Methyl radical, half-life

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

The enyne-allene 12 having a methyl substituent at the allenic terminus was likewise prepared from the corresponding enediynyl propargylic alcohol 11 (Scheme 20.4). The presence of a methyl group accelerates the rate of cyclization by approximately sixfold and 12 cyclizes with a half-life of -3.6 min at 78 °C. The formation of a more stable secondary benzylic radical is apparently responsible for the rate enhancement. 

With the presence of two methyl substituents at the allenic terminus of 20a, the a,3-didehydrotoluene biradical 21 having a tertiary benzylic radical center was generated after cycloaromatization (Eq. 20.1). As a result, the half-life of the reaction is only -70 min at 37 °C, which is significantly shorter than that of 8. 

Photolytic. Based on data for structurally similar compounds, acenaphthylene may undergo photolysis to yield quinones (U.S. EPA, 1985). In a toluene solution, irradiation of acenaphthylene at various temperatures and concentrations all resulted in the formation of dimers. In water, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, 1,2-epoxyacenaphthylene, and 1-naphthoic acid. In methanol, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, methyl 8-formyl-1-naphthoate, and dimethoxyacetal 1,8-naphthalene dialdehyde (Chen et al., 1979). Acenaphthylene reacts with photochemically produced OH radicals and ozone in the atmosphere. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with OH radicals (500,000/cm ) at 25 °C are 8.44 x lO " cmVmolecule-sec and 5 h, respectively. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with ozone at 25 °C are... 

Photolytic. Grosjean (1997) reported an atmospheric rate constant of 1.25 x lO" cmVmolecule-sec at 298 K for the reaction of methyl cellosolve and OH radicals. Based on an atmospheric OH concentration of 1.0 x 10 molecule/cm , the reported half-life of methyl cellosolve is 0.64 d (Grosjean, 1997). 

Chemical/Physical. Anticipated products from the reaction of methyl iodide with ozone or OH radicals in the atmosphere are formaldehyde, iodoformaldehyde, carbon monoxide, and iodine radicals (Cupitt, 1980). With OH radicals, CH2, methyl radical, HOI and water are possible reaction products (Brown et al., 1990). The estimated half-life of methyl iodide in the atmosphere, based on a measured rate constant for the vapor phase reaction with OH radicals, ranges from 535 h to 32 wk (Garraway and Donovan, 1979). 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

The enormous effect of perchlorophenyl groups on the stability of arylmethyl radicals has been well documented by a series of reports by Ballester. Thus, perchlorotriphenylmethyl has been shown to have a half-life on the order of 100 years in solution at room temperature in contact with air. It has therefore been termed an inert free radical. Even perchlorodiphenyl(chloro)methyl has been shown to be stable.The perchlorophenyl group may be expected to exert a similar stabilizing effect on triplet DPCs. 

This experiment showed that some volatile component was formed in the thermal decomposition of tetramethyllead and that this compound consumed a cold lead mirror with formation of a volatile product. If, instead, a zinc mirror was first deposited and allowed to be consumed by the volatile product from decomposition of tetramethyllead, dimethylzinc could be identified as the product. Paneth concluded that free methyl radical was formed in the thermal reaction and could determine its half-life to be 0.006 seconds under the reaction conditions employed. Also, free ethyl radicals could be formed in... 

The ESR spectra of polyisobutylene after irradiation with ultraviolet light (6) are different from those obtained after irradiation with ionizing radiation. The spectra consists mainly of two components one, a sharp quartet which has a half life of 1% hours at liquid nitrogen temperature, has been attributed to free methyl radicals (XI), in analogy with ultraviolet-irradiated polypropylene (51). The broad component is composed of many superimposed lines and was interpreted as caused by three different radicals, all stable at liquid nitrogen temperature. One of these radicals (XV) is the counterpart to the methyl radical (XI) while the others are the two radicals (XIII and XVI) which can both be formed by hydrogen abstraction. 

The thermal decay process of the radicals is a bimolecular reaction. The decay rate increases with increasing temperature. At 100° C, a half-life of 12 minutes has been observed for methyl methacrylate popcorn radicals. A fast decay rate takes place when the dry popcorn is swollen in a liquid such as benzene. That means that during the proliferous growth process, when the polymer is swollen by the monomer, a decay process also occurs, and a stationary radical concentration in the growing polymer popcorn results. A liquid that does not swell the polymer (for example, methanol for polystyrene) does not influence the decay rate. A much higher rate of radical decay is obtained with a benzene solution of diphenylpicrylhydrazil. The reaction rate between the polymer radical and inhibitor radical may be measured. 

Since the styrene-methyl acrylate and styrene-methyl methacrylate copolymers could be prepared in toluene solution at 25°C., it became possible to investigate the influence of free radicals on the ethylaluminum sesquihalide complexed system. 2-Methylpentanoyl peroxide (Lucidol R-226), having a half-life of 10 hours at 27 °C. in 0.2M solution in mineral spirits was used as the free radical source at 25 °C. 

Both Katayama (20) and Metz (24) have reported the observation of a radical anion of extremely short half-life in pulsed, dry a-methyl-styrene. This species is extremely sensitive to water and oxygen. 

Effect of Peroxides. In addition to benzoyl peroxide, lauryl-, acetyl-, 2,4-dichlorobenzoyl-, and methyl ethyl peroxide, and tert-hvXy hydroperoxide were studied and gave satisfactory results. The effectiveness of the peroxide is relatively independent of the half-life of the peroxide (Table X). By contrast, the catalyst AIBN is much less satisfactory, as found for methylvinylpyridine and acrylonitrile. The difference between these peroxides and AIBN suggests that the AFR polymer is not formed by a simple uncatalyzed free radical system which would give a graft polymer or a simple mixture of polypropylene and polyacrylate. It is well known that for the polymerization of acrylates AIBN is at least as good if not better than peroxide in initiating the free radical reaction (2). 

Procarbazine is rapidly and completely absorbed following oral administration. It readily decomposes by chemical and metabolic routes, with a half life of 7 to 10 minutes, to produce highly reactive. species including methyl diazonium ion, methyl radicals, hydrogen peroxide, formaldehyde, and hydroxyl radicals. ... 

MIBK has a short half-life in the atmosphere and is also biodegraded in water. It is not expected to bioaccumulate. Based on an experimental vapor pressure of 19.9 mmHg at 25°C, MIBK is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase MIBK is degraded in the atmosphere by reaction with photochemically produced hydroxyl radicals with an estimated atmospheric half-life of 27h. Methyl isobutyl ketone is expected to have high mobility in soils based upon an estimated Kqc value of 123. Volatilization from dry soil surfaces is expected based upon the vapor pressure of this compound. 

The initiation of the radical polymerization of reactive unsaturated monomers, e.g. methyl methacrylate, styrene and vinyl chloride, generally requires the use of a catalyst with a half life of 3-10 hrs at the reaction temperature. The polymerization may be carried out in bulk or in dilute or concentrated solutions. 

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