Methyl bromide, photolysis

Kobrinsky, P.C. and Martin, M.R. High-energy methyl radicals the photolysis of methyl bromide at 1850 A, / Chem. Phys., 48(12) 5728-5729, 1968. 

For some pesticide compounds, such as dini-troaniline herbicides (Weber, 1990), phototransformation occurs primarily in the vapor phase, rather than in the dissolved or sorbed phases. Perhaps the most environmentally significant pesticide phototransformation in the atmosphere, however, is the photolysis of the fumigant methyl bromide, since the bromine radicals created by this reaction are 50 times more efficient than chlorine radicals in destroying stratospheric ozone (Jeffers and Wolfe, 1996). Detailed summaries of the rates and pathways of phototransformation of pesticides and other organic compounds in natural systems, and discussions of the physical and chemical factors that influence these reactions, have been presented elsewhere (e.g., Zepp et al, 1984 Mill and Mabey, 1985 Harris, 1990b). 

Secondary reactions then follow involving these and subsequently formed radicals. The nature of the species formed in the primary photolytic act has been inferred from the product distribution and other kinetic evidence, and also from observations of transient spectra during flash photolysis studies. For example, spectra characteristic of CIO, BrO, and 10 have been observed following the flash photolysis of methyl chloride , methyl bromide and methyl iodide in the presence of oxygen, and the absorption spectrum of the free methyl radical has been ob-served during the flash photolysis of methyl iodide and methyl bromide. 

As a soil fumigant methyl bromide leaves no toxic residue in soils. The volatile gas rises into the atmosphere. Methyl bromide is an ozone-depleting substance. Although methyl bromide is very soluble in water, its high vapor pressure in various soil types indicates a low tendency to adsorb to soils and rapid evaporation. Methyl bromide has a half-life in air estimated from 0.3 to 1.6 years. Degradation is primarily due to photolysis. In soils, the half-life is 0.2-0.5 days. In water, a half-life of 3 h was calculated. 

PROBABLE FATE photolysis volatilized methyl bromide should photodissociate above the ocean layer, probably not significant in aquatic systems, reaction with photochemi-cally produced hydroxyl radicals has a half-life from 0.29-1.6 yrs, direct photolysis is the dominant fate in the stratosphere, but is not expected to be important in the troposphere oxidation atmospheric photooxidation by hydroxyl radicals releases inorganic bromide which is carried... 

The absorption spectrum of the methyl radical has been observed by Herzberg11 during the flash photolysis of methyl iodide and bromide, but... 

The photolytic and thermolytic decomposition of emetine have been studied. Photolysis has been shown to involve oxidation and fragmentation, whilst thermolysis involves only oxidation. Compounds identified were emetamine, O-methylpsychotrine, l, 2 -didehydroemetine, tetradehydroemetinium bromide, rubremetinium bromide, 6,7-dimethoxy-3,4-dihydroisoquinoline, 6,7-dimethoxy-l-methyl-3,4-dihydroisoquinoline, 3-ethyl-9,10-dimethoxy-l,4-dihydrobenzo[a]quinolizinium chloride, its 2-methyl-derivative, and the dimerized compound (112).141... 

The photolysis of (Me3Si)3SiPh in the presence of functional substituted olefins is of considerable interest (55). Irradiation of 20 with a low-pressure mercury lamp in the presence of vinyl chloride or l-bromo-2-methyl-propene affords the respective 1-alkenyl-1-halo-1-phenyltrimethyldisilanes as the sole volatile product. The fact that the reaction of trimethylsilylphenylsilylene with butyl bromide does not give any volatile products suggests that compound 23 and 24 must come from a 1,2-halogen shift of... 

The original o-nitrobenzyl bromide linker 1.19 (76) attached to aminomethyl PS resin has been used for both peptide and small organic molecule SPS. Cleavage by photolysis at 350 nm under anaerobic conditions gives carboxylic acids the insertion of an a-methyl allowed an easier cleavage (77). The linker 1.19 has also been prepared... 

Etzler and Rollefson reported carbon monoxide and methyl halide to be the major products from the continuous photolysis of acetyl bromide, iodide and chloride in the vapour phase. Similarly halogenated alkanes were found as major products from chlorodifluoroacetyl fluoride and from other polyfluoroacyl fluorides, chlorides and bromides . The mechanisms of these reactions are not clear. 

TABLE 20. Photolysis (x 224-194 nm) of methyl amine in the gas phase (226) (100 torr, room temperature). Quantum yields are based on the hydrogen bromide actinometer. 

Aryltelluroformates (533), accessible from the corresponding chloroformates, undergo cleavage of the acyl-tellurium bond on photolysis. In the presence of diphenyldiselenide, the oxyacyl radical (534) is trapped as the corresponding alkyl (phenylseleno)formate (535) in excellent yield. Irradiation of aryl bromides or iodides (536) with potassium tellurocyanate (537) in dimethyl sulfoxide, as both solvent and methyl source, yields aryl methyltellurides (538) in modest yields (9-34%). ... 

Photolysis of diethylthallium bromide in cyclohexane is a radical process involving cleavage of the thallium-carbon bond, which yields ethylcyclohexane and dicyclohexyl, as well as other products. Photoelectron transfer from benzyltributylstannanes to 10-methylacridinium ion results in cleavage of the metal-carbon bond, to give the corresponding benzyl radicals, rather than benzyl cations. Photochemical homolysis of Re- and Ru-alkyl bonds in Re(alkyl)-(CO)3(diimine) and Ru(I)(alkyl)(CO)2(diimine) complexes has been studied by Fourier transform ESR. In related manganese complexes, Mn(R)(CO)3(di-imine), elimination of CO is the predominant pathway when R = methyl, but Mn-alkyl homolysis occurs when R = benzyl. 

Photolysis of l-methylnaphtho[l,8-rfe]-l,2,3-triazine (23) afforded the biradical 24 which could be trapped by hydrogen, benzene or vinyl bromide.7 Photolysis of the 2-methyl isomer 25 gave the triaziridine 26 in a two-photon laser process at 10-85 K. Short-wave photolysis of 25 yielded 26 and naphtho[l,8-af]pyrazole (27).339... 

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