Methoxy radicals, abstraction reactions

Methanol is formed from every disproportionation reaction in which the methoxy radical abstracts a hydrogen atom... 

Methoxy radicals can abstract a hydrogen atom, and several such reactions have been studied. Wijnen examined the photolysis of CH3COOCH3 and CH3COOCD3 from 29° to 217° and 145° to 350°C., respectively (39, 41). The rate constant for abstraction of a hydrogen atom from the substrate by methoxy can be deduced from the competition... 

The methoxy radical may subsequently react to form formaldehyde (H atom abstraction) or methanol (H atom addition). The sequence of reactions (R15) through (R17) is strongly chain branching and serves to build up a radical pool. Once this radical pool is established, another chain-branching oxidation route becomes dominating. Methane consumption now occurs mainly by the reactions [254]... 

Methanol can scarcely be chiefly formed from methoxy radicals since these would abstract from hydrogen iodide to give methane and water in the same way as methyl and hydroxyl radicals. Therefore, methanol must be produced from methyl peroxy radicals by the reaction... 

A theoretical analysis of the reaction kinetics was given in the study by Jodkowski et a/.31 A hydrogen-bonded molecular complex was found for the hydroxymethyl reaction channel. The formation of this complex may be a ratedetermining process in the two-step reaction mechanism of the H-abstraction from methyl group. On the other hand, the reaction channel which produces methoxy radicals is a simple metathesis reaction. The profile of the potential energy surface obtained by Jodkowski et al at the G2 level is shown in Fig. 14. 

A novel method for generating semidione radicals has recently been reported by Monroe, Weiner, and Hammond 178) who found that the quantum yield for photoreduction of camphorquinone in 2-propanol was markedly enhanced when benzophenone was added and the solution irradiated at wavelength 3660 A where most of the light was absorbed by benzophenone. Instead of benzpinacol formation, the dione underwent photoreduction. Similar enhancement was not observed with jw-methoxy-acetophenone which does not abstract hydrogen from 2-propanol. The conclusion was that the ketyl radical, formed in the efficient H-abstraction reaction of benzophenone, transferred a hydrogen atom to camphorquinone to generate the semidione radical. It was suggested that this phenomenon be called "chemical sensitization . 

LTA-Ij)" Inversion at C-6 is likely to involve rupture of the Cfs, C(S) bond in a 6j8-oxyl radical to give a stabilised raesomeric aldehyde radical. Ring closure in the reverse stereochemical sense to give the less-hindered 6a-oxyl radical has precedents elsewhere, and can lead to the methylenedioxy-compound (703) by hydrogen abstraction from the suitably-placed 5a-methoxy-group. Similar reaction conditions converted the isomeric 5a-hydroxy-6/9-methoxy-compound into the 5,6-seco keto-aldehyde (704a) and its aldol derivative (704b). ... 

For methoxy radicals the abstraction pathway has been estimated to contribute between 8 and 23% to the total reaction (Wiebe et al, 1973 Barker et al, 1977). Both pathways are termination reactions. 

The kinetics of the reactions of alkoxy radicals have been reviewed and evaluated by Gray et al. [369]. Results for H-abstraction reactions of methoxy radicals are shown in Table 33. It is not possible with alkoxy radicals to monitor the radical concentrations by measuring the rates of formation of the dimers since these are peroxides which are difficult to analyse. The reactions of methoxy radicals with alkanes were studied indirectly by measuring the rates of consumption of the alkanes in competitive experiments involving pairs of reactants [366]. This yielded relative Arrhenius parameters which were put on an absolute basis by... 

Hydrogen atom abstraction reactions of NO3 with phenol, cresols, and methoxy-benzenes are particularly rapid, indicating that this free radical has electrophilic properties. Nitrophenols are among the products of this reaction. Aromatic hydrocarbons, however, do not appear to add -NOj unless they contain two or more fused rings (Atkinson et al., 1987 Atkinson, 1991). 

It can react either with NO or with HO2. The first reaction causes the abstraction of an oxygen atom and produces a methoxy radical, which reacts with oxygen to produce formaldehyde and HO2... 

Atmospheric reactions of MTBE are often initiated by hydroxyl radicals that abstract hydrogen from the methyl groups to form water and organic rascals. There are various kinds of models and mechanisms postulated. The following are two possible initial reactions. For hydrogen radical abstraction at the methoxy end ... 

Two reaction paths for RO-NO2 reactions exist. For methoxy+N02 the fraction of reactions proceeding by abstraction have been estimated from 0.08 to 0.23 (23>2 ). Rate constants for RO-NO2 have been inferred from measured values of the ratio of the rate constants of RO-NO to RO-NO2 reactions. For methoxy radicals, this ratio has been estimated from 1.2 to 2.7. (23,25.) Thus, the uncertainty in the RO-NO rate constant is compounded by the uncertainty in the RO-NO to RO-NO2 ratio. Uncertainties in RO-NO2 rate constants are probably at least a factor of foiir. 

The chemistry is clearly complex most pertinently, a methyl radical can be oxidized by flame species to formaldehyde either directly or via a methoxy radical intermediate (Eqns (7)—(9)). Subsequent hydrogen atom abstraction yields formyl radical (Eqn (10)) once collisionaUy stabilized, formyl radical can form CO (Eqn (11)) and, presuming complete combustion, it ultimately reacts to form CO2 (Eqn (13)). The methyl radical intermediate, alternatively, can combine with another methyl radical to form ethane (Eqn (12)), which can then yield ethyl radicals by chemically activated C—H bond fission or via H atom abstraction, opening up a wide variety of side reactions. (In unsaturated systems common to HC fuel combustion, another common initiation step involves addition of a reactive flame species to a double bond to yield a radical center.)... 

The hydrogen abstraction from alkoxyamidyl 102 has been modeled at B3LYP/ 6-31G level by the reaction of methyl radical (R = CH3 ), with methoxy-formamidyl 102e giving 103e and methane. Energies are presented in Table 13. 

Among the initiation methods available, photochemistry has already been used successfully for Pschorr reactions [122]. A new study, from which one example is shown in Scheme 24, revealed a dramatic influence of the substituents on the 2-bromobenzophenone 66. Methoxy groups in the radical ring led to a highly electrophilic cr-aryl radical which proved unreactive for hydrogen abstraction and cyclization. Substituents in the non-radical ring that stabilize the cyclohexadienyl... 

Another substitution reaction that is initiated by photochemical hydrogen abstraction is the replacement of the bromine atom in 2-bromo-8-methoxy-l, 4-naphthoquinone by an acyl group757. Irradiation of a solution in benzene of the quinone, butyraldehyde or capraldehyde and pyridine yields mixtures of acylated quinone and acylated hydro-quinone. In the first step, the excited quinone abstracts the aldehyde hydrogen atom and this is followed by bond formation between the acyl radical and C-2 of the quinone. The radical that is formed after departure of a bromine atom may either lose a hydrogen atom and yield acylated quinone or take up a hydrogen atom and become acylated hydro-quinone. 

This chapter discusses an entirely different approach to the generation and investigation of highly reactive transient intermediates. The high reactivity is usually due to an unusual electron distribution in the intermediate that was acquired in the course of the chemical reaction. This implies that for an electron rich intermediate there is a corresponding stable cation in which the electron density was lowered by ionization. Likewise, for an electron-deficient intermediate there is a corresponding stable anion in which the electron deficiency was alleviated by electron attachment. Equations (1) and (2) show simple examples of the methoxy and hydroxymethyl radicals, respectively, which are isomeric transient intermediates of hydrogen atom abstraction from methanol ... 

---