Methanol hydroxyl radical reaction

The gamma-radiation-induced oxidation of 2-propanol has been investigated. Acetone and hydrogen peroxide are the principal products and arise via a chain reaction in aqueous acid solutions at high concentrations of 2-propanol. In neutral solutions of 2-propanol and in solutions of methanol and ethanol, no such chain reactions are observed. The reasons for this are discussed along with the implications of the results for the hydroxyl radical yield in water radiolysis. 

Gas-phase oxidation of methane could be enhanced by the addition of a small amount of NO or N02 in the feed gas.1077 Addition of methanol to the CH4-02-N02 mixture results in a further increase in methane reactivity.1078 Photocatalytic conversion of methane to methanol is accomplished in the presence of water and a semiconductor photocatalyst (doped W03) at 94°C and atmospheric pressure.1079 The yield of methanol significantly increased by the addition of H202 consistent with the postulated mechanism that invokes hydroxyl radical as an intermediate in the reaction. 

The above Hammett correlation shows a direct correlation between the number of carbon atoms and the rate constant for alcohols. The kinetic rate constant is observed to increase with the number of carbon atoms on the alcohol chain. The reaction rate between alcohols and hydroxyl radicals can be described as follows methanol < ethanol < propanol < butanol < pentanol < hexanol < heptanol. Heptanol has the fastest reaction constant among all the alcohols and is about 10 times faster than the reference compound of methanol. The reaction pathway for substituted alcohols is shown in Figure 5.21. 

The effects of hydroxyl radical scavenging by methanol were predicted accurately. For more complex compounds and for high concentrations, a detailed knowledge of the reaction mechanism is necessary to obtain good agreement between empirical and modeled results. 

More recently, Barber et al.10 have suggested that methanol can be produced by the reaction of hydroxyl radicals with methyl peroxy radicals... 

The absence of methyl hydroperoxide in the results of Hanst and Calvert38 cannot be considered conclusive and it is very likely that methyl radicals will be oxidized to methyl hydroperoxide, in this system as in other systems (e.g., CH3I photooxidation), where a readily available hydrogen atom is present. Decomposition to give methanol and/or formaldehyde might quickly follow and Hanst and Calvert say that under their conditions formaldehyde would quickly be converted to formic acid. The chain ending steps that they postulate [(96) and (21)] are quite possible, but if one accepts the reaction between hydroxyl radicals and methyl peroxy radicals as put forward for CHSI photooxidation,10 one might equally accept a similar reaction... 

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... 

This scheme of interrelated primary photochemical and subsequent radical reactions is comphcated by the back reaction of hydrogen atoms and hydroxyl radicals with formation of water (Fig. 7-16, reaction 2) or the dimerization of the latter with formation of hydrogen peroxide (Fig. 7-16, reaction 3). Furthermore, hydroxyl radicals are scavenged by hydroperoxyl radicals with formation of oxygen and water (Fig. 7-16, reaction 5) or by hydrogen peroxide to yield hydroperoxyl radicals and water (Fig. 7-16, reaction 4). In addition, hydroxymethyl radicals (HOCH ) formed by reaction 1 (Fig. 7-16) are able to dimerize with formation of 1,2-ethane-diole (Fig. 7-16, reaction 7) or they disproportionate to yield methanol and formaldehyde (Fig. 7-16, reaction 8). 

Alternative sources of acidic species during the oxidation of isotactic polypropylene have been suggested from mass-spectrometric analysis of thermal-decomposition products from polymer hydroperoxides (Commerce et al, 1997). Acetone, acetic acid and methanol comprised 70% of the decomposition products, suggesting either a high extent of oxidation involving secondary hydroperoxides or direct reactions of hydroxyl radicals with ketones (derived through reactions discussed in the next section). 

Table III. Rate Constants for the Reaction of Hydroxyl Radicals with Methanol and Ethyl Alcohol Determined by Competition Kinetics...

The results were different in one important respect from the earlier ones for photolysis of Fe(III) complexes in solution. The Fe(II) yield here showed very little dependence on t-butanol concentration and indicated a primary quantum yield of 0.170, approximately half the extrapolated intercept for methanol of 0.330. In ferric perchlorate photolysis studied earlier, both alcohols gave a common extrapolated yield equal to the independently determined primary yield for hydroxyl radical production. In other words, at the TiOa surface we appear to generate more "OH radical for reaction with CHaOH than "OH radical" to react with t-butanol. From these results, we infer that a hole reaching the anatase surface may produce one of two distinct oxidants in approximately equal quantity. These are a species capable of abstracting hydrogen (e.g. the OH radical) and a second less reactive oxidant. Preliminary results from parallel experiments... 

The authors noticed no C-H/C-D isotope effect for the reaction of 13 with methanol and ferf-butanol, but saw a KIE k Jk = 1.4) for the O-H/O-D bond, suggesting that the stronger O-H bond is activated preferentially over the weaker C-H bonds (Pig. 12). In addition, the authors observed the formation of acetone upon the oxidation of tert-butanol. Upon comparison of rate constants (which have been normalized to account for the amount of hydrogens available for abstraction), tert-butanol reacts 50 times faster than cyclohexane. The authors propose a proton-coupled electron transfer event is responsible for the observed selectivity this complex represents a rare case in which O-H bonds may be homolyzed preferentially to C—H bonds. In further study, 13 was shown to oxidize water to the hydroxyl radical by PCET [95]. Under pseudo-first-order conditions, conversion of 13 to its one-electron reduced state was found to have a second-order dependence on the concentration of water, in stark contrast to the first-order dependence observed for aUphatic hydrocarbons and alcohols. Based on the theimoneutral oxidation of water (2.13 V v. NHE in MeCN under neutral conditions [96]) by 13 (2.14 V V. NHE in MeCN under neutral conditions) and the rate dependence, the authors propose a proton-coupled electron transfer event in which water serves as a base. While the mechanism for O-H bond cleavage of alcohols and water is not well understood in these instances, the capacity to cleave a stronger O-H bond in the presence of much weaker C-H bonds is a tremendous advance in metal-oxo chemistry and represents an exciting avenue for chemoselective substrate activation. 

Jasper AW, Khppenstein SJ, Harding LB, Ruscic B. (2007) Kinetics of the reaction of methyl radical with hydroxyl radical and methanol decomposition. J. Phys. Chem. A. Ill 3932-3950. 

Jodkowski JT, Rayez MT, Rayez JC, Berces T, Dobe S. (1998) Theoretical study of the kinetics of the hydrogen abstraction from methanol. 3. Reaction of methanol hydrogen atom, methyl, and hydroxyl radicals. J. Phys. Chem. A. 103 375(E3765. 

Irradiation of a suspension of anatase in normal primary alcohols (methanol, ethanol or n-butanol), in a nitrogen atmosphere, leads to the formation of Ti " " ions and corresponding aldehyde in solution. The hydroxyl radicals formed are responsible for the reaction [251, 345, 503, 1045, 1401] ... 

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