Chain process initiation

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

This is an intermolecular reaction, so it s going to be a chain process. Initiation has the AIBN-derived radical remove H from Si. In the propagation, the Si radical adds to C9. From there, two pathways are possible. Either we can make C4-C2, then cleave C3-C2, or we can cleave C2-C3, then make C4-C2. Either way, the final steps are the cleavage of C4-C5, then abstraction of H from Si-H to start the propagation again. 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

Alkyl radicals can be generated from alkyl iodides in a chain process initiated by a trialkylborane and oxygen.204 The alkyl radicals are generated by breakdown of a borane-oxygen adduct. 

At low temperatures in air, typically below 250°C, the polymer thermolysis (reaction 14.1) of thermostable polymers can be neglected. We are essentially in the presence of an oxidation chain process initiated by hydroperoxide decomposition. The main characteristics of this process are the following (Audouin et al., 1995) ... 

According to Semenov [11] at thermolysis of chain process initiators the energy of the weakest bond break in molecules of almost all substances fits the range of 209-419 kJ. In this regard, the chain initiation by initial reagents themselves may progress at a low rate. Therefore, substances promoting initiation of chain reactions are often used. For example,... 

The mechanism of the aerobic oxidation of alcohols depends on the particular catalyst used. Two general mechanisms can be considered (1) the direct oxygenation of alcohols by 02 through a free-radical chain process initiated by the catalyst, and (2) the direct oxidation of the alcohol by the catalyst, which is then regenerated by 02. Both mechanisms are well illustrated [6] by the aerobic oxidations catalyzed by the persistent tetramethylpiperidine-N-oxyl (TEMPO) radical 1 and the nonpersis-tent phthalimide-N-oxyl (PINO) radical 2. 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

A further observation was that the optical absorption by the N2O increased somewhat during the induction period. At the same time additional measurements of the IR emission at 4.75 pm showed a growth in the radiation from the oscillators CO, N2 0(1 3) and 02(1 3) before appreciable decomposition of the N2O had occurred. Both these effects can be associated with vibrational disequilibrium in the system (the absorption coefficient of the N2O being directly related to its vibrational temperature), and Zaslonko et al. use this evidence to support the view that the overall CO + N2O reaction occurs even at lower temperatures via a chain process initiated by the unimolecular dissociation of NjO- The overall CO + N2O reaction is strongly exothermic (AH — —87... 

The reaction is believed to be a chain process initiated by attack of a nucleophile (e.g., fluoride ion) on the sulfur atom. The fluorosulfonylacetyl fluorides are likely intermediates in the reaction of fluorinated 3-sultones with nucleophiles (water,hydroxide ion,900,908.928 893,898b,sosd,... 

Iodine accelerates the decomposition of acetaldehyde In the steady-state range, the order is approximately 1.0 and 0.5 in aldehyde and iodine, respectively. The experimental results of Rollefson and Faull have been reinterpreted and added to by O Neal and Benson . The iodine-catalysed reaction is a free radical chain process initiated by the attack of an iodine atom on the acetaldehyde molecule. The proposed mechanism fits the experimental data very well. The thermal decomposition of acetaldehyde is catalysed also by other halogens and halogen compounds . 

From the stated it follows that the use of highintensivity lasers (I 1024 28 photons/(cm2.s) results in the change of radical chain processes initiation mechanism as result of nonlinear effects rised dy AIBN emission absorption. It is to be noted that according to literature data [14,16,19,32,37] usually lasers with I<1020 photons/ (cm2.s) are being used in research work to initiate radical chain reactions. The laser intensity is changed in narrow interval. 

It is accepted generally that photooxidation follows the usual mechanism for the oxidation of hydrocarbons, that is, a chain process initiated by free radicals which are formed after the absorption of a quantum of UV light. The primary initiating processes are indicated in... 

Evidence has been provided for the intermediacy of A-methyl-A-phenylnitre-nium ion in the photolysis of iV-(methylphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate. Substituted aryltropylium ions have been generated by photolysis of a series of 7-methoxycycloheptatrienes, and found to have lifetimes strongly dependent on the donor capacity of the aryl substituent. Photolysis of a mixture of the two pinacols (96 and 97) in acetonitrile resulted in efficient fragmentation of both compounds, by cleavage of the central C-C bonds. This reaction has a quantum yield of 9 1, which suggest a chain process initiated by single electron-transfer quenching of excited (96) by (97). 

The same reactants, oxaziridine 301 and CsF, in acetonitrile at 22°C yield a single product, alkoxyimine 343. This surprising result can be explained in terms of a nucleophilic chain process, initiation of which entails formation of imine 338 and acyl fluoride 339 followed by fluoride addition to the latter to give alkoxide 344. Propagation involves attack by anion 344 on the oxaziridine to form 345, scission of the C-N bond to produce anion 346 plus 339, elimination of fluoride ion from 346,... 

Ligand substitutions of 18-electron complexes can also occur by radical-chain processes initiated by atom abstraction. These radical chains occur through 17-electron intermediates that imdei o facile associative substitutions. Substitutions of metal carbonyl hydrides, halides, and stannyl complexes by this mechanism are all known. These reactions are particularly prevalent in first-row metal hydrides because the M-H bond is weaker than the M-H bond in second- and third-row metal complexes, and hydrogen atom abstraction is one step of the radical chain. However, they have also been proposed to occiu with third-row metal-hydride complexes... 

Scheme 10.71 Carbocydization of secondary alkenylzinc reagents radical chain process initiated by traces of oxygen [55].

The majority of papers published in the field of template polymerization deals with the chain process initiated by radicals. 

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