Showing papers on "Hydrogen atom abstraction published in 1969"

Pulse radiolysis studies on the oxidation of organic radicals in aqueous solution

Abstract: Pulse radiolysis has been used to measure directly the absolute rates of oxidation by ferricyanide ion of various radicals produced by OH attack on organic solutes. These include mono, di- and polyhydroxylic compounds, hydroxy acids, polyethylene oxides of molecular weight 200, 6000 and 20 000 and the amino acid serine. Radicals produced by hydrogen abstraction from α carbon atoms in alcohols are oxidized at, or near, diffusion-controlled rates, whereas the reactions are much slower for radicals formed by OH-attack elsewhere. The technique has been used to measure the percentage OH-attack at the α position for a series of straight and branched-chain alcohols. Oxygen competes with ferricyanide for radical oxidation. The data for oxygen-containing solutions fit a simple radical-competition scheme which has been used to measure rates of peroxy-radical formation. These approach diffusion-controlled limits.

Thermal reactions of O− ions with saturated hydrocarbon molecules

Abstract: The thermal energy reactions of O− ions with saturated, hydrocarbon molecules have been investigated in a flowing afterglow of pure oxygen and a mixture of helium and oxygen. Hydrogen atom abstraction was found to be the dominant reaction channel. The reaction probability was found to increase for the higher members of the homologous series. The further reaction of the product OH− ions with saturated hydrocarbon molecules was not observed. This result was used to estimate an upper limit to the electron affinity of the free radicals CH3, C2H5, C3H7, and C4H9.

Absolute rate constants for hydrocarbon autoxidation. XIII. Aldehydes: photo-oxidation, co-oxidation, and inhibition

Abstract: The oxidations of acetaldehyde, heptanal, octanal, cyclohexanecarboxaldehyde, pivaldehyde, and benzaldehyde in chlorobenzene at 0 °C have been studied. These aldehydes oxidize at similar rates under similar conditions because there are compensating changes in the rate constants for chain propagation (kp) and chain termination (2kt). The termination rate constants increase from ∼7 × 106 M−1 s−1 for pivaldehyde and cyclohexanecarboxaldehyde to ∼2 × 109 M−1 s−1 for benzaldehyde. The propagation rate constants increase from ∼1 × 103 M−1 s−1 for pivaldehyde to ∼1 × 104 M−1 s−1 for benzaldehyde.The rate of oxidation of the aldehydes was decreased by the addition of 1,4-cyclohexadiene, tetralin, tetralin hydroperoxide, cumene, cumene hydroperoxide, t-butyl hydroperoxide, and 2,6-di-t-butyl-4-methylphenol. As a result of product analysis and absolute rate constant measurements, it is concluded that the peroxy radicals derived from aldehydes are considerably more reactive in hydrogen atom abstraction from hydrocar...

The reaction kinetics of gaseous hydrogen atoms with graphite.

Abstract: : The kinetics of reaction between solid graphite and gaseous atomic hydrogen was studied in the temperature range 450-1200K. The products of reaction are molecular hydrogen and methane. The rate exhibits an activation energy of 5.55 kcal/mole and is a function of the concentration of both hydrogen atoms and hydrogen molecules. Near 800K the rate goes through a maximum value, probably because of the thermodynamic instability of methane. A mechanism for the reaction is proposed. (Author)

Photochemische Reaktionen. 52. Mitteilung [1]. Zur Photochemie von α,β-ungesättigten cyclischen Ketonen: Spezifische Reaktionen der n,π*- und π,π*-Triplettzustände von O-Acetyl-testosteron und 10-Methyl-Δ1,9-octalon-(2)

Abstract: The photochemistry of the conjugated cyclohexenones O-acetyl testosterone (1) and 10-methyl-Δ1,9-octalone-(2) (24) has been investigated in detail. The choice of reaction paths of both ketones depends strongly on the solvent used. In t-butanol, a photostationary equilibrium 1 ⇄ 3 is reached which is depleted solely by the parallel rearrangement 1 5 (Chart 1; for earlier results on these reactions see [2a] [6] [7]). In benzene, double bond shift 1 16 (Chart 3) occurs instead, which is due to hydrogen abstraction from a ground-state ketone by the oxygen of an excited ketone as the primary photochemical process. In toluene, the major reaction is solvent incorporation (1 17, Chart 4) through hydrogen addition to the β-carbon of the enone, accompanied by double bond shift and formation of saturated dihydroketone as the minor reactions. Contrary in part to an earlier report [19], the photochemical transformation of the bicyclic enone 24 exhibit a similar solvent dependence. The corresponding products 25–29 are summarized in Chart 5 and Table 1. Sensitization and quenching experiments established the triplet nature of the above reactions of 1 and 24. Based on STERN-VOLMER analyses of the quenching data (cf. Figures 2, 4–8, and Table 3), rearrangement, double bond reduction and toluene addition are attributed to one triplet state of the enones which is assigned tentatively as 3(π, π*) state, and the double bond shift is attributed to another triplet assigned as 3(n, π*) state (cf. Figure 9). The stereospecific rearrangement of the 1α-deuterated ketone 2 to the 4β-deuterio isomer 4 shows the reaction to proceed with retention at C-1 and inversion at C-10. The 4-substituted testosterone derivatives 33–36 (Chart 8) were found to be much less reactive in general than 1. In particular, 4-methyl ketone 33 remains essentially unchanged on irradiation in t-butanol, benzene and toluene.

The photochemical Beckmann rearrangement

Abstract: The direct irradiation of aryl aldoximes gives the amide (the photochemical Beckmann rearrangement) by intramolecular oxygen migration. This has been shown by the rearrangement of 18O-labelled benzaldoxime in the presence of p-tolualdoxime: no exchange of label was observed.The rearrangement usually gives the amide, but in the case of the anisyl derivative the anilide was also obtained. The highest quantum yield for benzamide formation noted (in acetic acid) was 0.034, but the figure is dependent on irradiation time. The results of low-temperature irradiation suggest that an oxazirane is an intermediate. Phenyl-N-methyl oxazirane is converted into the amide on irradiation (though not thermally) and it seems likely that the reaction is induced, under the conditions of the reaction, by benzylic hydrogen abstraction by thermally produced small amounts of benzaldehyde; a fact which may explain the variation in quantum yield of amide formation with extent of irradiation. An additional route from oxazirane to a...

Direct Determination of Threshold Energies for the Reactions of Hydrogen Atoms with N2O and with CO2

Abstract: IT has been realized for some time that hydrogen atoms produced by photolysis of the hydrogen halides or of certain other hydrides possess kinetic energy in excess of that corresponding to thermal equilibrium1–5. This arises because the energy of the quantum absorbed is larger than the minimum required to disrupt the molecule into stationary atoms. For example, the dissociation energy of HI is 296 kJ moles−1 (ref. 6) and its first absorption maximum occurs at 220 nm7, corresponding to an energy of 539 kJ moles−1. As a result of conservation of momentum, most of the excess energy appears as translational energy of the hydrogen atom. Such hydrogen atoms are often described as “hot” and their energy is manifested in enhanced reactivity. Their reactions with HI1–3, H2S4, halogens1–4, D2 and several deuterated hydrocarbons (or the corresponding reactions of D or T with H2 or hydrocarbons)8–13 have been demonstrated, and in some cases the variation of reaction probability with wavelength of photolysis has been measured10–12.

ENDOR of biradicals: A chemical solution to the biradical paradox

Abstract: The ENDOR spectra of Tschitschibabin's hydrocarbon and certain of its specifically deuterated derivatives have been measured. To interpret these spectra the factors determining the ENDOR spectrum of a biradical are discussed in detail. The analysis reveals that the paramagnetic species present in solutions of Tschitschibabin's hydrocarbon and responsible for the solution electron resonance spectrum is a substituted p-biphenyldiphenylmethyl radical, thus resolving the biradical paradox. The substituent is thought to be the diphenylmethyl group in which case the radical could be formed through hydrogen abstraction by a thermally populated triplet.

Some hydrogen atom abstraction reactions of CF2H and CFH2 radicals, and the C ? H bond dissociation energy in CF2H2

Abstract: By photolyzing (CF2H)2CO and (CFH2)2CO the hydrogen atom abstraction reactions of CF2H radicals with (CF2H)2CO, H2, D2, CH4, C2H6, nC4H10 and isoC4H10, and the reactions of CFH2 radicals with (CFH2)2CO and nC4H10, have been studied. Arrhenius parameters for these reactions are compared with related systems. From a knowledge of the activation energies for the forward and reverse reactions a value of the bond dissociation energy, D(CF2HH) = 97.4 ± 1.3 kcal mole−1 at a mean temperature of 543°K is obtained. This value is subject to much uncertainty due to possible compensation effects in the Arrhenius parameters. These effects are discussed for this and the other reactions, and the data suggest that D(CF2HH) is approximately 100 kcal mole−1, and that D(CFH2H) is very similar. Other literature data tend to confirm these approximate values.

Electron Spin Resonance Study of Radicals Formed by Abstracting Hydrogen from Amides

Abstract: Electron spin resonance (ESR) studies were made on various radicals formed from formamide, acetamide, N-methylformamide, N-methylacetamide, N,N-dimethylformamide and propionamide by hydrogen abstraction with hydroxyl radicals produced by the reaction of hydrogen peroxide and titanium trichloride in aqueous solution, using a rapid flow technique. In each case, except for formamide, various π-electron radicals were found. For formamide, N-methylformamide, and N,N-dimethylformamide, carbamoyl and its methyl substituted σ electron radicals were detected. The g factor of the σ electron radical is qualitatively discussed.

Kinetics of gas-phase fluorination of halomethanes

Abstract: The rates of the hydrogen abstraction reactions of fluorine atoms with five halomethanes and two alkanes in the gas phase were studied using competitive methods. The main competitors were arranged in the following series in which adjacent pairs were fluorinated competitively: (i) CH3Cl, CH2Cl2, CHClF2, CHCl2F, CH3Cl (ii) CHCl3, CH2Cl2, CHClF2, CHCl2F, CHCl3(iii) CH3Cl, CH2Cl2, CHCl3, CHCl2F, CH3Cl. The relative rates obtained were all internally consistent. The results for n-butane and neopentane agreed with reported values. Absolute Arrhenius parameters for the halomethanes were derived from reported data for n-butane and the A factors compared with theoretical values. The activation energies in the chloromethane series pass through a minimum in going from methane to chloroform similar to the minima found in the chlorination and bromination of chloro and fluoromethanes. The results are discussed in terms of changes in bond strength and in the repulsive force methanes. The results are discussed in terms of changes in bond strength and in the repulsive force between the nascent products. This force depends upon the dipole moment of the hydrogen donor.

Struktur der Blei(IV)-carboxylate und ihre Zersetzung über elektronenspinresonanz-spektroskopisch nachweisbare Radikalzwischenstufen†

Abstract: The position of the asymmetric COO- vibration of lead4+ carboxylates (at 6.50 to 6.58 μ) clearly indicates the absence of covalent bonding between oxygen and lead. These salts show strong UV. absorption in the region of 220 to 270 nm which is assigned to a charge transfer from oxygen to lead. UV.-irradiation of lead4+ carboxylates at −196°C in a solid benzene matrix leads to decarboxylation and production of alkyl radicals which were identified by ESR. spectrometry. These radicals react in the solid matrix by hydrogen abstraction from the α-position of a neighbouring carboxylate. In solution however rapid oxidation to carbonium ions occurs.

Photochemische Reaktionen 49. Mitteilung [1] Spezifisch π → π*‐induzierte Keton‐Photoreaktionen: Die Doppelbindungsverschiebung von O‐Acetyl‐10α‐testosteron Protonisierung der Doppelbindung seines δ5‐Isomeren

Abstract: The α,β-unsatured ketone 10α-testosterone has been reported previously [6] to photoisomerize in t-butanol solution to the β,γ-unsaturated ketone. The irradiation had been carried out using a high-pressure mercury lamp in a quartz vessel. For structural reasons this double bond shift cannot proceed through a photoenolization mechanism involving an intramolecular hydrogen transfer from the γ-position to the enone oxygen as has been suggested to operate in several formally analogous cases of aliphatic enone isomerizations. In the present reinvestigation, O-acetyl 10α-testosterone (1) was used, employing selectively either excitation of its n π* (with wavelengths > 300 nm) or its π π* absorption band (with 253,7 nm). In t-butanol solution the doublebond shift 1 2 could be effected with π* excitation only. Experiments in deuterated solvent (t-BuOD) resulted in deuterium in corporation in both the δ5-ketone in the C(4)-position, cf.(3) and in the conjugated ketone. These results indicate that the reactions is initiated either in the, Sπ,π* state or in a high vibrational mode of the S0 or tππ*state. n π* Excitation of 1 in t-butanol gave essentially no over-all chemical change, while in benzene solution it resulted again in a double bond isomerization (1 2). In analogy to results with similar enones [28] under identical conditions the deconjugation in benzene may be the consequence of an intermolecular hydrogen abstraction of the Tn,π* excited state of the enone. Another specifically π π* induced photoreaction was observed on irradiation of the β, γ-unsaturated ketone 2 in t-BuOD with 253,7 nm. The olefinic hydrogen at C-6 of 2 was exchanged with deuterium and, to a small extent, isomerization to the conjugated ketone 1 with concomitant deuterium incorporation occurred. It is concluded that from the higher excited state of the β, γ-unsaturated ketone, but not from its Sn,π* state, an activation mode of the double bond is accessible to effect D+ addition at C-6 followed by deprotonation to 4 and to deuterated 1, respectively.