Hydrogen bonding with peroxy radicals

For methyl ethyl ketone—water and hydrocarbon—alcohol mixtures, the rate coefficients kp ti) and kUetr) determined were found to decrease with increasing concentration of H20 (R OH) in the system. The results 

The measured fep(e f) and ftt(e f) represent combinations of relevant elementary rate coefficients, viz. 

The individual rate coefficients and equilibrium constants found are collected in Table 24. 

The formation of hydrogen bonds in 2-methylpentene-2 oxidation makes the peroxy radicals more reactive (Table 24). These radicals are usually of a low reactivity due to their intramolecular 7r-bonds. It is suggested that the latter are broken by the formation of hydrogen bonds, the radical activity thus increasing, viz. 

A hydrogen bond usually lowers the peroxy radical activity since the approach of the peroxide group and the molecule is hindered. Hydroxy-peroxy radicals formed during the oxidation of alcohols apparently possess intramolecular hydrogen bonds these lower the reactivity of such radicals. 

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The effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

The formation of hydrogen bonds between polar molecules and peroxy radicals permits a new interpretation of reactions involving R02 with molecules having O—H and N—H bonds. The peroxy radical first forms a... 

The stoichiometric factors of inhibition and the rate constants of the ter-penephenols (TP) with isobornyl and isocamphyl substituents were determined by the reaction with peroxy radicals of ethylbenzene. The reactivity was found to decrease for o-alkoxy compared with o-alkyl substituent caused by the intramolecular hydrogen bond formation that is conformed by FTIR-spectroscopy. The inhibitory activity for mixtures of terpene-phenols with 2,6-di-ferf-butyl phenols in the initiated oxidation of ethylbenzene was also studied. In spite of the similar antiradical activities of terpenephenols with isobornyl and isocamphyl sunstituents, the reactivity of phenoxyl radicals formed from them are substantially different that is resulted from the kinetic data for mixtures of terpenephenols with steri-cally hindered phenols. 

The mechanism of O2 insertion into the Pd-C bond of 24 differs from the autoxidation of organic substrates due to the ability of Pd to attain high oxidation state intermediates. In hydrocarbon autoxidation, peroxy radical abstracts hydrogen without the intermediacy of hypervalent intermediates. In the oxidation of 24, the coordination number of Pd is proposed to increase from four to five upon reaction of Pd(II) complex 24 with peroxy radical (26). 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

With the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains in forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical (Reaction 4). [The possibility of such an intramolecular attack has been demonstrated in an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions (9)]. 

Solvent effects on the reaction of 0—H bonds and carbon radicals could, at least partly, be accounted for by considering initial and final state solvation. Since hydrogen-bond formation with an electron donor (but not proton donor) solvent can exist in the initial state, may exist in the transition state but cannot exist in the final state, a rate-retarding effect of hydrogen bonding is to be expected. In reactions of 0—H bonds with alkoxy or peroxy radicals, however, the solvent effect may be an indication for the existence of a long-lived intermediate. 

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