Rate constant alkoxy radicals

Under these conditions, a component with a low rate constant for propagation for peroxy radicals may be cooxidized at a higher relative rate because a larger fraction of the propagation steps is carried out by the more reactive (less selective) alkoxy and hydroxy radicals produced in reaction 4. 

Various /-alkoxy radicals may be formed by processes analogous to those described for /-butoxy radicals. The data available suggest that their propensities for addition vs abstraction are similar.72 However, rate constants for [3-scission of /-alkoxy radicals show marked dependence on the nature of substituents a to oxygen (Figure 3.6).210 420,421 Polar, steric and thermodynamic factors are all thought to play a part in favoring this trend.393... 

Figure 3.6 Relative rate constants for (3-seission of /-alkoxy radicals at 60 C,4 1...

Along with tertiary hydroperoxide of ether, the BDE of the O—H bonds of alkoxy hydroperoxides are higher than that of similar hydrocarbons. Very valuable data were obtained in experiments on ether oxidation (RiH) in the presence of hydroperoxide (RiOOH). Peroxyl radicals of oxidized ether exchange very rapidly to peroxyl radicals of added hydroperoxide ROOH and only R02 reacts with ether (see Chapter 5). The rate constants of alkylperoxyl radicals with several ethers are presented in Table 7.18. The reactivity of ethers in reactions with peroxyl radicals will be analyzed in next section. 

The rate constants for reaction of Bu3SnH with the primary a-alkoxy radical 24 and the secondary ce-alkoxy radical 29 are in reasonably good agreement. However, one would not expect the primary radical to react less rapidly than the secondary radical. The kinetic ESR method used to calibrate 24 involved a competition method wherein the cyclization reactions competed with diffusion-controlled radical termination reactions, and diffusional rate constants were determined to obtain the absolute rate constants for the clock reactions.88 The LFP calibrations of radical clocks... 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

The chemical details of the reactions of representative alkyl radicals, alkoxy radicals, and biradicals with oxygen should be established. Both the rate constants and the immediate products are needed to construct realistic mechanisms for the model. 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

Small alkylperoxy and alkoxy radicals can decompose uni-molecularly, though their rate constants are often in the second-order region. They abstract hydrogen atoms from alkanes, aldehydes, esters, and acids, add to olefins, and may react with 02. Furthermore, interactions with other radicals can lead to disproportionation or combination. These reactions are reviewed, and particular attention is given to CH 02 and CH30 a number of rate constants are estimated. 

The exact values for the rate constants for hydrogen abstraction by triplet benzophenone are not yet entirely certain. Three groups338-338 have reported a value of 108M-1 sec-1 for abstraction from 2-propanol in concentrated 2-propanol, while the combination of the data of three other groups333,338 339 for dilute benzene solutions yields a value of only 105M-1 sec-1. This discrepancy could well reflect a solvent effect such as that found in studies of the reactivity of alkoxy radicals.340 However, the hundredfold difference between the reported rates for attack of triplet benzophenone on toluene338,338 undoubtedly reflects experimental problems, because both values were measured in aromatic solvents. 

Ratios of Rate Constants for j3-Scission of (k ) and Proton Abstraction from Cyclohexane (A2) by Alkoxy Radicals in Carbon Tetrachloride at 40°... 

Photooxetane formation is quite inefficient, a fact which usually points to the presence of an intermediate which can partially revert to ground state reactants. Cleavage of the diradical must be responsible for some of the inefficiency in oxetane formation 129>. However, in the past few years convincing evidence has appeared that a CT complex precedes the diradical iso.isi). The two most telling pieces of evidence are the relative reactivities of different alkenes 130> and the absence of any measurable secondary deuterium isotope effect on quenching rate constants 131>. Relative quenching rates of sterically un crowded olefins are proportional both to the ionization potentials of the donor olefins 130> and to the reduction potentials of the acceptor ketones 131>, as would be expected for a CT process. Inasmuch as n,n triplets resemble electron-deficient alkoxy radicals, such substituent effects would also be expected on direct radical addition of triplet ketone to olefin. However, radical addition would yield an inverse isotope effect (in, say, 2-butene-2,3-d2) and would be faster to 1,1-dialkylethylenes than to 1,2-dialkylethylenes, in contrast to the actual observations. 

The first step in the pyrolysis of the alkyl nitrates has been supposed to be O N bond fission to give NO2 and an alkoxy radical. The activation energy is 39-5 kcal for methyl nitrate and 39-9 or 34-6 kcaP <> for ethyl nitrate. If the latter value for ethyl nitrate is taken, and assumed to be i)(EtO -NO2) a value for the heat of formation of the ethoxy radical in good agreement with that given by Rebbert and Laidler is obtained, so apparently we may put D(MeO -NO2) =40 kcal, and i)(EtO -NO2) =34 kcal. In the opinion of the present author, however, the mechanism of the reaction is not sufficiently well established to allow this to be done. The discrepancy between the result of Adams and Bawn and that of Phillips 390 is large and may well be because of the different pressure ranges in which these authors worked. A further examination of the effect of pressure on rate constant is necessary before it can be taken as established that the reaction is of the first order. 

For mesitylene and durene, the kinetics have been followed by specular reflectance spectroscopy [17]. The results indicated that mesitylene produces a fairly stable radical cation that dimerizes. That of durene, however, is less stable and loses a proton to form a benzyl radical, which subsequently leads to a diphenylmethane. The stability of the radical cation increases with increasing charge delocalization, blocking of reactive sites, and stabilization by specific functional groups (phenyl, alkoxy, and amino) [18]. The complex reaction mechanisms of radical cations and methods of their investigation have been reviewed in detail [19a]. Fast-scan cyclovoltammetry gave kinetic evidence for the reversible dimerization of the radical cations of thianthrene and the tetramethoxy derivative of it. Rate constants and enthalpy values are reported for this dimerization [19b]. 

Decompositions of substituted bis(cyclopropylacetyl) peroxides 3 in cyclohexane at 60 °C produced the corresponding unrearranged and rearranged radicals which were trapped by l,l,3,3-tetramethylisoindolin-2-oxyl (4) as 2-alkoxy-l,l,3,3-tetramethylisoindolines 5 and 6 in overall yields of approximately 70 %. The alkoxyisoindolines were quantitatively analyzed by reverse-phase HPLC, whieh enabled determination of the rate constants of the trapping reactions and the rearrangements. 

Peroxy radicals dimerise at diffusion rates to tetroxides, which in turn, above 150 K, decompose into alkoxy radicals and oxygen. Low-temperature ESR has established the linear tetroxides as real molecules, with mM to pM dissociation constants at 110-150 The combination is subject to a very large magnetic isotope effect, caused by the coupling to O to the electronic spin, which relaxes some of the spin-symmetry prohibitions on radical recombination a value of knjki of 1.8 is observed for combination of with of R- 0- 0. If, however, the peroxy radical is a to a hydroxyl... 

Gray, P., R. Shaw, and J. C. J. Thynne (1967). The rate constants of alkoxy radical reactions. Prog. React. Kinet. 4, 65-117. 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

Aminyl radicals are less reactive than carbon radicals, which in turn are less reactive than alkoxy radicals. For dialkylaminyl radicals, the reduction rate constant with tributyltin hydride 5 x 10 M s ) [14] is about ten times lower than for primary alkyl radicals [15] and a thousand times lower than for alkoxy radicals [16]. 

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