Showing papers on "Homolysis published in 1982"

Hydrocarbon Bond Dissociation Energies

Abstract: The best available values for homolytic bond dissociation energies (BDEs) of various classes of neutral compounds are considered in this review. (BDEs in ionic species is a legitimate subject that is touched on briefly and could easily be included in a longer review. The same can be said for heterolytic BDEs, which are not reviewed as such, although some of the ionic thermochemical data discussed yield values for these processes.) The major emphasis is on hydrocarbons and their nitrogen, oxygen, sulfur, halogen, and silicon-containing derivations, but limited data for inorganic molecules are included. The focus is particularly on prototypical radicals whose heats of formation, formerly thought to be well in hand, have recently been called into serious question. The intent is to include all the major types of sigma bonds, if not all specific cases where known or estimatable heats of formation allow bond dissociation energies to be generated. This review attempts to acknowledge all the standard techniques for measuring BDEs in polyatomic molecules and to offer critical analysis of selected portions of the literature. This leaves values that the authors recommend as the most likely to be correct at the time of this writing. 246 references, 9 tables.

Arrhenius parameters of elementary reactions involved in the oxidation of neopentane

Abstract: The reactions of neopentyl radicals in an oxidising environment have been studied by adding neopentane to slowly reacting mixtures of H2+ O2 over the temperature range 380–520 °C. Over a wide range of mixture composition, the only detectable initial products at these temperatures are 3,3-dimethyloxetan (DMO), acetone, i-butene, methane, and formaldehyde. A relatively simple mechanism involving the formation of neopentylhydroperoxide (QOOH) radicals gives a quantitative interpretation of the product yields. Although the major source of i-butene is the C—C homolysis of neopentyl radicals, a significant proportion is formed in reaction (6)(CH3)2C(CH2OOH)CH2→(CH3)2CCH2+ HCHO + OH. (6)From measurements of the product ratios ([acetone]+[DMO])/[i-butene] and [acetone]/[DMO] at each temperature used, Arrhenius parameters have been determined for a number of the elementary steps. The recommended value of k3= 1.20 × 1013 exp (– 120 kJ mol–1/RT) S–1 for the 1,5p H-atom transfer in neopentylperoxy radicals is compared with previously determined parameters for the 1,4p H-atom transfer in ethylperoxy radicals. With the Arrhenius expressions for these two transfers as a basis, thermochemical calculations are used to obtain a self-consistent set of Arrhenius parameters from values of rate constants at 480 °C for primary, secondary and tertiary H-atom transfers in alkylperoxy radicals involving ring sizes in the transition state varying from 4 to 8 (CH3)3CCH2O2→(CH3)2C(CH2OOH)CH2. (3)

Utilisation des pièges à radicaux en vue de mettre en évidence des intermédiaires dans la photolyse de complexes contenant une liaison CoIII—C

Abstract: Using the spin trapping technique with 5,5′-dimethyl 1-pyrroline N-oxide (DMPO), phenyl-N-tert-butyl nitrone (PBN), nitrosodurene (ND), and α-4-pyridyl 1-oxide N-tert-butl nitrone (4-POBN), or their mixtures, we have been able to detect two types of radicals, one is a hydrogen atom spin adduct and the other is the corresponding alkyl of the alkylcobaloximes, salens or cobalamines.By the use of selective deuteration and the preparation of the benzyl bis(diphenylglyoximato)-pyridinato cobaloxime, we have shown that the spin trapped hydrogen atom comes from the chelated hydrogen of the dimethylglyoximato anion of the equatorial ligand of CoIII complexes. Using a mixture of two spin traps gives rise to an esr spectrum containing, at the same time, the hydrogen atom and alkyl spin adducts.To explain such an efficient spin trapping reaction while the homolysis is in competition with a β-elimination process, it should be postulated that the photolysis of such compounds proceeds through a solvent cage environment...

Effects of pressure and isotopic substitution on the rate of reaction of coal with Tetralin

Abstract: The molar activation volume for the reaction of coal with tetralin at 344/sup 0/C is -27 +/- 3 mL. This result suggests a transition state which is bimolecular and possibly ionic. It is inconsistent with the hypothesis that the rate is controlled by homolysis of the coal molecule. The H/D kinetic isotope effect for the reaction of coal with tetralin containing deuterium at the ..cap alpha..-positions is 2.1 +/- 0.1 at 335/sup 0/C. This result reinforces the conclusions based on the activation volume.

Bond homolysis in high‐temperature fluids

Abstract: Rate constants for the homolysis of 1,2-diphenylethane have been determined in tetralin, in dodecahydrotriphenylene, and in the gas phase at temperatures above 350/sup 0/C. The least-squares-derived Arrhenius expression for this reaction in the gas phase is k/sub 1//s/sup -1/ = 10/sup (15.25+-0.15)-(62.31+-0.50)/theta (theta = 0.004576T/K) and is consistent with available thermokinetic data. In liquid tetralin up to its critical temperature and in liquid dodecahydrotriphenylene Arrhenius parameters for this reaction were found to be distinctly higher than gas-phase values, and rate constants to be somewhat lower. These differences between gas- and liquid-phase kinetics are attributed to recombination of nascent free radicals in solution (the cage effect), the probability of which decreases with decreasing viscosity.

Thermal chemistry of cyclopropyl-substituted malonyl peroxides. A new chemiluminescent reaction

Abstract: : This document investigates the thermal properties and chemiluminescent behavior of 4-cyclo-propyl-4-methyl-1,2-dioxolane-3,5-dione (2) and 4-spiro ((2'-spirocyclopropyl)-cyclohexyl)-1,2-dioxolane-3,5-dione. The gross chemical properties of these malonyl peroxides are analogous to other alkyl substituted examples. Thermolysis leads to rate limiting homolytic rupture of the oxygen-oxygen bond, loss of CO2 and then cyclization to an alpha-lactone. This sequence of reactions can be catalyzed by aromatic hydrocarbons through an apparent electron transfer process. Chemiluminescence from these peroxides can be observed in the presence of the aromatic hydrocarbons. The mechanism for light generation is postulated to be chemically initiated electron-exchange luminescence (CIEEL) proceeding through a cyclopropylcarbinyl to allylcarbinyl rearrangement.

HOMOLYSIS AND ACIDOLYSIS REACTIONS OF (α‐HYDROXYALKYL)‐ AND (α‐ALKOXYALKYL)CHROMIUM(III) COMPLEXES: KINETICS, STERIC EFFECTS, AND BOND ENERGIES

Abstract: The rates of homolysis of 12 organochromium complexes were determined by the addition of oxidizing scavengers to prevent recombination of Cr/sup 2 +/ and the carbon-centered radicals. The rate constants for complexes derived from aliphatic alcohols span the range 3.7 x 10/sup -5/to approx. 3 x 10/sup 2/s/sup -1/ CrCH/sub 2/OH/sup 2 +/s/sup -1/ for CrC(CH/sub 3/)(t-C/sub 4/H/sub 9/)OH/sup 2 +/ at 25/sup 0/C. There is a good correlation between the free energies of activation for homolysis of the organochromium complexes and corresponding substituted symmetrical ethanes. On the basis of the activation parameters and the literature data, estimates of the Cr-C bond dissociation enthalpies were made. Rate constants for the acidolysis reaction, previously known for some of these complexes, were evaluated for the others. Means were developed, by control of reaction conditions, to permit independent measurements of homolysis and acidolysis. The organochromium complex derived from isopropyl ether, CrC(CH/sub 3/)/sub 2/OCH(CH/sub 3/)/sub 2//sup 2 +/, undergoes a rapid (0.05 < k < 2 s/sup -1/) conversion to CrC(CH/sub 3/)/sub 2/OH/sup 2 +/ in dilute perchloric acid. 5 figures, 6 tables.

Phosphororganische Antioxidantien. I. Kinetik und Mechanismus der Zersetzung von Alkyl‐hydroperoxiden durch o‐Phenylen‐phosphite und ‐phosphate

Abstract: Organophosphorus Antioxidants. I. Kinetics and Mechanism of the Decomposition of Alkylhydroperoxides by o-Phenylene Phosphites and Phosphates The reaction mechanism of 2-(2, 6-di-tert-butyl-4-methyl-phenoxyl)-1,3,2-benzodioxaphosphole (1) with cumyl and t-butyl hydroperoxide has been studied kinetically by means of 31P-n.m.r. spectroscopy and high pressure liquid chromatography. 1 reacts with cumyl hydroperoxide to give the corresponding 2-oxide (2) which with more hydroperoxide and/or water forms the open chained phosphate ester 5. This acidic phosphate decomposes hydroperoxide catalytically. The kinetic parameters of the separate reaction steps are given. The ionic mechanism of hydroperoxide decomposition is accompanied by a homolytic one in a minor proportion.

Homolytic displacements at carbon. Part 8. Synthesis of 1-cyano-2-(trichloroethyl)cyclopropanes from allylcobaloximes via but-3-enylcobaloximes and 1-bromo-1-cyanobut-3-enes

Abstract: Several 1-cyano-2-(trichloroethyl)cyclopropanes have been synthesised by a sequence of reactions involving homolytic displacement of cobaloxime(II) from allylcobaloxime(III) complexes by bromocyanomethyl radicals and homolytic displacement of cobaloxime(II) from but-3-enylcobaloxime(III) complexes by trichloromethyl radicals : the latter complexes being formed from the 1-bromo-1-cyanobut-3-enes formed in the reactions of the allylcobaloxime(III) complexes. In cases where the but-3-enylcobaloxime(III) complex could not be synthesised, alternative routes to 1-cyano-2-(trichloroethyl)cyclopropanes have been demonstrated, including the homolytic displacement of bromine atoms from 1-bromo-1-cyanobut-3-enes by trichloromethyl radicals.

Reactions of aliphatic radicals and benzyl(aqua)cobaloxime: kinetics of coupling reactions as studied by novel kinetic competition methods

Abstract: Reactions of aliphatic radicals with benzylcobaloximes were found to be similar to those previously postulated for the catalytic reactions of selected organocobaloximes with tetrahalomethanes and arenesulfonyl chlorides. Data indicated that these reactions were biomolecular reactions, and their rate constants were determined with new methods of chemical competition. Although homolytic displacement has been considered the descriptive phrase for the reactions, it is pointed out that a mechanism consisting of radical addition and reductive elimination also agrees with the experimental results. (BLM)

Synthesis of alkanesulphonyl chlorides from alkylcobaloximes and trichloromethanesulphonyl chloride

Abstract: Alkyl- and substituted alkylcobaloximes react photochemically with trichloromethanesulphonyl chloride to give high yields of alkanesulphonyl chlorides erythro-2-Phenyl[1-2H1,2-2H1]ethylcobaloxime gives an equimolar mixture of erythro- and threo-2-phenyl[1-2H1,2-2H1]ethanesulphonyl chloride and diastereoisomerically pure 1-phenyl [1-2H1] propan-2-ylcobaloxime gives a 1 : 1 mixture of diastereoisomers of 1-phenyl [1-2H1]propane-2-sulphonyl chloride The reaction is believed to proceed through a non-chain process initiated by photolysis of the carbon–cobalt bond The product alkanesulphonyl chloride is then formed, either by a direct attack of the alkyl radical on the sulphur of trichloromethanesulphonyl chloride, or through capture of the alkyl radical by sulphur dioxide and halogen abstraction by the so-formed alkanesulphonyl radical from trichloromethanesulphonyl chloride The latter process seems the more likely No evidence could be obtained for an intermolecular homolytic attack of trichloromethyl radicals on the saturated alkyl ligand

Photocyclization of aryl halides. Part 3. Phenanthridone photosynthesis from 2-halogenobenzanilides

Abstract: 2-Chlorobenzanilides cyclize to phenanthridones in deaerated cyclohexane solution on 254 nm irradiation, whereas 2-bromo- and 2-iodo-benzanilides undergo dehalogenative reduction. These observations are understandable in terms of the assisted homolysis model as applied to configurationally flexible molecules. Cyclization in hydrocarbon media is retarded by triplet quenchers and fails in polar solvents. The cyclization can also be singlet and triplet sensitized. 4′-Substituents influence the cyclization quantum yield by altering the π-donor ability of the aniline ring and the rotational barrier about the amide bond.

Photolytic reactions of cyclopentadienylmetallic compounds

Abstract: Bonds to the cyclopentadienyl ring readily undergo homolysis because the cyclopentadienyl radical is resonance stabilized. Two aspects of this reactivity are discussed. First, a wide variety of cyclopentadienylmetallic compounds undergo photolysis with ultraviolet light, providing a route to metal—centred radicals. The properties of some of these radicals have been studied by e .s .r. spectroscopy.

Mechanism of oxidation of diphenyl sulfide by peroxyanions

Abstract: Reactions of diphenyl sulfide with the two oxidants peroxydisulfate and peroxydiphosphate are described. With each oxidant, the reaction is first order in the oxidant and zero order in the substrate. The rate of the reaction is also independent of the effect of the substituent. Hydrogen ion catalyzes the reactions. The lack of inhibition of the reaction rate by added acrylamide rules out the possibility of a radical reaction initiated by the homolysis of the peroxyanions. From the effect of [H+] on the oxidation rates, the active species involved in the reactions have been determined. The redox reaction is essentially proceeding via hydrolysis of the protonated peroxy anions in a rate-determining step, followed by a fast step involving the oxidation of diphenyl sulfide to diphenyl sulfoxide. This has been confirmed by estimating the hydrolyzed products in each case and also by the product analyses.

Electron spin resonance detection of the intermediate radicals occurring in homolytic aromatic substitutions of furan and thiophen

Abstract: A number of radicals generated by photolysis add to furan and thiophen giving radicals involved in homolytic aromatic substitution: the corresponding e.s.r. spectra were observed. Photolysis of (PhCOO)2 in furan or thiophen gives the spectra of both (A) and (B). On the other hand photolysis of (MeCOO)2 in furan gave the spectrum [graphic omitted] of (C) at –110° but that of (D) at higher temperature (–10°). The interpretation of the aH hyperfine splitting constants suggests that the structure of these radicals is not planar. They seem rather to prefer envelope-like conformations with the substituents either in pseudoaxial or in pseudoequatorial positions or in equilibrium between the two, depending on the substituent.

Effects of ionizing radiation and photolysis on methyl-cobalamin in low temperature matrices: an e.s.r. study

Abstract: Electron capture by methyl-cobalamin in CD3OD at 77 K occurs into a π* corrin orbital with subsequent transfer ro the Co dx2– y2 orbital: in contrast, photolysis causes homolysis of the methyl–cobalt bond giving some methyl radicals and ·CD2OD redicals separated from the CoII product and some radicals trapped ca. 8.3 A from the CoII as indicated by the triplet-state e.s.r spectrum.

Photo-oxidation of oxazolidones and hydantoins in the presence of benzophenone

Abstract: Irradiation in the presence of benzophenone and oxygen of nitrogen-containing heterocycles having an NCO group yields products arising out of regioselective oxidation α to the nitrogen atom. Direct irradiation (in the absence of benzophenone and oxygen) of 5-methyl- and 5,5-dimethyl-hydantoins yields allophanates. The first step of this reaction involves the homolysis of the C(4)–C(5) bond of the hydantoin.

Homolytic ring fission reactions of cycloalkylmethyl and bicycloalkyl radicals

Abstract: The geometries and enthalpies of formation of cyclopropyl, cyclopropylmethyl, cyclobut-2-enylmethyl, cyclobutylmethyl, the isomeric bicyclo[2.1.0]pentyl, bicyclo[1.1.1]pentyl, and spiropentyl radicals have been studied by the semi-empirical MINDO/3 method and compared with the calculated and experimental data for the corresponding cyclo- or bicyclo-alkanes. The ring fission reactions of the radicals have been investigated by calculating the energies for successive increments of a chosen reaction co-ordinate. For the cycloalkylmethyl radicals the calculated enthalpies of activation show the same trend as the experimental activation energies for β-scission. The MINDO/3 calculations for the bicycloalkyl radicals are also in agreement with such experimental evidence as is available. For the bicycloalkyl radicals which have not been examined by experiment MINDO/3 suggests that ring fission should be favoured for bicyclo[2.1.0]-pent-5-yl radicals. The calculations indicate that overlap of the SOMO with the bond to be broken is usually the most important factor in controlling which bond breaks and the rate of the process. Release of ring strain may, however, oppose and overrule the stereoelectronic effect, e.g. with bicyclo[2.1.0]pent-2-yl radicals. If overlap of the SOMO with the β,γ bond is prevented then ring fission is predicted to have a high energy barrier, e.g. in cyclopropyl and spiropentyl radicals.