Reactive radical species

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

These reactive species initiate the linking process by adding to the double bond of the chosen acrylate monomer (isooctyl acrylate) to create another reactive radical species. This linking process may be represented by... 

Radical-anions of carbonyl compounds are basic and undergo protonation at the oxygen centre generating highly reactive radical species. 

O2- or other reactive radical species could be one of the crucial factors which determines the lifetime of G(-H) radicals in double-stranded DNA. 

Direct reaction between an adsorbed species A(s) and a gas-phase molecule B is sometimes proposed. This reaction pathway is called the Eley-Rideal mechanism. Although such a mechanism may seem as reasonable as the Langmuir-Hinshelwood model discussed above, very few heterogeneous reactions are still thought to occur by the Eley-Rideal mechanism. (An exception seems to be when species B is a very reactive radical species, e.g., a gas-phase H-atom reacting with an adsorbed species, as is discussed in Problem 11.10, in which an Eley-Rideal pathway initiates the growth process.)... 

Figure 10.8 Peroxidation of lipid molecules by reactive radical species such as the hydroxyl radical, HO.

A major factor to consider in pitch carbonizations is the facility for hydrogen transfer reactions. These stabilize otherwise reactive radical species and permit the growth of mesophase in a fluid of low viscosity. Hydroaromaticity and methylene linkages are able to transfer hydrogen without creating radicals. 

Thus, it can be suggested that DPPH is unable to abstract hydrogen from a CH3 group and that in the mechanism assumed by Proll and Sutcliffe (1963) not only DPPH but a more reactive radical species has to be involved. This means a modification of the direct abstraction mechanism. A possible change in the mechanism of DPPH reactions could also be conjectured from the work of Hogg et al. (1961) who presented a very sharp break in the Hammett plot correlating the... 

Iron and copper catalyse the formation of oxyradicals. Three reactions are relevant in this context (1) Autoxidation of metal complexes may yield the superoxide radical which by itself is not very reactive, but is a precursor of more reactive radical species. (2) The one-electron reduction of hydrogen peroxide -the Fenton reaction - results in hydroxyl radicals via a higher oxidation state of iron [2]. (3) A similar reaction with organic peroxides leads to alkoxyl radicals, although a recent report alleges that hydroxyl radicals are also formed [3]. There is a fourth radical, the formation of which does not require mediation by a metal complex. This is the alkyldioxyl radical, ROO , which is formed at a... 

Haem proteins and the potential for the formation of reactive radical species in pathological states... 

Most of the reductive cyclizations of epoxides use Ti(III) species to reduce the epoxide and generate the reactive radical species. Nickel can also be used to effect this type of reductive cyclization <07JOC7451>. The reaction of epoxide 35 with Ni(cod)2 provides pumiliotoxin 209F in good yield. 

Among the major cellular and extracellular targets for reactive radical species are proteins, unsaturated fatty acyl components of lipids and lipoproteins, and DNA constituents including carbohydrates, as is depicted in Fig. 2.9. 

This reaction was one of the first very fast or explosive reactions to be studied in detail by research chemists. The reaction makes use of a very short-hved species called a free radical. These are particles with an odd number of electrons associated with them, made by the symmetrical splitting of a covalent bond (CI2 —> Cl + Cl ) hence they are very reactive as they have a great tendency to link up with another electron from other molecules. The mechanism goes in three very fast steps initiation, when the free radicals are formed, propagation of the reaction by producing the various products and more of the reactive radical species, and then termination when one of the reactants runs out or if the free radicals join together. 

Griller, D. Ingold, K. U. Acc. Chem. Res. 1976, 9, 13. In the context of this review, radicals are called persistent if their lifetimes in liquid solution exceed those of reactive radical species by many orders of magnitude. They may self-terminate slowly or disappear by other reactions, but these processes do not compete with the cross coupling with usual transient radicals. Stable radicals can be isolated in pure form. They are included in our definition of persistence. 

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