Showing papers on "Hydrogen atom abstraction published in 1968"
TL;DR: In this paper, the rate constants for two-stage chain self-reactions with both alkyl radicals and triorganotin radicals were determined using the rotating sector technique, and they were shown to be near the diffusion-controlled limit for both propagation and termination.
Abstract: : The kinetics of the photochemically initiated reactions between some alkyl halides (chlorides, bromides, and iodides) and some triorganotin hydrides have been examined in cyclohexane at 25C. Chains are generally terminated by bimolecular radical-radical reactions. In general, with alkyl chlorides, the rate-controlling step for chain propagation involves chlorine abstraction by a triorganotin radical, while with bromides and methyl iodide, the rate-controlling step involves hydrogen abstraction from the hydride by the alkyl radicals. Absolute rate constants have been determined by the rotating-sector technique. Chain termination rate constants are near the diffusion-controlled limit for the self-reactions of both alkyl radicals and triorganotin radicals. The evaluation of the rate constants for both propagation reactions and for two out of the three possible termination reactions means that these processes are now among the best understood two-step chain reactions. (Author)
147 citations
TL;DR: In this article, the half-lives of triplet nitrenes are determined over a range of concentrations in hexane, ethanol and in several polymeric matrices (polystyrene, sucrose benzoate, polymethylmethacrylate) and are interpreted in terms of three processes : nitrene recombination, hydrogen abstraction from the solvent and reaction of the nitrene with an azido group.
Abstract: The transients in the flash photolysis of several aromatic azides are identified as triplet nitrenes by their spectra. Their half-lives are determined over a range of concentrations in hexane, ethanol and in several polymeric matrices (polystyrene, sucrose benzoate, polymethylmethacrylate) and are interpreted in terms of three processes : nitrene recombination, hydrogen abstraction from the solvent and reaction of the nitrene with an azido group. Recombination is diffusion-controlled (rate constant ∼109 1. mole–1 sec–1); the same applies to the reaction with an azido group. In contrast, hydrogen abstraction by an aromatic nitrene is a comparatively slow process, corresponding to a life-time (in ethanol) of about 2 × 10–4 sec. This value is corroborated by experiments in soft polystyrene matrices (half-life 10–3sec) and by the rate of intramolecular hydrogen abstraction in the cyclization of 2-nitrenobiphenyl to carbazole (half-life 8 × 10–4 sec). In hard polymeric matrices, hydrogen abstraction is considerably slower (half-lives from 0.1 to 2 sec). Here the rate-determining step is configurational diffusion of CH bonds towards the electron-deficient nitrogen.
105 citations
TL;DR: The parent carbonyl Ru6C(CO)17 has also been prepared by heating Ru3(CO), under reflux in n-octane or n-nonane.
Abstract: The compounds Ru6C(CO)17 and Ru6C(CO)14(arene)(arene = Me3C6H3, m-Me2C6H4, or MeC6H5) have been prepared by the reaction of Ru3(CO)12 with the appropriate arene. The parent carbonyl Ru6C(CO)17 has also been prepared by heating Ru3(CO)12 under reflux in n-octane or n-nonane. The hydrides H2Ru4(CO)13 and β-H4Ru4(CO)12 were also products of this reaction indicating that hydrogen abstraction from the alkane has occurred. On the basis of mass spectral studies the new compounds are formulated as ‘carbide’ compounds of the class first exemplified by Fe5(CO)15C. Their n.m.r. and i.r. specrta are discussed in the light of the known structure of the mesitylene compound.
45 citations
TL;DR: In this article, it was shown that aromatic nitro-compounds, when irradiated with u.v. light in organic solvents containing abstractable hydrogen atoms, give rise to reduction products, the relative proportion of which depends on the solvent and the wavelength of exciting radiation.
Abstract: Aromatic nitro-compounds, when irradiated with u.v. light in organic solvents containing abstractable hydrogen atoms, give rise to reduction products, the relative proportion of which depends on the solvent and the wavelength of the exciting radiation. The overall reaction is shown to involve a sequence of photochemical and thermal steps, the first of which is a hydrogen abstraction probably by a 3(ππ*) excited state of the nintrocompound. SCF–MO and zero-order HMO calculations lend support to this view.
43 citations
TL;DR: The photolysis of hydrogen peroxide in the presence of phenols in aqueous solution was used for the hydroxylation of various phenols on a preparative scale.
41 citations
33 citations
TL;DR: In this paper, the rates of hydrogen abstraction by methyl radicals from the three different sites in ethanol have been measured by studying three isotopically different ethanols : CH3CH2OH, CH3 CD2 OH and CH3 CH2 OD.
Abstract: The rates of hydrogen abstraction by methyl radicals from the three different sites in ethanol have been measured by studying three isotopically different ethanols : CH3CH2OH, CH3 CD2 OH and CH3CH2 OD. Photolysis of acetone and acetone-d6 over the temperature range 130–250° has been used as the source of methyl radicals and velocity constants have been obtained for five elementary reactions.
Abstraction takes place from all three sites. The methylene group is the most reactive site in the molecule, and the methyl group the least. At 150°C, 75 % of the hydrogen comes from CH2, 20 % from OH and about 5 % from CH3. Arrhenius parameters for methyl radical attack based on Shepp's rate-constant for methyl radical combination are as follows : [graphic omitted] Isotope effects have been determined for the methylene and hydroxyl groups; at 150°, (kCH2/kCD2) is 6.3 and (kOH/kOD) is 3.2. The activation energy difference ECD2–ECH2 is somewhat greater than would be expected from the zero-point-energy difference, and tunnelling may contribute to this.
A hydrogen exchange reaction between acetone and the hydroxyl group of ethanol has been found to occur. Its importance is to limit the validity of abstraction studies to isotopically compatible systems. Such pairs are studied here.
31 citations
24 citations
23 citations
23 citations
TL;DR: In this paper, rate constants have been measured for hydrogen abstraction from four aralkanes and addition to two aralkenes by five secondary and one primary peroxy radical, showing a slight dependence on the structure of the attacking peroxy radicals which may be due to the inductive effects of the substituents on the radical.
Abstract: Rate constants have been measured for hydrogen abstraction from four aralkanes and addition to two aralkenes by five secondary and one primary peroxy radical. These rate constants appear to show a slight dependence on the structure of the attacking peroxy radicals which may be due to the inductive effects of the substituents on the radical. The secondary and primary peroxy radicals are generally about 2–4 times more reactive than tertiary peroxy radicals in abstraction and 4–8 times more reactive in addition. The lower reactivity of tertiary peroxy radicals is probably due to steric factors.
TL;DR: In this paper, the results were discussed in terms of a combination of an inductive effect, an electron-releasing and an electron sharing conjugation, and were shown to be positively correlated with Hammett σ values.
Abstract: Hydrogen abstraction reactions of various α-substituted compounds were examined in chlorobenzene using di-t-butyl peroxide. The reactivities are not satisfactorily correlated with σ* values of the substituents. The relative rates of substituted anisoles, N,N-dimethylanilines and phenyl methyl sulfones give good correlations with Hammett σ values with ρ values of −0.39, −0.42 and −0.2 respectively, but those of thioanisoles requires an additional term due to conjugative effect as described in logk⁄k0=−0.11(σ+0.75ΔσR+) in order to get a good Hammett correlation. These results were discussed in terms of a combination of an inductive effect, an electron-releasing and an electron-sharing conjugation.
TL;DR: In this paper, the authors showed that the addition of benzene or cyclohexene as solutes caused drastic reductions in the rate of decomposition and a significant fraction of the decreases in the product yields was attributed to scavenging of hydrogen atoms by the solute.
Abstract: Liquid cyclohexane and cyclohexane solutions were photolyzed at 1236 and 1470 A. The products and quantum yields for cyclohexane photolysis at 1470 A are hydrogen (1.0), cyclohexene (0.94), dicyclohexyl (0.06), ethylene (0.003) and butadiene (0.001). Products and relative yields at 1236 A are hydrogen (1.0), cyclohexene (0.93), dicyclohexyl (0.05), ethylene (0.06) and butadiene (0.02). The addition of benzene or cyclohexene as solutes caused drastic reductions in the rate of cyclohexane decomposition. A significant fraction of the decreases in the product yields is attributed to the scavenging of hydrogen atoms by the solute. The ratio of the rate constant for hydrogen atom addition to benzene to that of hydrogen abstraction from cyclohexane is about 300. Neutral excited cyclohexane molecules are found to be important intermediates, and the major effects of benzene and cyclohexene as solutes are accounted for by quenching of these excited species. The rate‐constant ratio for energy transfer to the solute and dissociation of the excited intermediate is about 3.2 liter/mole.
TL;DR: In this article, the reaction of nitrogen atoms, produced by an electric discharge, with acetaldehyde has been studied in a flow system, a mass spectrometer being used to follow the course of the reaction.
Abstract: The reaction of nitrogen atoms, produced by an electric discharge, with acetaldehyde has been studied in a flow system, a mass spectrometer being used to follow the course of the reaction. Sampling was carried out through a small hole in a gold diaphragm. The main stable products were HCN, H$\_2$ and CO; a small amount of glyoxal was also formed. In addition appreciable amounts of a substance yielding ions of m/e = 43 were obtained. Arguments are presented for identifying this with the radical CH$\_2$CHO. Small amounts of a product giving ions of m/e = 86 were also produced. The nature of this material is discussed. No evidence was obtained for any hydrogen abstraction by nitrogen atoms. Experiments were also carried out with CH$\_3$CDO to clarify certain aspects of the proposed reaction mechanism. A few experimental results obtained with propionaldehyde can be understood in similar terms. The rate constant of the reaction N + CH$\_3$CHO $\rightarrow$ HCN + H$_2$ + HCO was deduced to be 1.20 $\pm$ 0.15 x 10$^{10}$ mole$^{-1}$ cm$^3$ s$^{-1}$ at 296 $^\circ$K.
TL;DR: In this paper, the effects of a radical scavenger (C2H4) and electron scavengers (N2O or SF6) on hydrogen formation in the radiolysis of liquid n-butane at room temperature have been studied.
Abstract: The effects of a radical scavenger (C2H4) and electron scavengers (N2O or SF6) on hydrogen formation in the radiolysis of liquid n‐butane at room temperature have been studied. Further, the isotopic composition of the hydrogen from n‐C4H10–n‐C4D10, n‐C4H10–n‐C4D10—0.3M N2O, and n‐C4H10–n‐C4D10—0.15M C2H4 mixtures has been measured, and the ratios of unimolecular to bimolecular hydrogen formation determined for each mixture. Combining these results, the yields of six primary processes by which hydrogen is formed have been determined separately with the results that G (thermal hydrogen atom), G (hot hydrogen atom), and G(molecular hydrogen) are, respectively, 0.7, 1.4, and 0.6 by direct excitation and ionization and 0.5, 0.6, and 1.0 by neutralization, with probable errors ± 0.2. It is noticeable that the largest contribution is from hot hydrogen atoms produced by direct excitation and ionization.
TL;DR: In this article, the reactions of trifluoromethyl radicals with methanol and trideuteromethanol have been studied in the temperature range 84-162°C in order to determine the rate at which hydrogen and deuterium atom abstraction occurs.
Abstract: The reactions of trifluoromethyl radicals with methanol and trideuteromethanol have been studied in the temperature range 84–162°C in order to determine the rate at which hydrogen and deuterium atom abstraction occurs. Over this temperature range, abstraction from the hydroxyl group is slightly favoured over abstraction from the methyl group. Substitution of deuterium for hydrogen alters the positional reactivity by an amount in accord with the zero-point energy difference.
Arrhenius parameters (based upon a value of 1013.34 mole–1 cm3 sec–1 of the rate constant of combination of trifluoromethyl radicals) have been measured for the reactions : [graphics omitted] and have been calculated for [graphic omitted]
(units of A mole–1 cm3 sec–1 and E kcal mole–1).
TL;DR: In this article, it was shown that the unpaired spin density on the carbonyl carbon atom of the ketyl radical-anions is only 70% of that in the corresponding alkyl radicals in which the oxygen is replaced by hydrogen.
Abstract: Sodium or potassium atoms have been deposited on a variety of aliphatic ketones at 77°K in a rotating cryostat and the products examined by electron spin resonance spectroscopy. The spectra show that for acetone, biacetyl, cyclopentanone, and cyclohexanone the corresponding ketyl radical-anions are formed and trapped at 77°K. The unpaired spin density on the carbonyl carbon atom of the ketyl radical-anions is only 70% of that in the corresponding alkyl radicals in which the oxygen is replaced by hydrogen. This reduction shows that there is an appreciable unpaired spin density at the carbonyl oxygen atom.The spectra from ethyl methyl, diethyl, dipropyl, and di-isopropyl ketones show that the ketyl radical-anion is partially, or completely, replaced by a secondary radical which has been identified as that formed by hydrogen abstraction from the parent ketone. It is postulated that in all cases the ketyl radical-anions are formed initially, but that they react immediately in some of the ketones.
TL;DR: THE AUTHORS have examined the action of hydrogen atoms on bacteriophage T7 in aqueous suspension, and found the mechanism of inactivation to be premature release of the DNA.
Abstract: WE have examined the action of hydrogen atoms on bacteriophage T7 in aqueous suspension, and found the mechanism of inactivation to be premature release of the DNA.
TL;DR: In this article, photolytically generated hydroxy-radicals from benzophenone and benzaldehyde have been extended to radicals from acetophenone, xanthone, thioxanthone and p-hydroxybenzophenone.
Abstract: Previously reported work on photolytically generated hydroxy-radicals from benzophenone and benzaldehyde has been extended to radicals from acetophenone, xanthone, thioxanthone, and p-hydroxybenzophenone. In each case, hyperfine structure in the e.s.r. spectrum indicates the corresponding hydroxy-radical which is produced via the (nπ*) triplet. A radical from p-hydroxybenzophenone was observed in aprotic solvents and not in protic solvents in which the ‘charge transfer’ excited states are more accessible; these tend not to favour hydrogen atom abstraction from the solvent.All proton coupling constants except that for the hydroxy-group proton, aOH, are relatively independent of temperature and of solvent. This is also the case for the previously reported hydroxyperinaphthenyl and monoprotonated p-benzosemiquinone radicals. The temperature dependence of aOH suggests torsional rotation of the hydroxy-group about the C–OH bond.
TL;DR: In this article, the inductive effect of the methoxycarbonyl group and a (ω − 1) effect which seems to be steric were observed in the chlorination of methyl decanoate by N-chlorodialkylamines.
Abstract: The high selectivity observed in the chlorination of methyl decanoate by N-chlorodialkylamines is due to the inductive effect of the methoxycarbonyl group and a (ω– 1) effect which seems to be steric. This second effect occurs also in the chlorination of n-heptane, in which the methylene at position 2 is more reactive than the others, and it is increased by use of an N-chlorodialkylamine with higher steric requirement.
TL;DR: After exposure of pyridinium hydrochloride in solid, anhydrous hydrogen chloride to γ-rays, a well-resolved e.s.r. spectrum was obtained.
Abstract: After exposure of pyridinium hydrochloride in solid, anhydrous hydrogen chloride to γ-rays a well-resolved e.s.r. spectrum was obtained which has been shown by study of the corresponding radicals formed from certain deuteriated pyridines to be due to a radical produced by addition of a hydrogen atom to the 4-position of the pyridinium cation. The results are compared with those for the isoelectronic cyclohexadienyl radical.