Radical chain-propagating

Particularly valuable for the viable nature of oxidations is the flame-arrestor ef feet of micro reactors affecting radical-chain propagation. 

In the following sections, the types of radical chain-propagating and chainbranching reactions responsible for combustion under both low- and high-temperature conditions will be discussed. 

During the course of radical chain propagation, several types of transfer reaction can occur, leading to the interruption of the "material" chain, while allowing the "kinetic" chain to continue. "Different" end groups can appear as a result of such transfer reactions (Figure 20). 

Termination is by the recombination of H and Cl. There is complete corroboration of this mechanism from photochemical studies. This is undoubtedly a free-radical chain propagation mechanism. 

Mechanism II. Reaction R02 + RH occurs slowly and tertiary peroxyl radicals react more rapidly with the formation of alkoxyl radicals. Chain propagation includes the following steps ... 

Ruhho, H., Parthasarathy, S., Barnes, S., Kirk, M., Kalyanaraman, B., and Freeman, B. A., 1995, Nitric oxide inhibition of hpoxygenase-dependent hposome and low-density hpoprotein oxidation termination of radical chain propagation reactions and formation of nitrogen-containing oxidized hpid derivatives, Arclr. Biochem. Biophys. 324 15-25. 

Figure 4. Radical chain propagation steps for the pyrolysis of silica-immobilized I, 3-diphenylpropane. Bracket notation denotes species exists in both gas-phase and surface-attached forms.

In general, what is the activation energy in free radical chain propagation of polymer chains ... 

In the atmospheric free radical reactions involving hydrocarbon species, molecular products of interest are formed via either radical chain propagation or termination steps. 

Because radical brominations are so selective, they can be used successfully in the lab to make alkyl bromides. There are relatively few ways of functionalizing an unfunctionalized centre, but radical allylic bromination is one of these. Just as tertiary radicals are more stable than primary ones, so allylic radicals are even more stable than tertiary ones (see the table on p. 1026). In the presence of a suitable initiator, bromine will therefore selectively abstract an allylic hydrogen atom to give an allylic radical that can then be trapped by a molecule of bromine to regenerate a bromine radical (chain propagation) and produce the allylic bromide, initiation Br2 ----------- 2 x Br ... 

Unlike hydrosilation (Section 8-5), hydrostannation proceeds via radical chains propagated by R3Sn radicals intermediate radicals R3SnCR2CR2 have been characterized. The rates for hydrostannation reactions are greatly increased (100-600 times) under sonication conditions.84... 

Another key point to note is that chain transfer and termination by radical combination leads to radicals and molecules with more carbon atoms than the feed (propane). Subsequent involvement of these moieties in the radical chain propagation leads to larger molecules. In practice this maimer of radical cracking of ethane and propane cracking leads to some C4, C5 and Ce+ products forming pyrolysis gasoline. 

There are four major mechanisms for radical chain propagation by alkoxyl radicals. The mechanism dominating in a given system is determined largely by double bond structure, solvent conditions, and steric factors (21) ... 

As a result, the rate of oxidation of alkylaromatic compounds is mainly dependent on the ratio of the rate of radical chain propagation and the rate of radical chain demolition with [ p/(2 t)° Jrei as a value for a relative oxidizability. For further discussion, cf. [11b, 19b, 23a, 27-30]. 

When alkyl iodides and ROH were irradiated under CO pressure in the presence of a base such as potassium carbonate, good yields of carboxylic acid esters were obtained (Scheme 4-42) [71]. In the absence of a base, no carbonylation took place. The role of photo-irradiation is to initiate this hybrid radical/ionic reaction by effecting the homolysis of an R-I bond. The thermal initiation process involving allyltin and AIBN has also been found to be useful, as demonstrated by two examples of amide synthesis which are shown in Scheme 4-42 [72], The likely mechanism involves (i) radical initiation via either irradiation or thermal initiation, (ii) radical chain propagation, composed of two reversible type radical reactions (carbonylation and iodine atom transfer) and (iii) ionic quenching to shift... 

Even easier is the a-fluoroalkylation of alcohols [32b]. The a-hydroxyalkyl radicals are also resonance-stabilized. Because of their nucleophilicity, they are highly reactive towards perfluoropropene and have the right reactivity for the radical chain propagation step (Scheme 2.108). 

Interfere with the free-radical chain propagation mechanism. 

For basic oxidations using silver oxide, addition of copper or iron oxide (Cu20, Fe203) enhances oxidation [22] but, under these conditions, the reaction occurs by a catalytic mechanism without radical chain propagation. 

Nitric oxide can act as a vitamin E-like inhibitor of radical chain propagation reactions via radical-radical reaction with cytotoxic species such as lipid peroxyi (LOO-) and aikoxyl (LO-) radicals. Nitric oxide reacts with yLO- and LOO- at near-diffusion-limited rates (for LOO-, ife = 1.3 X 10 M sec" ) (Padmaja and Huie, 1993). This rate constant... 

Hindered amine light stabilisers (HALS) are the main stabiliser type (as a scavenger to inhibit free radical chain propagation), while organo-nickel compoimds (as a quencher to prevent initiation of polymer degradation) are used for UV stabilisation. 

Vanderzande showed that activation of the starting material 6 proceeds via 1,6-elimination of HX, and provided experimental evidence of radical chain propagation of the p-quinodimethane-type active monomer 8 [55-57]. However, while high-molecular-weight polymers form in the polar-protic... 

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