental data leave no alternative but to assign larger proton hyperfine coupling constants to one aryl group in 12 than to the other. This non-equivalence was interpreted in terms of the Linnett theory. The picrylhydrazyl 14 does not give completely resolved ESR spectra. Structures were ascertained by IR, UV-vis, 1H NMR and 13C NMR spectra.

Effect of evolved hydrogen fluoride on radiation‐induced crosslinking and dehydrofluorination of poly(vinylidene fluoride)

AbstractPoly(vinylidene fluoride) (PVdF) was irradiated by 60Co γ‐rays with and without potassium hydroxide (KOH) under vacum. KOH tablets were added to absorb completely hydrogen fluoride (HF) which is the main volatile product of radiolysis of PVdF in the irradiation cell. In the presence of HF, the rates of radiation‐induced crosslinking and dehydrofluorination of PVdF were lower than those in the absence of HF. The experimental results are discussed from the standpoint of stabilization of alkyl free radicals in PVdF by reaction with hydrogen fluoride.

Stable free radicals. Part I. A new principle governing the stability of organic free radicals

The concept of merostabilisation, the stabilisation of free radicals by simultaneous conjugation with electron donor and acceptor groups, is developed by valence bond, MO, and Linnett double quartet theories. The concept is used to rationalise the stabilities of known radicals, and to predict those of new ones, some of which have been synthesised.

Factors affecting stability and equilibria of free radicals—VIII : 1-Cyano-2,2-diphenylhydrazyl

The oxidation of 3,3-diphenylcarbazonitrile (1-cyano-2,2-diphenylhydrazine) with lead dioxide or tetraacetate affords the stable violet free radical 1-cyano-2,2-diphenylhydrazyl whose ESR spectra in solution and solid state are reported and discussed.

Merostabilisation — A New Principle Governing the Stability of Organic Free Radicals

Merostabilisation — A New Principle Governing the Stability of Organic Free Radicals

Factors affecting stability and equilibria of free radicals: VI. Oxidation of cyclic hydroxarnic acids to nitroxides

Oxidation of cyclic hydroxamic acids (1-hydroxy-2-indolinone, II, 4-hydroxy2H-1,4-benzoxazin-3(4H)-one, V, and their derivatives with a gem-dimethyl group adjacent to the carbonyl group) affords nitroxides III, III-Me 2, VI and IX, whose ESR spectra are presented and discussed. On recrystallization of the hydroxamic acids from D 2O, only II exchanges the methylene protons, assisting in the interpretation of the ESR spectrum of III.

Electron Spin Resonance Study of Phenolic Antioxidants. Correlations of Organic Free Radical Stability with Antioxidant Activity

Abstract Electron spin resonance (ESR) is a well established, sensitive spectroscopic technique for determining the molecular structure, as well as the lifetime or stability of organic free radicals. Consequently, ESR spectroscopy provides a logical method for gaining insight into the role that free radicals play in the stabilization of rubber by organic antioxidants. We used ESR spectroscopy to determine the molecular structure of the phenoxyl free radicals derived from a series of twelve phenolic antioxidants. We also used ESR to measure relative radical concentrations of the free radicals and evaluated the twelve phenolic antioxidants relative to each other. We have found that the ESR measurement of relative radical concentration is a measure of relative radical stability, and that the relative radical stability is temperature dependent. By plotting relative radical concentration (or stability) as a function of temperature, we can rank these twelve antioxidants relative to each other. This ranking is in good agreement with the activity of these antioxidants in EPDM. These data were discussed in relation to antioxidant mechanism.

E.P.R. of radiation-induced radicals in A- and Se-amino acids.

SummaryThe formation and stabilization of free radicals in x-irradiated sulphur and selenium amino acids have been investigated by electron paramagnetic resonace (E.P.R.). An attempt has been made to identify the main radical species present. It is shown that selenium compounds have lower radical yields than those for sulphur compounds. An explanation is suggested for this difference.

Cпeктpы элeктpoннoгo пapaмaгни tcy;нoгo peзoнaнca oблyчeнньч зaмopoжeнньiч вoдньч pacтвopoв — Xi paдикaльныx пpoдyктыx paдиoлизa вoдныч paтвopoв cepнoй и фcфopнй киcлoт пpи 77 к и ич cвйcтвa

Results of studies of free radicals in γ-irradiated aqueous solutions of sulphuric and phosphoric acids at 77 K by electron spin resonance and optical spectroscopy are described. Besides hydrogen atoms, the following radicals are formed in these systems: SO 4 − radical anions and OH radicals in sulphuric acid solutions; PO 4 2− radical anion and OH radical in phosphoric in phosphoric acid solutions. In concentrated solutions of acids the radicals SO 3 − and PO 3 2− are also formed. The yield of radicals were measured in dependence upon the concentration of acids. The possible mechanism of radicals formation is discussed. Photo- and thermal stability of free radicals have been studied. Photochemical bleaching of radical anions obeys a first-order law. Reversible photo- and thermal transformations of radical anions and OH radicals are confirmed.