Radical chains, mixed

Fluorine reacts explosively by a radical chain reaction as soon as the gases are mixed. A mixture of hydrogen and chlorine explodes when exposed to light. Bromine and iodine react with hydrogen much more slowly. A less hazardous laboratory source of the hydrogen halides is the action of a nonvolatile acid on a metal halide, as in... 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

The presence of chlorine radical chain reactions in a photocatalytic reaction system may significantly increase reaction rates and photocatalytic efficiencies. These enhancements would appear to have the potential to overcome the shortcomings typically associated with the photocatalytic oxidation of aromatic contaminants if a chlorine radical chain reaction could be initiated in conjunction with an aromatic photocatalytic reaction and if the chlorine radicals were capable of reacting with (and thus accelerating the conversion of) the aromatic contaminant of interest. Two potential configurations for combining chlorine radical promotion with the photocatalytic oxidation of aromatic contaminants have been examined in some detail mixed contaminant feeds and prechlorinated catalysts. 

Growth of long chains (n > 102) in mixed 1 1 crystals of ethylene with chlorine or bromine at 20-70 K has been studied in detail by Wight et al. [1992a, 1993]. Active radicals were generated by pulsed laser photolysis of Cl2 or Br2. The rate constant has been found to be kc = 8-12 s-1 below Tc = 45 K. The chain grows by the radical chain mechanism... 

The mixed behaviour of such catalysts, in terms of oxo-type and allylic oxidation, was also confirmed in the oxidation of a-pinene, yielding a mixture of the epoxide and the allylic oxidation product (D-verbenone). The epoxide stems from the existence of a high valent Ru(V)=0 intermediate, while D-verbenone formation points to the presence of a radical chain involving peroxoruthenium as intermediated128,1291 The activity of encapsulated H, Cl, Br, nitro-substituted Ru on and Co(salophen) (structure of ligand see insert also known as saloph) is always at least a factor of two higher than in solution. Comparable Co/Si ratios are obtained from XPS and TGA, indicated no significant amounts of complex at the external surface. 

The mechanism for the formation of the ion pair [Co(CO)3L2][Co(CO)4] has been investigated in detail with L = P(n-Bu)3 and was found to be extremely complicated. Mixed reaction orders, between 1 and 1.5 in Co2(CO)g and 0.3 to 0.6 in L, which depend not only on the ligand chosen but also on concentrations and temperature, show that a number of parallel reaction rontes are followed where one or the other pathway has preponderance. The principal scheme, however, given by eqnations (4 10), is initiated by cleavage of a dinuclear species (equation 5) to start a radical chain see Radicals). 

In particular, this chapter wiU stress the need to look beyond the classic radical chain reaction. Lipid oxidation mechanisms have been proposed based on kinetics, usually of oxygen consumption or appearance of specific products (e.g., LOOK) or carbonyls (e.g., malonaldehyde), assuming standard radical chain reaction sequences. However, when side reactions are ignored or reactions proceed by a pathway different from that being measured, erroneous conclusions can easily be drawn. The same argument holds for catalytic mechanisms, as will be shown in the discussion about metals. In the past, separation and analysis of products was laborious, but contemporary methods allow much more sensitive detection and identification of a broad mix of products. Thus, multiple pathways and reaction tracks need to be evaluated simultaneously to develop an accurate picture of lipid oxidation in model systems, foods, and biological tissues. 

Another approach to carrying out tin-free radical fragmentation processes, developed by Fuchs, utilizes trifluoromethyl sulfone, or triflone, derivatives. Fuchs first reported examples of free radical alkynylation reactions using acetylenic triflone 102 [62]. What is most remarkable about these reactions is that the radicals being alkynylated are formed from the cleavage of C-H bonds standard radical precursors are not required. For example, when tetrahydrofuran is mixed with triflone 102 at room temperature, alkynylation occurs a to the ether oxygen in 92% yield (Scheme 21). In this case, the radical chain process is most likely initiated by traces of peroxides in the THF. Similarly, unactivated alkanes such as cyclohexane will react with triflone 102 in good yield (83% for cyclohexane) when heated with a catalytic amount of AIBN. 

Trialkylboranes react with organic disulfides to give thioethers (sulfides) but air or light is required because tbe reaction proceeds by a radical chain process.This reaction is very slow in the dark and ultraviolet (UV) light is required for reasonable reaction rates. Tri octyl borane reacts with dimethyl disulfide under these conditions to give methyl 1-octyl sulfide in 95% yield. 6 The mixed borane B-cyclohexyl-3,5-dimethylborinane (220), gave 94% of cyclohexyl methyl sulfide (221) under the same conditions. ... 

Mixed radical chains, where the attacking species are both Z and X, are sometimes observed when HX can readily generate X2 from reaction with the reagent ZX. Such processes have been identified in the reactions of r-BuOCl, NBS and CI2O. On the other hand, iV-chlorosuccinimide seems to react always by a chlorine atom chain by virtue of reaction 10. 

In producing a block copolymer with interesting applications in polymer blends, Chapiro produced a block copolymer of acrylonitrile and acrylonitrile-co-styrene by gamma irradiation of the mixed monomers in dimethylformamide/benzyl alcohol. It was reported that irradiation induced the formation of radical-anions. Acrylonitrile then added exclusively to the anionic chain ends, and acrylonitrile-styrene added randomly to the free radicals chain ends. The resultant polymer was found to be poly(acrylonitrile-b-acrylonitrile-co-styrene). 

Monitoring of both the photochemistry and the dose requirements indicated that the mixed systems such as PZT may react more efficiently than at least some of the individual components react in pure films. This may be due to the effect of the mixture on the structure of the films or due to a radical chain component to the reaction. If a radical chain component is present then radicals photogenerated by a precursor such as lead which reacts efficiently may serve to initiate the reaction of less photosensitive systems such as the zirconium precursor. This result is consistent with the higher dose requirement for the lead in the presence of the other components. These results indicate that a mixture of precursors may improve the efficiency of photodecomposition. Further enhancements may be available by using additives which are photoradical generators, however, these may also alter the purity of the products obtained. 

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