Methanol rate constant with hydroxyl radical

For substituted alcohols reacting with hydroxyl radicals, the least substituted alcohol, methanol, was used as a reference compound for Hammett correlation analysis. The o values were taken from Hansch et al. (1995) and rate constants were taken from the U.S. Department of Commerce (USDOC, 1977). These values are believed to be the most accurate rate constants in... 

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. 

Rate constants for methanol and ethyl alcohol relative to those for benzoate ion, phenylacetate ion and p-nitrobenzoate ion are shown in Table III. Each value in the table consists of experiments at five separate concentration ratios. The random uncertainty in each value is less than 10%. In determining these rate constants from optical density ratios it was necessary to make a small correction for the contribution to the optical density by the H-adduct free radical. The molar extinction coefficients at 340-350 m/x for the H-adduct and OH-adduct are similar for benzoic acid (22) and were assumed to be comparable for the other two aromatic ions in the table. The correction is necessary since the rate constants for the reaction of hydrogen atoms with the alcohols used are two orders of magnitude lower than the rate constants for hydrogen atom addition to the aromatic ring, while the analogous hydroxyl rate constants are roughly comparable. 

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

Figure 8. Aqueous solution versus gas phase ratio of rate constants for addition of H and Mu atoms to benzene. Open symbols apply to solutions using tert butanol, crossed squares to methanol as hydroxyl radical scavengers. The brokoi line gives the e q)ected values based on the poative free energy of solvation of hydrogen atoms in water, the solid line involves a further correction ([28], reprinted with permission).

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. 

GreenhUl, P.G., and B.V. O Grady (1986), The rate constant of the reaction of hydroxyl radicals with methanol, ethanol and (D3) methanol, Aust. J. Chem., 39, 1775-1787. 

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