Free alkyl radical

Methyl free radicals, generated either by thermolysis of lead tetracetate in acetic acid solution (401) or by radical cleavage of dimethylsulfoxide by H2O2 and iron (II) salts (408), afford 2- and 5-methylthiazole in the proportion of 86 and 14%, respectively, in agreement with the nucleophilic character of alkyl free radicals and the positive charge of the 2-carbon atom of the thiazole (6). 

HNO conversions to nitroparaffins pass through a maximum at paraffiniHNO molar ratios of approximately 4 1 to 6 1 (32). At higher ratios, a high fraction of the HNO reacts to form alkyl free radicals. At lower ratios, a large fraction of HNO decomposes, as ia reaction 5. 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decaibonylation and sidsoetprait reactions of the alkyl free radicals that result ... 

Any substance capable of reacting with free radicals to form products that do not reinitiate the oxidation reaction could be considered to function as free-radical traps. The quinones are known to scavenge alkyl free radicals. Many polynuclear hydrocarbons show activity as inhibitors of oxidation and are thought to function by trapping free radicals [25]. Addition of R to quinone or to a polynuclear compound on either the oxygen or nitrogen atoms produces adduct radicals that can undergo subsequent dimerization, disproportionation, or reaction with a second R to form stable products. 

The reactions of alkyl halides with Fe(II) deuteroporphyrin IX have been examined (Wade and Castro 1973). Three classes of reaction were observed (i) hydrogenolysis, (ii) elimination to alkenes, and (iii) coupling of alkyl free radicals. Further discussion has been given in Castro (1998). 

One of the fundamental questions about the mechanism is whether the radical is really free in the sense of diffusing from the metal surface.7 For alkyl halides, there is considerable evidence that the radicals behave similarly to alkyl free radicals.8 One test for the involvement of radical intermediates is to determine whether cyclization occurs in the 6-hexenyl system, where radical cyclization is rapid (see Part A, Section 12.2.2). 

When an alkyl free radical 9 is generated by homolytic cleavage of a C—X bond in its precursor 8... 

In this context, an elegant alkylative carbonylation of alkenes accompanied by phenylselenyl transfer is described by Ryu and Sonoda. This three-component coupling involves the addition of a methoxycarbonylmethyl radical to an alkene, the trapping of the produced alkyl free radical by CO, and termination of the reaction by phenylselenenyl group transfer from the starting material. Yields are... 

There are two important considerations concerning free-radical mechanisms. One of these factors is the identity of the halogen and the other is the stability of the alkyl free radical. The more substituted the carbon atom is, the more stable the free radical. The stability of the alkyl free radicals increase in the following order Me < 1° < 2° < 3°. The relative stabilities rarely lead to rearrangement of the free radical. 

Alkyl Free Radicals. The primary alkyl free radical, RCH./, has been postulated to exist in gamma-irradiated polyethylene (23), but its presence has never been unambiguously demonstrated. If all the a and p protons interacted equally with the unpaired electron, the primary alkyl free radical ESR spectrum should consist of five lines, in contrast to those of the secondary and tertiary alkyl radicals whose ESR spectra should consist of six and eight lines, respectively (the latter calculated for the tertiary free radical in polypropylene). The existence of the secondary... 

On heating, migration of a hydrogen atom, alpha to a polyene group, to an alkyl free radical site would also convert a polyene to a polyenyl free radical, a transformation similar to transformation of the allyl free radical described above ... 

Substitution 1 if R/R = t-C 4 6 alkyl/ Free radicals formed more stable and... 

The past decade has been an extremely fruitful one in the field of quantitative free radical kinetics. Two papers can be identified as the starting point of much of this work. The first of these is the acetone photolysis study by Noyes and Dorfman1 which gave confidence to the use of acetone as a reproducible source of methyl free radicals in a fairly simple kinetic environment. The second is the study of Gomer and Kistiakow-sky2 of the absolute rate of recombination of CH3 radicals. The latter study made it possible to give absolute values for the Arrhenius parameters for the reactions of alkyl free radicals with stable molecules. It also opened the way for putting the reactions of methyl radicals with other alkyl radicals on an absolute basis. 

Rate Constants for the Primary Reactions of the Isobutyl Free Radical with Oxygen. The major primary reaction of the simple alkyl free radicals near room temperature is the association with oxygen, Reaction 7a. 

Most of the other products found in irradiated meat volatiles except those containing sulfur or aromatic rings may also be accounted for by mechanisms associated with alkyl free radical formation in the fat. Oxygenated compounds are far less abundant than hydrocarbons, but appreciable amounts of a homologous series of n-aliphatic alcohols up to hexanol are found. Of these, only ethanol is detected in the unirradiated controls. Since the water content of meat averages nearly 60%, the formation of alcohols may be thought to occur by reaction of the alkyl free radical with water. Such a mechanism is supported by the fact that only traces of alcohols are found in irradiated dry butterfat and were undetected in irradiated triglycerides or methyl esters of fatty acids. 

The aldehydes and ketones are least abundant of all the compounds found which may be considered as derived from the fat. The carbonyl compounds are probably produced by an indirect route, which is most likely similar to that involved in autoxidation of a fat. The alkyl free radical can absorb oxygen, form a hydroperoxide, and then follow the many decomposition paths which are familiar in the oxidation chemistry of fats. The more abundant aldehydes found are unsaturated, which further agrees with the hypothesis that they are derived from the decomposition of hydro-... 

The most stable alkyl free radicals are tertiary. The tertiary free radical having the formula C5HU has the same skeleton as the carbocation in Problem 4.13. 

Thermolytic Reactions. At the higher temperatures, nonoxidative reactions also occur as for example the formation of tetrasubstituted cyclohexenes via Diels-Alder reaction, or the formation of dehydrodimers and mono- or polycyclic dimers via combination of alkyl free radicals or free radical attack on double bonds. 

In the majority of these reactions, higher valent (Mn,v or Mnv) compounds have been implicated in the respective reactions (vide supra). As mentioned previously, two p-oxo Mn,v porphyrin species have been characterized from the reaction of iodosylbenzene with Mn111 compounds of the type [MnX(TPP)].678,690 These latter compounds are capable of oxidizing alkanes in good yield at room temperature, apparently via the intermediacy of alkyl free radicals.693... 

Alkenes, perfluoro-cycloadditions to benzyl azide, 60, 35 pyridine imines and ylids, 60, 36 Alkenes, perfluoro-, as heterocyclic precursors, 59, 10 Alkylation, free-radical, of 1,3-dimethyluracil, 55, 227 A-Alkylation, [ 1,2,4]triazolo[ 1,5-u)-pyrimidines, 57, 110 Alkynes, photocycloaddition to uracils, rearrangement with HCNO elimination, 55, 149 Alkynes, ethoxy-, cyclaoddition to hexafluoroacetone azine, 60, 32 Alkynes, perfluoro-, as heterocyclic precursors, 59, 10 Allenes... 

Some variants of these techniques have been described by Melville/ Benington, and Bartlett. " The technique described by Melville involves the ability of II atoms and alkyl free radicals to reduce the blue oxide coat on a polished molybdenum surface. By photometrically studying the change in optical color of the surface it was shown possible to make calculations of the relative concentrations of radicals diffusing through a vapor stream to the metal oxide surface. The mathematical calculations are involved, and the difficulties already described for mirror techniques occur here too. 

Group VIA -Cr, Mo, W. The stable, water-soluble metal alkyl [(H20)5-Cr( y -<7-CH2C6H5)] is obtained by one-electron oxidative additions to [Cr-(H20)J with reactive alkyl halides. The rate-determining, halogen-transfer step generates an alkyl free radical, which then rapidly reacts with a second molecule of Cr(ll) ion or undergoes radical coupling and reduction to alkane ... 

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