Carbon hydrogen radical

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of medrane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation duough the formation of higher hydrocarbons such as CaHe before CO is formed. The chain reactions include... 

Free radicals are initially generated whenever polymer chains are broken and carbon radicals are formed. These effects occur during manufacture and in service life. Many elastomers are observed to oxidize at relatively low temperature (about 60°C), where carbon-hydrogen and carbon-carbon bond cleavages are highly unlikely. It has been demonstrated [52] that traces of peroxides impurities in the rubber cause low-temperature oxidation of rubber. These initiating peroxides are present in even the most carefully prepared raw rubber polymer [53]. 

Sixteen non- to trimethyl-substituted thiazoloquinolines of these types were inveshgated under electron impact mass spectra thiazoloquinolines lose carbon monosulhde (CS) from the thiazole and HCN from both heterocyclic nuclei. When 2-methylthiazoloquinolines fragment, the hydrogen radical and a loss of a neutral... 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

Studies with simple radicals show that carbon-centered radicals react with phenols by abstracting a phenolic hydrogen (Scheme 5.14). The phenoxy radicals may then scavenge a further radical by C -C or C-O coupling or (in the case of hydroquinones) by loss of a hydrogen atom to give a quinone. The quinone may then react further (Section 5.4.4). Thus two or more propagating chains may be terminated for every mole of phenol.I9j... 

In the second step, the activated surface-carbon radical reacts with the carbon-hydrogen species (acetylene as a monomer unit) in the gas phase to become the site for carbon addition ... 

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