Reaction, methane + hydroxyl

A slightly more complicated example, yet one which is useful to illustrate a problem which can occur when analyzing isotope effects is the reaction between hydroxyl radical and methane ... 

Hsu, K. J., and W. B. DeMore, Rate Constants and Temperature Dependences for the Reactions of Hydroxyl Radical with Several Halogenated Methanes, Ethanes, and Propanes by Relative Rate Measurements, J. Phys. Chem., 99, 1235-1244 (1995). 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

Whereas several transient species have been observed for dioxygen activation by MMOH, no intermediates were found by rapid-mixing spectroscopic methods for the actual methane hydroxylation step. Mechanistic probes, i.e. certain non-natural substrates that are transformed into rearranged products only if the reaction proceeds via a specific intermediate such as a radical or a cation, give ambivalent results Some studies show that products according to a pathway via cationic intermediates are obtained in sMMO hydroxylations and at least one study suggests the presence of a radical intermediate [40]. Computational analyses of the reaction of MMOHq with methane suggest a so-called radical recoil/rebound mechanism in which MMOHq... 

An additional pathway of similar energy was found to proceed via a concerted oxygen atom insertion [45]. In summary, it appears as if different substrates can be differently activated and the reaction pathway hence proceeds via different intermediates. Clearly, more studies are necessary before the mechanism of methane hydroxylation by MMOH can be fully understood. 

Mueller, C.R., Ignatowski, J. (1960) Kinetics of the reaction of hydroxyl radical with methane and nine chloride- and fluorine-substituted methanes. I. Experimental results, comparisons, and application. J. Chem. Phys. 32, 1430-1434. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

The radical rebound mechanism has been proposed and proved in several cases in reaction of hydroxylation catalyzed by cytochrome 450 and methane monooxigenase (Section 3.2)... 

In microorganisms utilizing methane, a methane-hydroxylating system, metan monooxigenase, (MMO) has been detected, which catalyzed the reaction ... 

Thus the lifetime of a constituent with a first order removal process is equal to the inverse of the first order rate constant for its removal. Taking an example from atmospheric chemistry, the major removal mechanism for many trace gases is reaction with hydroxyl radical, OH. Considering two substances with very different rate constants for this reaction, methane and nitrogen dioxide... 

It is estimated that about 500 million tons of methane are being added to the air each year (Craig and Chou, 1982), largely by anaerobic production in rice paddies and wetlands as well as from the metabolism of ruminant domestic animals and, possibly, African termites (Rasmussen and Khalil, 1981 Zimmerman et d., 1982). This gas is slowly oxidized by reactions with Hydroxyl free radical. Its atmospheric content is around 5 gigatons, indicating that the residence time in the atmosphere is about 10 years. As Figure 12 shows, since 1965 the atmospheric concentration of methane has increased by about 3096. If this rate continues, the methane concentration will have doubled early in the 21st century. 

The dominant sinks for atmospheric methane, accounting for about 90% of its loss from air, are the reaction with hydroxyl radical, OH, (reaction (1))... 

There has been a good deal of study of the polyhalogenated methanes in hydrogen atom abstraction reactions toward hydroxyl (HO ) and chlorine radicals. These reactions are involved in both the atmospheric destruction of such compounds as well as their involvement in ozone depletion. Information is needed about these reactions to model the environmental impact of the compounds. 

Is the ruthenium analogue of compound I of cytochrome P450 an efficient oxidant A theoretical investigation of the methane hydroxylation reaction. J. Am. Chem. Soc. 125, 2291—2300. 

Methane is the most abundant hydrocarbon in the atmosphere. Table 2.9 summarizes the global sources of CH4, which are estimated at 535 Tg(CH4) yr (range 410 to 660) (IPCC, 1995). Of the estimated global annual emissions, 160 Tg(CH4) yr is attributed to natural sources, with the most prominent contribution being emissions from wetlands. Of the estimated 375 Tg(CH4) yr from anthropogenic sources, 100 Tg(CH4) yr comes from fossil fuel combustion, and the remainder from biospheric sources. Methane is removed from the atmosphere through reaction with hydroxyl radicals (OH) in the troposphere, estimated at 445 Tg(CH4) yr, and by reaction in the stratosphere, estimated at 40 Tg(CH4) yr. Microbial uptake in soils contributes an estimated 30 Tg(CH4) yr removal rate. The im-... 

It is important to understand that these values provide a conceptual base for evaluating the environmental behavior of compounds that one does not gather from simply comparing rate constants. However, it must be recognized that assuming a steady-state concentration for the hydroxyl radical is somewhat artificial, since it has been noted that this parameter varies with time and latitude, and so on. In addition, the calculation applies only for the oxidant in question. If other reactions occur, a lifetime calculated for just the hydroxyl radical would overestimate persistence. On the other hand, for a compound such as methane where the reaction with hydroxyl radical is the major reaction, the calculated lifetime would be a realistic estimate of persistence. 

Fig. 1. Arrhenius plot of rate coefficients for the reaction of hydroxyl radical with methane. Symbols are experimentally determined rate coefficients from several different investigations. Solid line is a fit of equation (25) to the data, yielding the equation k (cm molecule Aec ) = 5.7 x 10 T ° exp[—2007(cal)/i 7]. (Reproduced from Smith (1980) with permission of the author.)...

Gierczak, T., Talukdar, R.K., Herndon, S.C., Vaghjiani, G.L., Ravishankara, A.R. Rate coefficients for the reactions of hydroxyl radicals with methane and deuterated methanes. J. Phys. Chem. A 101, 3125-3134 (1997)... 

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