Showing papers on "Homolysis published in 1990"

Chemical Kinetics and Combustion Modeling

Abstract: ion of H atoms from fuel or other species by R02 produces alkyl hydroperoxides ROOH that then decompose to produce RO and OH radicals. However, for hydrocarbon fuels more complicated than n-butane, a more rapid process for R02 is isomerization via internal abstraction of H atoms ( 1 94). The general features of this R02 isomerization theory provide the kinetic basis for global reaction schemes for engine knock in internal combustion engines ( 1 96-1 98). The major steps consist schematically of R02 +=t QOOH (internal H atom abstraction) QOOH � QO + OH (0-0 homolysis). At sufficiently low temperatures, molecular oxygen can add further to the QOOH radicals, leading eventually to an overall reaction QOOH + 02 = products + OH + OH. Both alternatives are important since they produce OH radicals through reaction sequences with relatively low energy barriers. ROz isomerization rates are determined primarily by the size of the ringlike intermediate transition state, by the bond energy of the H atom being abstracted internally, and by the equilibrium constant of the R02 addition reaction. For fuels of larger hydrocarbons many isomerizations are possible, and 0-0 homolysis of the QOOH product of the iso­ merization reaction yields a different stable oxygenated species for each isomerization reaction. Thus for n-alkanes, a 1 ,4-H -atom abstraction, followed by 0-0 bond fission, leads to a 3-membered oxygenated ring, an oxiran. Similarly, 1 ,5-processes lead to oxetans, 1 ,6-abstractions produce tetrahydrofurans, and l ,7-abstractions produce tetrahydropyrans. Cur­ rent models generally use activation energies tabulated by Baldwin et al ( 199), but with A factors slightly lower than the 1 0 1 2. 1 S 1 recommended by Baldwin et aI, closer to the value of 1 0 1 1 . 5 S 1 recommended by Benson ( 1 89) for unimolecular reactions involving a cyclic transition state. The isomerization reactions are reversible, and activation energies for the reverse isomerizations are easily computed from the activation energy of the forward (endothermic) reaction and the AH of the reactions ( 1 94). ROz isomerization through internal abstraction of an H atom from a site adjacent to the C-O bond, followed by breakage of the C-O bond, will lead to a conjugate olefin and H02_ Direct abstraction paths leading to the same products have been discussed by Gutman and co-workers (200, 20 1 ), favoring a path proceeding through R02 isomerization. The current work of Wagner et al (20 I) provides some insight into the diffi382 MILLER, KEE & WESTBROOK culties of this reaction, but this is one of the simplest of the R02 iso­ merizations, and there are many more such reactions for which complex analyses are needed to understand fully the detailed reaction rates and mechanisms. Reactions of the product epoxide and other oxygenated species must be included in kinetic models, but very few quantitative studies of H atom abstraction or other reactions for these species have been reported. Current models must estimate both the rates and products for reactions of the epoxides, primarily attributed to H atom abstraction by OH or H02•

Photo-heterolysis and -homolysis of substituted diphenylmethyl halides, acetates, and phenyl ethers in acetonitrile : characterization of diphenylmethyl cations and radicals generated by 248-nm laser flash photolysis

Abstract: Para-substituted diphenylmethyl halides, acetates, and ethers RPh(R'Ph)CH-X (R, R' = CF3 to OCH3), upon photolysis with -250-nm light in acetonitrile solutions, undergo homolysis and heterolysis of the C-X bond to give the radicals, RPh(R'Ph)CH' (abbreviated as C), and the cations, RPh(R'Ph)CH+ ((2'). Whereas the quantum yields for homolysis (0.2-0.4) are rather independent of the nature of the substituent on the benzene ring, those for heterolysis increase with increasing electron-donator strength from 50.07 for CF3 to 0.3 for OMe. The cationxadical ratios are also dependent on the nucleofugal properties of X. For the halides, the observed hetero1ysis:homolysis ratios correlate with the pK, values of the conjugate acids HX and not with the electron affinities of X'. In acetonitrile, heterolysis is much less endothermic than homolysis. Homolysis and heterolysis can also be effected indirectly by reaction with triplet acetophenone (produced by 308-nm photolysis). Unless stabilized by one or more MeO, the cations decay predominantly by reaction with acetonitrile to give nitrilium ions. However, since this reaction is reversible (shown for the benzhydryl cation), the nitrilium ion contributes only to an insignificant degree to the formation of the final (cation-derived) products, which result from reaction with trace water (main product, benzhydryl alcohol; minor, benzhydrylacetamide). The rate constants for addition of C+ to CH$N are in the range 3.5 X lo5 to 3.8 X IO7 s-I for the cations with R = R' = Me to R = H, R' = CF3. The rate constants for reaction of C+ with halides (ion recombination) are -2 X 1O1O M-l s-' (diffusion control). The radicals C' disappear by dimerization and disproportionation, for which a complete mass balance has been achieved by product analysis for the case of the benzhydryl system. At laser-pulse powers > IO mJ electronically excited radicals, C", are additionally formed in many cases, via absorption of a light quantum by ground-state C'.

Acidities and homolytic bond dissociation energies of the αC—H bonds in ketones in DMSO

Abstract: Equilibrium acidities in DMSO are reported for nine cycloalkanones, acetone, acetophenone, and 19 of their α-substituted derivatives. Oxidation potentials in DMSO for the conjugate bases of most of...

Determination and interrelation of bond heterolysis and homolysis energies in solution

Abstract: The present paper provides a methodology for the quantitative evaluation of the energies required for the homolysis of many of the carbon-carbon, carbon-oxygen, carbon-nitrogen and carbon-sulfur bonds for which we have previously measured DH heterolysis 's

Homolytic bond dissociation energies for the cleavage of .alpha.-nitrogen-hydrogen bonds in carboxamides, sulfonamides, and their derivatives. The question of synergism in nitrogen-centered radicals

Abstract: Estimates of homolytic bond dissociation energies (BDEs) of the N-H bonds of acetamide, benzamide, and benzenesulfonamide by measurements of their equilibrium acidities and the oxidation potentials of their conjugate bases in dimethyl sulfoxide (DMSO) solution place them within experimental error of that of ammonia (107 kcal/mol)

Ab initio MO study on adsorption of a hydrogen molecule onto MgO(100) surface

Abstract: Mechanisms of dissociative adsorption of a hydrogen molecule and migration of a hydrogen atom on the MgO(100) surface were investigated by means of the ab initio molecular orbital method using a Mg{sub 4}O{sub 4} cluster model. For adsorption, both the homolytic path resulting in two O-H bond formation and the heterolytic path resulting in Mg-H and O-H bond formation were found to give stable products, but only the latter reaction proceeded with no activation energy. The second H{sub 2} molecule was adsorbed attractively to the site adjacent to the Mg-H and O-H bonds. The optimized Mg{sub 4}O{sub 4}H{sub 4} structure had C{sub 2v} symmetry, which suggested a possibility of H{sub 2}-D{sub 2} exchange reaction among adsorbed hydrogen atoms.

Reaction of hydroxide ion with manganese(III) tetramesitylporphyrin and the oxidation states of manganese tetramesitylporphyrins

Abstract: The autoreduction reaction of the non-μ-oxo-dimer-forming chloromanganese(III) tetramesitylporphyrin, (TMP)Mn III (Cl), with methanolic tetra-n-butylammonium hydroxide (YO − species = OH − or CH 3 O − ) in acetonitrile and dimethyl sulfoxide under anaerobic conditions has been examined by UV/visible and 2 H NMR spectroscopies, where the spectral changes accompanying the reaction could be associated with changes in the oxidation and ligation states of the manganese porphyrin. Two intermediates, (TMP)Mn II and [(TMP)Mn II (OY)] − , may be directly observed to form during the reaction by a mechanism thought to involve homolytic bond cleavage

Heterolysis and homolysis energies for some carbon-oxygen bonds

Abstract: Methods described previously for obtaining heterolysis (ΔH het ) and homolysis (ΔH homo ) enthalpies for bonds that can be cleaved to produce resonance-stabilized carbenium ions, anions, and radicals are extended to the study of carbon-oxygen bonds through the reactions of resonance-stabilized carbenium ions with substituted phenoxide ions. Titration calorimetry was used to obtain the heat of heterolysis, and the second-harmonic ac voltammetry (SHACV) method was used to obtain reversible oxidation potentials for the anions

Acid–base behaviour of the ferricyanide ion in perchloric acid media. Spectrophotometric and kinetic study

Abstract: A wide study was carried out of the acid–base behaviour of ferricyanide ions in aqueous perchloric acid media in the range 0.1–11.5 M. This study shows that protonation and decomposition of the ferricyanide ions occur simultaneously. The three successive protonation equilibria were found in the acidity ranges 0.1–4.0, 3.5–8.5, and 7.0–11.5 M HClO4, respectively. A kinetic study was also made of the decomposition reaction at 60 °C. The kinetic data are explained by considering a reaction mechanism involving homolytic and heterolytic dissociation steps. The homolytic and heterolytic rate constants corresponding to each of the four protonated species were determined, along with the acid–base protonation constants, pK1 = −6.25 ± 0.10; pK2 = −3.23 ± 0.03; and pK3 = −0.60 ± 0.02. Keywords: acid–base behaviour, protonated species, decomposition, homolytic dissociation, heterolytic dissociation.

Acid-catalyzed thermal decomposition of 1-aryl-3,3-dialkyltriazenes in the presence of nucleophiles

Abstract: The acid-catalyzed thermal decompositions of a number of substituted 1-aryl-3,3-dialkyltriazenes (1) in the presence of nucleophiles have been conducted with a view to understanding the underlying mechanisms of these reactions. These reactions parallel the corresponding diazonium ion reactions in that a heterolytic dediazoniation path competes with a homolytic route. The relative proportion of the products arising due to these two competing mechanisms depends upon (a) the electron-transfer efficiency of the nucleophile, (b) the redox potential of the aryl diazonium substrate, and (c) the reaction solvent.

Chemical Bond-Making, Bond-Breaking, and Electron Transfer in Solution

Abstract: A method is described for determining heats of heterolysis, ΔHhet, from the heats of reaction of resonancestabilized carbenium ions with carbanions in solution. These results can be converted into heats of homolysis, ΔHhomo, for the same bonds by combining ΔHhet values with electron transfer energies, δGET (δGET ≈ δHET), obtained from redox potentials of the reacting ions. The results provide a comprehensive tabulation of ΔHhet, ΔHhomo, and ΔGET values for an extended series of organic compounds and are examined in terms of their interrelationships and a number of related properties of the ions and radicals.

Homolytic reactions of ligated boranes. Part 15. Comparative studies of amine–boranes as donor polarity reversal catalysts for hydrogen-atom abstraction

Abstract: ESR spectroscopy has been used to characterise the reactions of the amine–boryl radicals produced by hydrogen-atom abstraction from a variety of amine–borane complexes by photochemically generated t-butoxyl radicals. The complexes Me3N→BH2R (R = Me2CHCMe2, Bun, Bui, Bus), 1,1-dimethyl-1,2-azaborolidine, 1-methyl-cis-1-azonia-5-boratabicyclo[3.3.0]octane, Me2NCH2CH2NMe2·2lpcBH2(Ipc = isopinocampheyl), Me3SiCH2NMe2→BH3, and Me3N→BH3 were investigated. All the amine–boryl radicals rapidly abstract halogen from alkyl bromides and chlorides at 170 K. Specific alkyl radicals can be generated for ESR studies at low temperature by UV irradiation of a solution containing ButOOBut, Me3N→BH2Bun, and the corresponding alkyl chloride. The amine–borane complexes act as donor polarity reversal catalysts for the overall abstraction of acidic hydrogen from HCC(O) groups in esters, lactones, ketones, imides, and related compounds. Relative rates of catalysed hydrogen-atom abstraction from MeCO2Et, MeCH2CO2Et, and Me2CHCO2Et have been determined and competitive abstraction from the two different types of α-CH groups in MeC(O)CHMe2 has been similarly quantified. The relative reactivities of the amine–boryl radicals can be understood in terms of a balance between enthalpic, polar, and steric factors and the merits of the different amine–boranes as polarity reversal catalysts for the overall abstraction of hydrogen from acidic C–H groups by alkoxyl radicals are assessed. The origin of the polar effects observed in hydrogen-atom abstraction reactions is discussed in terms of the electronegativity difference between the attacking and departing radicals and a simple approach for the quantitative description of polar effects is outlined.

Free radical reactions in synthesis. Homolysis of alkylcobalt complexes in the presence of radical-trapping agents

Abstract: Irradiations of the alkylcobalt salophen complexes (14), (21), (22), and (23) in the presence of radical-trapping agents, e.g. molecular oxygen, tetramethylpiperidine oxide, nitrogen monoxide, diphenyl disulphide, diphenyl diselenide, methanesulphonyl chloride, bromotrichloromethane, or iodine, leads to oxgen-[e.g., (18), (19), and (25)], nitrogen-[e.g., (27) and (28)], sulphur/selenium-[e.g., (30) and (31)] or halogen-(34) functionalised products. When these radical-trapping methodologies are combined with cobalt-mediated radical cyclisation reactions (Scheme 2) a powerful synthetic procedure, i.e. radical carbon-to-carbon bond formation with simultaneous functionalisation of the product radical centre, becomes available.

The selective homolytic cleavage of primary over secondary carbon-oxygen bonds in cyclic thionocarbonates: the effect of bond angle strain energy

Abstract: Several cases of the selective homolytic, tri-n-butylstannyl radical promoted fragmentation of primary over secondary carbon-oxygen bonds in 2-membered cyclic thionocarbonates have been uncovered. Molecular mechanics calculations (MM2) simulate the transition states for these reactions and have revealed that bond angle distortion is a prime contributor to the abnormal course of these reactions

Cobalt-mediated intermolecular radical additions to carbon-to-carbon double bonds leading to new functionalised alkenes

Abstract: Alkyl radicals generated by photolytic homolysis of cobalt salophen reagents, e.g.(6) and (15), are shown to add to activated carbon-to-carbon double bonds, i.e. ethyl acrylate, methyl vinyl ketone, acrylonitrile, and styrene, producing preparatively useful yields of new alkene products, viz.(9), (10), (12) and (14)[from (6)], and (16)[from (15)]. The reactions proceed via radical (Michael) additions, followed by ‘dehydrocobaltation’ from the presumed organocobalt intermediates (2)(Scheme 4). By use of this chemistry, in combination with the Schrauzer ‘hydrocobaltation’ reaction of alkenes, a new method for the cross-coupling reactions between two alkenes, leading to new functionalised alkenes, e.g.(24), (25), (26), (27), (28), and (29), is developed [Equation (2)].

Homolytic allyl transfer reactions of 1- and 3-alkyl-substituted allyltributylstannanes

Abstract: L'allylation de l'ester methylique de la (N-benzoyl bromo) glycine passe par un mecanisme radicalaire

Cobalt-mediated reactions in synthesis. The degradation of carboxylic acids to functionalised noralkanes via acylcobalt salophen intermediates

Abstract: Arylmethyl- and allyl-carbonylcobalt salophen complexes, e.g.(7), (8), (9), and (22), readily undergo carbon-to-cobalt-bond homolysis and in situ decarbonylation, producing new alkyl radical centres which can be intercepted with oxygen-, nitrogen-, halogen-, sulphur-, and selenium-containing radical-trapping agents leading to functionalised noralkanes. The sequence constitutes a useful, and in some cases more flexible, variant of the classical Hunsdiecker reaction, and amounts to a cobalt equivalent of the Barton radical decarboxylation reaction of carboxylic acids via their corresponding thiohydroxamic esters. In a similar manner, irradiation of the oxy-substituted acylcobalt salen reagents (25) and (26) in the presence of tetramethylpiperidine oxide produces the products (14) and (27), respectively, resulting from homolysis–decarboxylation and alkyl-radical trapping. In the absence of radical-trapping agents, irradiation of (7c) produces (18), and irradiation of (29) leads to the but-2-enolide (32).

Dimethyl sulfoxide phase carbon-hydrogen bond dissociation energies for phenalene and benzanthrene

Abstract: Comparisons of fluorene, phenalene, and benzathrene sp 3 C-H homolytic bond dissociation energy data, data collected with the aid of a cycle that utilizes acid-base and redox data collected in dimethyl sulfoxide solution, indicate that the sp 3 C-H bonds in phenalene and benzanthrene are about 18 and 16 kcal/mol weaker (in a homolytic sense) than the sp 3 C-H bond in fluorene

Photodissociation of Halonaphthalenes in Solution: Comparative Photochemistry of 1‐Iodo‐, 1‐Bromo‐, and 1‐Chloronaphthalenes

Abstract: Fluorescence measurements have been used to follow the build-up of photoproducts during the direct and benzophenone-sensitized irradiation of the title compounds 1-IN, 1-BrN, and 1-ClN (HN = naphthalene). Compounds 1-IN and 1-BrN react by homolytic dissociation through their lowest triplet and singlet excited states, respectively. Compound 1-ClN does not undergo C–Cl bond fission, except through electron transfer in the presence of an amine A, In the absence of this electron transfer, 1-ClN reacts only through substitution and oxidation processes.

Homolytic reactions of ligated boranes. Part 14. ESR studies of ring opening of cycloalkylaminyl–borane radicals and reactions of aminyl–borane radicals with silicon-containing compounds

Abstract: ESR spectroscopy has been used to characterise reactions of the aminyl–borane radicals RNH→BH3(2) in solution. The relative rates of β- hydrogen -atom transfer from [2; R = H, Me, Bus, ButC(H)Me, or But] to 2,3-dimethylbut-2-ene and to furan depend on the nature of the group R, confirming that H-atom transfer is bimolecular and does not involve prior β-scission of (2) to give a free hydrogen atom. Because of favourable polar factors, β-H-atom transfers from (2) to the allylsilanes Me3SiCH2CHCH2 and Me3SiCH2CMeCH2 are particularly rapid. The cycloalkylaminyl–borane radicals (2; R = cyclo-C3H5 or cyclo-C4H7) undergo rapid ring opening at 282 K, while no spectroscopic evidence was found for opening of the cyclopentyl or cyclohexyl analogues. Alkylaminyl–borane radicals transfer a β-hydrogen atom to hexamethyldisilane to bring about homolytic Si–Si bond cleavage. These radicals also react with trialkylsilanes R3SiH to give R3Si˙. Approximate absolute rate coefficients for the reactions of (2) have been determined at 282 or 292 K. Of the primary amine–boranes investigated, s-butylamine– and cyclopentylamine–boranes appear to be the most suitable complexes for ESR spectroscopic work in liquid solution.