Showing papers on "Homolysis published in 1981"

Very‐low‐pressure pyrolysis of ethylbenzene, isopropylbenzene, and tert‐butylbenzene

Abstract: The thermal unimolecular decomposition of ethylbenzene, isopropylbenzene, and tert-butylbenzene was studied using the very-low-pressure pyrolysis (VLPP) technique. Each reactant decomposed by way of β CC bond homolysis, producing methyl radicals and benzyl or benzylic-type radicals. RRKM calculations show that the observed rate constants, when combined with thermochemical estimates, are consistent with the following high-pressure rate expressions: for ethylbenzene between 1053 and 1234 K, for isopropylbenzene between 971 and 1151 K, and for tert-butylbenzene between 929 and 1157 K, where θ (kcal/mol) = 2.303RT. Resulting activation energies combined with heat capacity and heat of formation data led to the following dissociation enthalpies and enthalpies of formation at 298 K: DH° (oCH(CH3)CH3) = 73.8 kcal/mol, ΔHf° (oCCH(CH3)) = 39.6 kcal/mol, DH° (oC(CH3)2CH3) = 72.9 kcal/mol, and ΔHf° (oC(CH3)2) = 32.4 kcal/mol. Derived high-pressure rate constants are in good accord with results of lower temperature toluene- and aniline-carrier experiments.

Spectroscopic studies of cutaneous photosensitizing agents—ii. spin trapping of photolysis products from sulfanilamide and 4‐aminobenzoic acid using 5,5‐dimethyl‐1‐pyrroline‐1‐oxide

Abstract: — The photodecomposition of sulfanilamide, 4-aminobenzoic acid and related analogs in aqueous solution has been studied with the aid of spin traps 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and CH3NO2 as well as by direct electron spin resonance techniques. The NH2 radical was trapped by DMPO during the photolysis of aqueous solutions of sulfanilamide with a Xe arc lamp. Studies with [15N1]-sulfanilamide indicated that the NH2 radical was generated by homolytic fission of the sulfur-nitrogen bond. Under the same conditions DMPO trapped the H and SO−3 radicals during photolysis of sulfanic acid. Direct photolysis of sulfanilamide, sulfanilic acid and Na2SO3 in the absence of any spin trap yielded the SO−3 radical. Photolysis of 4-aminobenzoic acid at pH 7 gave the H radical which was trapped by DMPO. At low pH values OH and C6H4COOH radicals were generated during the photolysis of 4-aminobenzoic acid. No e−aq were trapped by CH3NO2 when acid (pH 4) and neutral aqueous solutions of sulfanilamide or 4-aminobenzoic acid were photoirradiated. The mechanism of formation of known photoproducts from the free radicals generated by sulfanilamide and 4-aminobenzoic acid during irradiation are discussed. The free radicals generated by these agents may play an important role in their phototoxic and photoallergic effects.

An electron spin resonance study of pentadienyl and related radicals: homolytic fission of cyclobut-2-enylmethyl radicals

Abstract: Pentadienyl radicals were generated from penta-1,4-diene and cis and trans-1-bromopenta-2,4-diene, and were observed in the E,E(1) and E,Z(2) conformation in hydrocarbon solution by e.s.r. spectroscopy. The E,Z-radicals are converted into the E,E-radicals at T > ca. 170 K, but the E,E-radicals are not converted into the E,Z-radicals in the accessible temperature range. From the estimated barrier to rotation in E,Z-pentadienyl radicals the methane based stabilization energy (E8Me – H) was estimated to be 104 kJ mol–1. Pentadienyl radicals can also be obtained from ring-opening of cyclobut-2-enylmethyl radicals (3). Bromine abstraction from cyclobut-2-enyl methyl bromide by triethylsilyl radicals gave cyclobut-2-enylmethyl radicals at temperatures below ca. 230 K. Above this temperature homolytic fission of the cyclobutene ring occurred and pentadienyl radicals in the E,E-conformation were detected by e.s.r. Initially, E,Z-pentadienyl radicals must be formed, but at the temperature of ring fission these are converted into the E,E-radicals and so are not observed. Hydrogen abstraction from neither 3-methylcyclobutene nor from bicyclo[2.1.0]pentane yields (3): instead 3-methylcyclobutenyl radicals and cyclopent-3-enyl radicals are formed, respectively. E.s.r. parameters are also reported for a range of substituted pentadienyl radicals generated from the corresponding 1,4-dienes. Of these radicals only 3-trimethylsiloxypentadienyl was observed in two conformations.

Coal structure cleavage mechanisms: scission of methylene and ether linkages to hydroxylated rings

Abstract: The kinetics of the thermolysis of the coal-model compounds (hydroxyphenyl)phenylmethanes and p-hydroxyphenyl phenyl ether in tetralin at 400/sup 0/C are described. The observed rates for (hydroxyphenyl)phenylmethanes are shown to be quantitatively consistent with thermochemical data and an enol-keto tautomerization followed by rate-determining homolysis of the cyclohexadienone intermediate. For p-hydroxyphenyl phenyl ether the breakdown of this quantitative agreement and the effects of various additives indicate that the tautomerization is rate determining and most probably involves electrophilic attack on the enolate ion. Implications for coal liquefaction and catalysis are briefly discussed.

Metal-hydrogen bond energies in protonated transition-metal complexes

Abstract: Proton affinities of 20 organotransition-metal complexes in the gas phase are reported. For 16 of these complexes protonation occurs on the metal center. The corresponding metal-hydrogen homolytic bond dissociation energies were determined and these data summarized. All proton affinities were determined by the techniques of ion cyclotron resonance spectroscopy, by examining proton-transfer reactions in mixtures with compounds of known base strength. Ionization potentials are taken from a variety of sources and experimental procedures, as noted. The site of protonation in several of these compounds has been determined by either gas-phase or solution-phase studies. These results and their interpretations are presented. (AT)

Chemiluminescence of secondary peroxy esters

Abstract: : The thermolysis of 1-phenylethylperoxyacetate and a series of substituted 1-phenylethylperoxybenzoates was investigated. Thermolysis in benzene gives acetophenone and the corresponding carboxylic acid. The study of the reaction kinetics and kinetic isotope effect indicate that the unimolecular thermolysis proceeds by homolysis of the oxygen-oxygen bond. Electronically excited states are formed in the thermolyses of these peroxyesters. These are detected by their characteristic direct chemiluminescence or by indirect chemiluminescence. In the presence of easily oxidized catalysts these peresters give excited states by the chemically initiated electron-exchange luminescence (CIEEL) path. The mechanism of these luminescent reactions was investigated. (Author)

An ESR and ENDOR study of tunneling rotation of a hindered CH3 group in C2H5 radicals trapped in xenon matrices at 4.2 K: Environment effect on internal rotation

Abstract: Two kinds of ethyl radicals have been observed in Xe matrices depending upon the reactions of radical formation. The CH3 group in C2H5(I) formed from H abstraction by H atoms at temperatures below 50 K exhibits an ESR hyperfine pattern characteristic of tunneling rotation in a threefold hindering potential, whereas that in C2H5(II) formed from homolytic scission of the C–H bond exhibits a conventional spectrum typical of free or random hopping rotation with a small barrier. The g and hyperfine coupling tensors indicate that both the ethyl radicals possess a conventional planar or nearly planar structure. It is concluded that the difference in the internal motion arises from an environmental effect which lowers the rotational symmetry from a six‐ to threefold potential. The tunneling splitting has been determined to be 450 MHz for C2H5(I). From this the barrier to internal rotation is estimated to be about 1 kcal/mol. In estimating the barrier height the influence of the interaction of the CH2 group with t...

Intramolecular cyclisations of biphenyl-2-carboxyl radicals: evidence for a II-state aroyloxyl radical

Abstract: Biphenyl-2-carboxyl radicals generated by homolysis of acyl hypoiodites cyclise intramolecularly giving mainly δ-lactones through Ar2-6 cyclisation. 2′-Alkoxybiphenyl-2-carboxyl radicals do not give the expected Ar1-5 cyclisation product but undergo a homolytic ipso-substitution of the 2′-substituent. The phenanthrene-4-carboxyl radical gives 5H-phenanthro[4,5-bcd]pyran-5-one. Consideration of the molecular orbitals involved suggests that the biphenyl-2-carboxyl radicals are in the π-ground state and have a higher energy, and, therefore, a less thermally accessible Σ-state than the corresponding amido-radicals. It is suggested that acyloxyl radicals which readily decarboxylate have either a Σ-ground state or a thermally accessible excited Σ-state.

Photolysis of the carbon–hydrogen bond in pentamethylcyclopentadiene. Properties of the pentamethylcyclopentadienyl radical

Abstract: Irradiation of pentamethylcyclopentadiene (A) in liquid solution with u.v. light results in homolysis of the ring C–H bond to give the pentamethylcyclopentadienyl radical (B)[a(15 H) 6.4, a(13C) 3.5 G], and atomic hydrogen which [graphic omitted] abstracts hydrogen from a second molecule of (A) to give molecular hydrogen and a second radical (B). The radicals (B) self-react at a diffusion-controlled rate (2kt 2 × 109 I mol–1 s–1 in hexane at 25°C) by two different routes. The first, which is thermally and photolytically reversible, is the combination to give the dehydrodimer (C), and the second, which is irreversible, is the disproportionation to give the parent cyclopentadiene (A) and the tetramethylfulvene (D).

Addition of 2,2,3-trimethylbutane to slowly reacting mixtures of hydrogen and oxygen at 480 °C

Abstract: The oxidation of 2,2,3-trimethylbutane (TRIMB) has been studied by adding traces of the alkane to slowly reacting mixtures of H2+ O2 in aged boric-acid-coated vessels at 480 °C. Rate constants for H and OH attack on TRIMB have been obtained and, by use of an additivity principle, Arrhenius parameters of log10(A/dm3 mol–1 s–1)= 9.32 ± 0.13 and E= 460 ± 950 J mol–1 are suggested for OH attack at a tertiary C—H bond in alkanes from a combination of the present results with studies at lower temperatures.Propene, isobutene, and 2,2,3-trimethylbut-1-ene (TRIMB-1) are the major initial products, the yield of the latter increasing markedly at high O2 pressures. The proportions of the three species of trimethylbutyl radicals formed in each mixture have been estimated. Two of the species of trimethylbutyl radicals react almost completely by homolysis of the strained central C—C bond to give propene and isobutene as products. The third radical (CH3)3CC(CH3)2 is removed by either reaction (1 C) or (2 C), and it is suggested that (1 C) occurs by a concerted mechanism: (CH3)3CC(CH3)2→ i-C4H8+ i-C3H7(1 C)(CH3)3CC(CH3)2+ O2→(CH3)3CC(CH3)= CH2+ HO2. (2 C)Rate constants have been obtained for a number of the homolysis reactions of the trimethylbutyl radicals, for which there is a good correlation between log k and ΔU, the internal energy change.

Grafting by Chemical Activation of Cellulose

Abstract: Activation of cellulose, that is, introduction of active centers into the cellulose molecule, has been successfully achieved by a number of chemical means. By and large, the mechanisms proposed for graft initiation and chain growth have been either free radical or ionically based, the radical methods being the more common. Most of the recent work in this field has featured reactions in special systems where oxidation of cellulose precedes grafting via a free radical intermediate1). In the homolytic oxidation of cellulose, a hydrogen atom is abstracted, not by a growing chain, but by an independent chemical reaction. The formation of active centers in cellulose can also be achieved by the influence of certain metallic halides such as BF3, i.e., cationic initiation, or in the presence of a variety of bases such as sodium alkoxides, i.e., anionic initiation. In this section acrylic and vinyl graft copolymerization by chemical activation of cellulose, by introduction of free radicals, and by ionic mechanisms are independently discussed.

Thermal and photochemical reactions of organocobaloximes with diphenyl disulphide and diphenyl diselenide

Abstract: Alkyl- and benzyl-bis(dimethylglyoximato)pyridinecobalt(III) complexes react with diphenyl disulphide or diphenyl diselenide, thermally and photochemically, to give good yields of alkyl or benzyl phenyl sulphides or selenides. Chiral alkylcobalt(III) complexes react to give racemic alkyl phenyl sulphides or selenides. Allyl- and propadienyl-bis(dimethylglyoximato)pyridinecobalt(III) complexes react similarly to give the thermodynamically more stable allyl, propadienyl, or propynyl phenyl sulphides or selenides. These reactions are interpreted in terms of free radical processes in which organic radicals formed as a result of thermal, photochemical, or chemically induced homolysis of the organocobalt complex, attack the diphenyl dichalcogenide to give the product organo phenyl chalcogenide. No evidence could be obtained for the direct attack of phenylthyl or phenylselenyl radicals on the organic ligand of the organocobalt complexes, though this process cannot be exluded in the case of allyl-cobalt complexes.

Cage effect in the solid phase calculation of quantum yields of stabilized radicals on the basis of a free volume model

Abstract: A method of evaluating the quantum yield of stabilized radicals in solid phase molecular homolytic photodecomposition is suggested. The calculation is based on a free volume model. It is assumed that the radical escape from the matrix cage occurs only where a free volume larger than the volume of the fragment coming out of the cage is concentrated near the decomposed molecule. The available experimental data on the quantum yields of the stabilized radicals are in good agreement with the calculated ones.

Synthesis and stereomutation of optically active α-cyanosulphoxides

Abstract: Reaction of (–)-menthyl(S)-toluene-p-sulphinate with nitriles and lithium NN-di-isopropylamide (LDA) in 1:1:1 and 1:2:1 ratios affords optically active α-cyano- and α-cyano-β-imino-sulphoxides, respectively. α-Cyanobenzyl sulphoxide racemizes through a homolytic process in a temperature range (35–50 °C) well below that required for benzyl aryl sulphoxides.