Propagation cycle

Chain-breaking antioxidants which interrupt the propagation cycle by reacting with the radicals R and R02, introducing new termination reactions. 

It is assumed that for each initiation there are many propagation cycles before termination. The main reaction is therefore given by the addition of the propagation steps alone, which gives the correct stoichiometric equation. A small amount of ethane, C2Hg, is expected due to the termination reaction. 

Prior-equilibrium approximation, 86-89 Product-catalyzed reactions, 36-37 Propagation cycle, 182... 

In the presence of an excess of non-reducible soft nucleophile (for example PhS- or PhSe -), a new reducing species may be formed which allows in certain cases a propagation cycle (electron chain catalysis). 

Applying the steady state approximation to the partial pressures of the O and N atoms is valid if the average number of propagation cycles prior to termination is large. Assuming this to be the case we find... 

The initiation step in the chain oxidation, reaction (43), is not affected by the presence of oxygen. SO4 radicals formed in (43) give arsenic(fV), reaction (45), initiating the following propagation cycle which leads to the reformation of As(IV)... 

In the presence of oxygen, reactions (46) and (46 ) will be replaced in the chain propagation cycle by reactions (63) and (55) consequently, there will be no change in the overall stoichiometry of the reaction. The chain reactions will be terminated by steps (52), (53), (54) and (58)... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Irradiation of mixtures of hydrocarbons and chlorine at suitable wavelengths leads to chlorination of the organic molecule (Scheme 1.3). Reactions have overall quantum yields in excess of 106 (>106 propagation cycles for each termination step). 

Figure 7.10 Repetitive nature of the propagation cycle in photobromination of a hydrocarbon (RH) by a free-radical chain reaction...

These reactions, if of sufficient magnitude, would result in the formation of non-radical, nitrogen-containing species, thereby stopping the propagation cycle of lipid peroxidation. 

Kinetic studies of migratory insertion reactions of the ligands that are involved as (P-P)Pd" fragments in either the propagation cycle of ethene/CO copolymerisation or ethene dimerisation to butenes have been reported by Brookhart [28] and Bian-chini [5e, fj. 

T he primary chain-propagating cycle during the gaseous oxidation of an alkane is essentially a mechanism for oxidizing alkyl radicals and ... 

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

The more effective a termination step is, the shorter will be the propagation cycle and the less product will be produced per initiation event. In the limiting case, if each initiation event was terminated, then no product would be produced. This is the role of antioxidants added to many products and most processed food. These additives scavenge free radicals produced by the reaction of oxygen with C-H bonds and prevent them from participating in oxidation propagation cycles—thus oxidative degradation is stopped or slowed markedly. 

As a consequence of the fact that free-radical reactions are chain processes, they are very well suited for the preparation of polymers rather than single products. That is, products are obtained whose size is determined by the number of propagation cycles that occur before a termination event stops the growing chain. 

If the number of propagation cycles is between 200 and 300, then the product mixture will contain molecules which contain between 200 and 300 monomers. It is more reasonable to describe the product mixture in terms of the average molecular weight rather titan a single product with a discrete molecular weight. The physical properties reported for a polymer are those of a mixture of polymeric molecules rather than of a single polymeric compound. 

These radical-neutral molecule reactions tend to be chain reactions, which is an important facet of a large percentage of radical mechanisms. As with all chain mechanisms, there are initiation, propagation and termination steps. The efficiency of the chain is defined by the number of propagation cycles (chain length) and, in many reactions, termination becomes unimportant so is not commonly considered, except for conventional radical polymerisations or reactions in viscous solvents. Rate constants for propagation help to determine the synthetic utility and mechanism of radical reactions. Initiation is covered in Section 10.2. 

This produces the major product HBr and a second radical which must undergo reaction to regenerate Br and form the other major product, CH3CHBrCH3, along with a similar reaction to produce the less substantial major product BrCH2CH2CH3. These must be produced by reaction of the two alkyl radicals produced in the two possible first steps of the propagation cycle. 

Figure 6.6 a Propagation cycle in the synthesis of polyethylene according to the Cossee mechanism b calculated energy/reaction coordinate diagram for the ethene insertion step. 

Scheme 1. Radical propagation cycle for single-electron transfer-initiated production of CX3 radicals from CX, (X= Hal) and functionalization of an alkane (RH).

Hydride transfer steps leading to the formation of propane, n-butane, and isopentane are involved in propagation cycles of the surface chain reactions, whereas hydride transfer steps that produce heavier alkanes lead to termination of surface chain reactions. 

Substituted 2,3-dihydro-l-H-indoles 26 are accessible by the versatile application of a 5-exo ring closure process during the propagation cycle in the SRN1 reaction [67]. Following the initial ET and fragmentation of the radical anion of the substrate, the... 

The photostimulated reaction of 1,8-diiodonaphthalene with p-methyl-benzenethio-late ions in DMSO yields the substituted cyclized product 10-methyl-7-thia-benzo[de] anthracene (31) in moderate yield (Scheme 10.58) [54], The mechanism proposed to explain product 31 involves an intramolecular radical cyclization after monosubstitution in the propagation cycle of the SRN1 process. 

Overall, Eqs. (l)-(3) depict a nucleophilic substitution Eq. (4) in which radicals and radical anions are intermediates. Once the radical anion of the substrate is formed it fragments into a radical and the anion of the leaving group (Eq. (1)). The aryl radical can react with the nucleophile to furnish a radical anion (Eq. (2)), which by ET to the substrate forms the intermediates needed to continue the propagation cycle (Eq. (3)). The mechanism has termination steps that depend on the substrate, the nucleophile and experimental conditions. Not many initiation events are needed, but in this case, the propagation cycle must be fast and efficient to allow for long chains to build up. 

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