Showing papers on "Hydrogen atom abstraction published in 1978"

Formation and reactivity of the amino radical

Abstract: The amino radical has been produced by reaction of the OH radical with ammonia in irradiated aqueous solutions. At pH 11.4 the unprotonated radical, NH/sub 2/, is produced with a rate constant k = (9 +- 1) x 10/sup 7/ M/sup -1/ s/sup -1/, while in acidic solution the corresponding reaction of OH with NH/sub 4//sup +/ is too slow to be observed. The SO/sub 4//sup -/ and PO/sub 4//sup 2 -/ radicals are found to react more slowly with NH/sub 3/ than does OH. It is concluded that these radicals react with ammonia by hydrogen abstraction and not by electron transfer oxidation. The method is not useful for the production of the protonated NH/sub 3//sup +/ radical. The subsequent reactions of NH/sub 2/ are followed by monitoring the kinetics of formation of the more strongly absorbing radical products. Rate constants for the reactions of NH/sub 2/ with various substituted phenoxide ions have been measured; with phenoxide itself at pH > 11, k = 3.0 x 10/sup 6/ M/sup -1/ s/sup -1/. These rate constants are strongly affected by the ring substituent. When the rates are plotted against the Hammett sigma values, a slope corresponding to rho approximately = -3.3more » is derived. The mechanism is considered to be oxidation of the phenoxide ions by electron transfer. Although the NH/sub 2/ radical was found to be relatively unreactive toward addition to benzene, its adduct radical to the fumarate anion has been observed by ESR. The NH/sub 2/ radical may also be produced by reaction of e/sup -//sub aq/ with NH/sub 2/OH. An attempt was made to generate the NH/sub 3//sup +/ radical in the corresponding reaction with the acid form of hydroxylamine, /sup +/NH/sub 3/OH, which is especially fast. On the basis of several pieces of evidence, we postulate that NH/sub 3//sup +/ cannot be the main product of this reaction but that /sup +/NH/sub 3/OH splits preferentially into the OH radical and NH/sub 3/. 1 table, 1 figure.« less

Photochemical Oxidation of Benzene, Toluene, and Ethylbenzene Initiated by OH Radicals in the Gas Phase

Abstract: Photochemical oxidation of benzene, toluene, and ethylbenzene initiated by OH radicals in the presence of NO and NO2 was studied in the air by using a 67 dm3 reaction chamber made of Pyrex glass. OH radicals were produced by photochemical decomposition of nitrous acid, the products being phenol and nitrobenzene for benzene; cresols, benzaldehyde, m- and p-nitrotoluenes, and benzyl nitrate for toluene; ethylphenols, benzaldehyde, acetophenone, and m- and p-nitroethylbenzene for ethylbenzene. The reaction mechanisms are proposed based on the addition of OH radicals to aromatic rings and the hydrogen abstraction of OH radicals from alkyl groups.

Surface reactions of oxygen ions. 2. Oxidation of alkenes by oxygen(1-) ion on magnesium oxide

Abstract: Ethylene, propylene, 1-butene, and cis-2-butene were adsorbed on magnesium oxide containing O/sup -/ and the product distributions of their temperature-programed desorption compared with those of the desorption of possible intermediates (e.g., acetaldehyde) from untreated magnesium oxide. The results and ESR and IR spectroscopic studies suggested the alkenes reacted initially via hydrogen abstraction to form radicals; the 1-butene radical is oxidized to the alkoxide ion and forms mainly butadiene by a mechanism similar to that previously reported for alkane dehydrogenation; ethylene and propylene radicals form carboxylate ions which yield methane and carbonate ions as the main products.

Photochemical reactions of chlorpromazine; chemical and biochemical implications.

Abstract: — The photoexcited chlorpromazine reacts with methanol to yield promazine and 2-methoxypromazine by two different reaction pathways: hydrogen atom abstraction and nucleophilic attack. respectively. When the photoexcitation of chlorpromazine is performed in the presence of protein or nucleic acids, chlorpromazine binds to the biopolymer. This binding is drastically pH-dependent and correlates to the phototoxic effect exhibited in chlorpromazine—photosensitization of E. coli. No photodynamic damage of E. coli attributed to CPZ-sensitization of molecular oxygen could be detected.

The influence of protonation or alkylation of the phosphate group on the e.s.r. spectra and on the rate of phosphate elimination from 2-methoxyethyl phosphate 2-yl radicals.

Abstract: SummaryThe e.s.r. spectra of 1-yl, 2-yl and 3′-yl methoxyethyl phosphate radicals derived from CH3OCH2CH2-OPO3H2 by hydrogen abstraction have been measured in aqueous solutions and the hyperfine constants determined. The coupling constants vary strongly with protonation or alkylation of the phosphate group.The 2-yl radicals eliminate phosphate. The rate-constants for the elimination (ke) have been estimated by e.s.r. measurements and by product studies as a function of pH using 60Co γ-radiolysis. The ke values vary from ∼0·3 s−1 for the CH3OĊHCH2OPO—3 radical and ∼103 s−1 for CH3OĊHCH2OPO3H−, to ∼3 × 106 s−1 for CH3OĊHCH2OPO3H2. Alkylation of the phosphate group increases the elimination rate-constant to a similar extent as protonation. The results support a recent mechanism which described the OH-radical-induced single-strand breaks of DNA in aqueous solution starting from the C-4′ radical of the sugar moiety. It is further concluded the C-4′ radical of DNA eliminates the 3′-phosphate group faster than t...

Absolute rate constants for hydrocarbon autoxidation. 25. Rate constants for hydrogen atom abstraction from alkanes by the tert-butylperoxy radical

Abstract: Rate constants for abstraction of secondary and tertiary hydrogens from structurally different alkanes by the tert-butylperoxy radical in solution at 30 °C have been determined by competitive exper...

Quenching of triplet states of aromatic ketones by indole and 3‐methyl indole in benzene solution. evidence for hydrogen‐atom abstraction, for quenching due to favourable charge transfer interaction and for the lack of electronic energy transfer

Abstract: — Quenching of the triplet states of the aromatic ketones (KCO), benzophenone, acetophenone and xanthone, by indole and 3-methyl indole gives rise to the neutral radicals resulting from hydrogenatom transfer with variable efficiency (40–100%). Thus in addition to the reaction, 3KCO*+RH KCOH +R. some other quenching path or paths occur. There is no evidence for any triplet energy transfer even when this is energetically favourable, and it is suggested therefore that quenching to give ground state species following favourable charge-transfer interactions accounts for the proportion of quenching without reaction. The spectra of the indole radicals, R., were determined and the kinetics of their decay in aerated and deaerated solution were investigated and reaction schemes proposed to explain these observations.

Relative rate constants for hydrogen atom abstraction by the cyclohexanethiyl and benzenethiyl radicals

Abstract: Relative values of the rate constants k (eq 1) for hydrogen atom abstraction from a number of organic substrates by cyclohexanethiyl and benzenethiyl radicals at 80/sup 0/C are reported. Good correlations with both sigma and sigma/sup +/ constants were found for ring-substituted ethylbenzene and cumene derivatives, and some limited data for toluenes also are reported. Two new methods were developed to obtain these data; the key feature of both is that tritium-labeled thiol (RSH*) is used as a solvent. In this environment, reversal of the hydrogen abstraction reaction (eq 2) leads to labeling of the hydrogen donor (QH), and k is related to the radioactivity incorporated into the recovered QH*. Isotope effects are involved in the calculations, but they can be evaluated independently. Thiyl radicals are found to be extremely selective, more so than even bromine atoms or CCL/sub 3/. radicals. Surprisingly, both cyclohexanethiyl and benzenethiyl radicals, and also bromine atoms, show remarkably similar polar effects; this is not what would be expected on the basis of heats of reaction or electron affinities. It is suggested that this similarity might be attributable to the similar polarizabilities of bromine atoms and thiyl radicals. 6 tables, 3 figures.

Absolute rates of hydrogen abstraction by tert-butoxy radicals

Abstract: The reaction of tert-butoxy radicals with diphenylmethanol and with cumene was examined using nanosecond laser flash photolysis techniques in order to determine the absolute values of the rates of reaction of the radicals. The absolute rate measured for both substrates compare favorably with the values for the benzophenone triplet. (JSR)

Organic photochemistry in the solid state. The distance and geometric requirements for intramolecular hydrogen abstraction reactions. Structure-reactivity relationships based on x-ray crystallographic data

Abstract: Die unterschiedlichen Reaktionsweisen der Naphthochinone (I), (III), (V) bzw. (VII) bzw. (X) bzw. (XII) bei Bestrahlung mit 340 nm im kristallinen Zustand zu den im Schema angegebenen Produkten (keine Ausb.-Angabe; Fuhrung der Reaktion bei Temp. unterhalb der eutektischen Temp. der Gemische aus Substrat und Produkt; Versuchsapparatur beschrieben) wird eingehend und ausfuhrlich mit Strukturunterschieden der Substrate im festen Zustand 31 erlautert.

An e.s.r. and spin-trapping study of the reactions of the SO4- radical with protein and nucleic acid constituents.

Abstract: Reactions of the SO4- radical, generated by U.V. photolysis of Na2S2O8, were studied in aqueous solutions of amino acids, dipeptides, nucleic acid bases, nucleosides and nucleotides. The transient free radicals so formed were spin-trapped by t-nitrosobutane and identified by e.s.r. spectroscopy. The amino acids primarily undergo oxidative decarboxylation. The pKs of the ammonium groups of the spin-trapped decarboxylated radicals of glycine and alanine in D2O were determined to be 8.3 +/- 0.2. An oxidation product, which is the precursor of the decarboxylated radical, is tentatively identified for alanine, valine and isoleucine. Radicals formed by hydrogen abstraction by SO-4 are identified for leucine, serine, phenylalanine and 4-hydroxyproline. In dipeptides, SO-4 produces decarboxylation of the amino acid located at the carboxylate terminal residue. For gly-ala and ala-ala, radicals generated by hydrogen abstraction from the carboxylate terminal residue alanine were also characterized. Radicals centered on the C(5) carbon were observed for uracil, cytosine and thymine. For nucleosides and nucleotides, radicals situated on the base and/or the sugar moiety were assigned.

Zur Photochemie von Allylaryläthern. 55. Mitteilung über Photoreaktionen

Abstract: The photochemical reactions of different allyl aryl ethers (Scheme 3) were investigated in hydrocarbons (Chap. 3.1) and in alcoholic solvents (Chap. 3.2). The composition of the photoproducts depended very much on the nature of the solvent. Irradiation (3-95 h) of different methyl substituted allyl aryl ethers (1, 3, 5, 7 and 11) with a low pressure mercury lamp (lambda(Emiss.) = 254nm; 6 or 15 Watt) under argon (quartz vessel) resulted in the formation of 2-, 3- and 4-substituted phenols, dienones and products of consecutive reactions (Tables 1-4 and 6). The results suggested that all products were formed by homolytic cleavage of the C-O bond in the singlet state of the ethers to intermediate radical-geminates (Scheme 5) followed by radical recombination of the two fragments. No products were formed by concerted processes (Table 5, Schemes 5 and 6). Upon irradiation of allyl aryl ethers lacking alkyl substituents at position 4 (1 and 5) in protic solvents, mainly 2- and 4-allylated phenols were obtained (Tables 1 and 4); 3-allylated phenols were formed only in small amounts (0.02%). However, in aromatic hydrocarbons or cyclohexane 3-allylated phenols were obtained from 1, 5 and 11 in significant amounts (3-11%; Tables 1, 4 and 6). E.g., upon irradiation of allyl-2,6-dimethylphenyl ether (5) in toluene, the main photoproduct was 6-allyl-2,6-dimethy1-2,4-cyclohexadien-1-one (6) besides 3- and 4-ally1-2,6-dimethylphenol (23 and 24). Irradiation of 5 in methanol afforded 23 and 6 only in traces, whereas 24 was the main product. Ethers alkylated at position 4 (3 and 7) yielded 3-allylated phenols after irradiation in hydrocarbons and in methanol (Tables 2 and 3). The time independent equilibration of deuterium labelling in the allyl chain of dienone d3-6 obtained upon irradiation of 2,6-dimethylphenyl-2’, 3’, 3’-trideuterioallyl ether (2’,3’, 3’-d3-5) (cf. Table 5) demonstrated that the photolysis of aromatic allyl ethers did not occur by a [1s,3s]-sigmatropic process (cf. Chap. 3.1.4.2). For the photochemical formation of 3-allylated phenols, the following two mechanisms may be envisaged: 1. According to CIDNP measurements [12] at least one portion of the 3-allylated phenols 14, 20 and 23 is produced via a direct recombination of the triplet-geminate, which is formed from the singlet-radical-geminate via intersystem crossing (Scheme 5, pathway d). 2. Formation of 4-allyl-2,5-cyclohexadien-1-ones (III, IV and 12) could occur via a singlet-geminate (Scheme 5, pathway c). These dienones undergo photochemical excitation and give bicyclic intermediates, which after further photoexcitation are finally transformed into the 3- allylated phenols 14, 20 and 23 (cf. Scheme 6, pathway g, h and i). This path allows the formation of significant amounts of 3-allylated phenols (3- 11%) during photo-Claisen-rearrangement of allyl phenyl ethers lacking a substituent at position 4. The lifetimes of the initially formed 4-monosubstituted dienones III and IV in hydrocarbons were long enough to permit photochemical isomerization to 3- allylated phenols. In protic solvents however, a fast enolization of 3 and IV to 4-allylated phenols is expected, so that the photochemical isomerization is interrupted. The presence of bases which catalyse this heterolytic enolization further suppress the photochemical isomerization (Table 1). The very small amount (0,01- 0,1%) of 3-allylphenols, which were still formed after the irradiation of allyl aryl ethers lacking a substituent at position 4 in protic solvents or under basic conditions, must be produced by direct radical recombination within the triplet-geminate (cf. Scheme 5, pathway m and n). Free phenoxy and allyl radicals were also formed from the triplet-geminate (cf. ESR. experiments, Chap. 5). The former yielded the observed phenols after hydrogen abstraction from the solvent. A crossover experiment with tritiated ether 3’-t-3 and ether 1 suggested that recombination of free phenoxy and allyl radicals during photolysis did occur only to a very small extent (2 - 0,1%; cf. Chap. 4). The photochemical transformation of ((E)- or (Z)-2’-buteny1-2,6-dimethylphenylether ((E)- and (Z)-11, respectively) to (E)- or (Z)-6-(2’-butenyl)-2,6-dimethyl-2,4-cyclohexadien-1-one ((E)- and (Z)-26, respectively) at 7° showed, that the configurational integrity of the allyl radical was maintained (90-95%) until the recombination had occurred (Table 6).

Exchangeable proton couplings in free radicals: Radiation products of hydroxyproline HCl

Abstract: Free radicals were generated by X irradiation in single crystals of hydroxyproline HCl irradiated at low temperature. Proton hyperfine couplings were deduced from ENDOR measurements on two species, namely, a primary reduction product and a secondary free radical formed in a hydrogen abstraction process. These two radicals are representative of two general types of radicals, one nearly planar and the other markedly pyramidal, in which the unpaired electron interacts with nonexchangeable β protons attached to carbon and exchangeable β protons attached to oxygen. The characteristics of the hyperfine coupling tensors of these protons are discussed.

The Photochemical and Radiation-chemical Stability of Molecules. Unimolecular Reactions Involving the Abstraction of a Hydrogen Atom

Abstract: The ways in which electronically excited molecular systems are deactivated are examined, a theory of the non-adiabatic and predissociation mechanisms for the decomposition of molecular systems with elimination of a hydrogen atom is described, and the isotope effect in this reaction is discussed and compared with experimental data. The photochemical stability of molecules of different classes, radical-cations, and radicals is explained on the basis of the theoretical results. The evolution of atomic hydrogen (formation of radicals) in low-temperature radiation-chemical experiments is associated with the position of the lowest triplets state of the molecules, their ionisation potentials, the polarity of the medium, and the energies of the C–H bonds in the radical-cations. The bibliography includes 95 references.

Metal-catalyzed Organic Photoreactions. The Photooxidation of Olefins in the Presence of Uranyl Acetate

Abstract: The photooxidation of olefins in pyridine in the presence of uranyl acetate afforded β-hiydroxy hydroperoxides. It was confirmed by spectroscopic studies and isotope-incorporation experiments that (1) the stereochemistry of the hydroxyl and hydroperoxyl groups in the β-hiydroxy hydroperoxides was trans, (2) the hydroxyl group was added to the less substituted carbon of the double bond, and (3) the oxygen atom in the hydroxyl group originated mainly from the water molecule, while the oxygen atoms in the hydroperoxyl group originated mainly from molecular oxygen. From these results, we proposed a reaction scheme of: (i) the formation of the hydroxyl radical from water by uranyl acetate-catalyzed photolysis, (ii) the addition of the hydroxyl radical to the double bond, (iii) the combination of the resulted radical with molecular oxygen, and (iv) the hydrogen abstraction of the peroxyl radical to give the product. The scheme was further supported by the photoreaction of 2-methyl-2-butene with bromotrichlorome...

Tunneling effect on hydrogen abstraction by hydrogen atoms and their trapping in some hydrocarbons irradiated at low temperatures

Abstract: The tunneling corrections for the rate constants of hydrogen abstraction by hydrogen atoms below 77/sup 0/K have been calculated for the reactions with methane, ethane, propane, and isobutane assuming gas phase activation energies and frequency factors. It has been shown that the use of the unsymmetrical Eckart potential function is effective in reproducing half-lives which are consistent with the experimental results that thermalized hydrogen atoms are not trapped even at 4.2/sup 0/K in irradiated alkanes other than methane. Qualitative trends for the effects of the structural and isotopic difference on the tunneling abstraction rate constants have been discussed based on these calculations. The results indicate that thermal H and D atoms may not be easily trapped even at 4.2/sup 0/K in protiated alkanes other than CH/sub 4/, while H and D atoms might be trapped in some deuterated alkanes other than CD/sub 4/. This was confirmed by detecting the ESR signal from trapped D atoms in C/sub 2/D/sub 6/ irradiated at 4.2/sup 0/K.

Selective Hydrogen Atom Abstraction by H Atoms in Radiolysis and Photolysis at 77 K: Decane-d22, Tetramethylsilane, and Xenon Matrices

Abstract: The selective formation of HD and solute radicals in the radiolysis of n-C10D22 doped with n-C10H22 at 77 K is explained quantitatively by the mechanism of a selective hydrogen-atom-abstraction rea...

Photochemical Reductive trans-Elimination from trans-Diacidotetracyanoplatinate(IV) Complexes

Abstract: Upon CT excitation the complex ions trans-[Pt(CN)₄N₃X]²⁻ and trans-[Pt(CN)₄X₂]²⁻ (X = Cl and Br) undergo a reductive trans-elimination with formation of [Pt(CN)₄]²⁻ and two ligand radicals in the photoprimary step. The formation of a Pt(III) intermediate is not observed. Due to the stability of [Pt(CN)₄]²⁻, recombination reactions regenerating the starting complex are efficient if the ligand radicals are not scavenged. For the azide complexes the high quantum yields for the production of [Pt(CN)₄]²⁻ are explained by the instability of azide radicals. For trans-[Pt(CN)₄X₂]²⁻, the recombination is efficient in aqueous solution, while in ethanol the halogen atoms are scavenged by hydrogen abstraction. The sequence of steps following CT excitation can be explained by a potential energy diagram.