Hydrogen abstraction from substituted aromatic compounds

The reactivity of tert-butoxy radicals with methyl substituted aromatic compounds is almost exclusively determined by the aromatic moiety and almost independent of the methyl group position. The hydrogen abstraction from the carbon α to the aromatic ring is hardly sensitive to the produced radical stabilization. This lack of dependence can be explained in terms of the large exothermicity of the process.     

Hydrogen abstraction by fluorine atoms under conditions of thermal initiation: Hydrocarbons and fluorinated hydrocarbons

Modified nuclear recoil techniques have been used to obtain accurate relative bimolecular rate coefficients for thermally induced hydrogen abstraction reactions by atomic fluorine. New results are reported for 12 hydrocarbons and partially fluorinated hydrocarbons studied at 303°K.     

Hydrogen abstraction by chlorine atom from amino acids: remarkable influence of polar effects on regioselectivity

Quantum chemistry computations have been used to investigate hydrogen-atom abstraction by chlorine atom from protonated and N-acetylated amino acids. The results are consistent with the decreased reactivity at the backbone α-carbon and adjacent side-chain positions that is observed experimentally. The individual effects of NH(3)(+), COOH, and NHAc substituents have been examined and reveal important insights. The NH(3)(+) group in isolation is found to be deactivating at the α-position, while the acetamido group is activating. For the COOH group, polar effects lead to a contrathermodynamic deactivation of the thermodynamically most favorable α-abstraction. In the N-acetylamino acid, the α-position is deactivated by the combined inductive effect of the substituents and the presence of an early transition structure, again overriding the greater thermodynamic stability of the α-centered radical product. Deactivation of the α-, β-, and γ-positions results in a peculiar stability for amino acids and peptides and their derivatives with respect to radical degradation.     

Hydrogen abstraction from copolymers of fluorinated olefins and allyl or vinyl ethers by hydroxyl radicals: a computational quantum semi-empirical study

The theoretical study of hydrogen abstraction by hydroxyl radicals on two substrates (copolymers of fluorinated olefins and allyl or vinyl ethers) was carried using the MNDO, AM1 and PM3 quantum semi-empirical methods. This study was performed as a function of the site of hydrogen abstraction and of the computational method. The results of the calculations clearly show that the transition state is early along the reaction coordinate and pinpoint that the reactions are not under enthalpic control. The results provide evidence of the importance of the polar effects due to the fluorine atoms.     

Enzymatic hydrogen atom abstraction from polyunsaturated fatty acids

The oxidation of polyunsaturated fatty acids such as arachidonic and linoleic acid initiates a plethora of cell signaling pathways in animals and plants. The chemistry of the enzymatic oxidation has been investigated for several enzymes, most notably prostaglandin synthase and the lipoxygenases, revealing many surprises and impressive examples of enzymatic control of hydrogen atom abstraction and subsequent oxygenation.     

Hydrogen abstraction from substituted benzyl chlorides by the trichloromethyl radical

A series of ring substituted benzylchlorides have been reacted under photolytic conditions with bromotrichloromethane at 70°. Competitive kinetics show the bromination reaction to be substantially dependent upon ring substituents. A rho value of ?1.02 (correlating with σ⁺) was obtained with a correlation coefficient of ?0.996. This is greater than has been observed for other α-substituted toluenes and is attributed to the slight radical destabilizing effect of the adjacent chlorine atom. A correlation between the ring substituent effects in various series of α-substituted toluenes and the electronic and steric requirements of the α-substituent has been developed.     

Hydrogen abstraction from ionic liquids by benzophenone triplet excited states

The activation energy for hydrogen abstraction from imidazolium-based ionic liquids is significantly higher than that observed in conventional solvents.     

Synthesis of aliphatic amino dicarboxylic acids from thiophene

Summary By hydrogenolysis of the corresponding thiophene derivatives, the following aliphatic amino dicarboxylic acids were prepared: 2-aminodecanedioic, 2-aminoundecanedioic, and 5-aminotridecanedioic acids.     

Mass spectra of some partially fluorinated aliphatic compounds

The mass spectra of several fluorohydrocarbons are shown and discussed. The fragmentation of these compounds displays a close correlation with the molecular structure and points to the importance of the charge-stabilisation concept for the ion intensities. The iodo-derivatives show a large abundant molecular ion.     

Hydrogen abstraction from trichlorosilane by the trichloromethyl and 1,1-dichlorotrifluoroethyl radicals

The Arrhenius parameters for the reactions and have been measured. Reaction (2a) was generated and studied by the photolysis of tetrachloromethane in the presence of trichlorosilane and reaction(2b) by the photolysis of 1,1,1,-trichlorotrifluoroethane in the presence of trichlorosilane. The measured data are given by the expressions .     

Hydrogen abstraction from deoxyribose by a neighbouring uracil-5-yl radical

Hydrogen abstraction from the C1' and C2' positions of deoxyadenosine by a neighbouring uracil-5-yl radical in the 5'-AU*-3' DNA sequence is explored using DFT. This hydrogen abstraction is the first step in a sequence leading to single or double strand break in DNA. The uracil-5-yl radical can be the result of photolysis or low-energy electron (LEE) attachment. If the radical is produced by photolysis the neighbouring adenine will become a cation radical and if it is produced by LEE the adenine will remain neutral. The hydrogen abstraction reactions for both cases were investigated. It is concluded that it is possible for the uracil-5-yl to abstract hydrogen from C1' and C2'. When adenine is neutral there is a preference for the C1' site and when the adenine is a radical cation the C2' site is the preferred. If adenine is positively charged, the rate-limiting step when abstracting hydrogen from C1' is the formation of an intermediate crosslink between uracil and adenine. This crosslink might be avoided in dsDNA, making C1' the preferred site for abstraction.     

Hydrogen atom abstraction by a chromium IV oxo complex derived from O2

The Cr(III) hydroxide [TptBu,MeCr(OH)(pz'H)] BARF (1) is produced by reaction of [TptBu,MeCr(pz'H)]BARF with [TptBu,MeCr(O2)(pz'H)]BARF or oxygen atom donors ONMe3 or PhIO in Et2O. However, reaction of [TptBu,MeCr(pz'H)]BARF with PhIO in pure CH2Cl2 yields the Cr(IV) oxo complex [TptBu,MeCr(O)(pz'H)]BARF (2). 2 abstracts hydrogen atoms from organic molecules with weak C-H bonds to form 1. Both 1 and 2 have been structurally characterized.     

Kinetics of the acid-catalysed formation of aliphatic peracids from hydrogen peroxide and aliphatic acids in dioxan

The acid catalysed formation of peracids from substituted acetic acids and hydrogen peroxide has been studied kinetically in dioxan. The rates of peracid formation together with equilibrium constants increase with increasing concentration of sulphuric acid and are correlated with the acidity of the media. The substituent effect suggests that the reaction is controlled by steric rather than polar effects. A mechanism similar to acid-catalysed esterification is discussed.     

Catalytic decomposition of aliphatic peroxy acids

The catalytic decomposition of peroxy acids is studied in various solvents in the presence of manganese, cobalt, chromium, nickel, and copper acetates and cerium benzoate. The catalytic activity of the salts in peroxy acid decomposition decreases in the order Mn²⁺ > Co²⁺ > Ce³⁺ > Cr³⁺ > Ni²⁺ > Cu²⁺. The apparent activation energies of the catalytic decomposition of peroxydecanoic acid range between 45.9 and 88.0 kJ/mol. The reaction medium has an effect both on the apparent rate constant and on the activation energy of the reaction. The reaction mechanism includes the fast formation of a catalyst-peroxy acid intermediate complex, which decomposes to release a catalyst molecule and forms the reaction products. The oxidation state of the metal ion of the catalyst can change. The introduction of compounds capable of forming complexes with metal ions of the catalyst substantially slows down the catalytic decomposition.