Rate constant alkoxy radical with

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. 

Table 14 Rate constants for the reactions of alkoxy radicals with O2 at 298 K.

Since the unimolecular decomposition rate constants for the secondary alkoxy radicals with > 4 carbons, are 10" s (Atkinson and Arey 2003), the reactions of the types (7.23), (7.24) and (7.25) can occur in parallel to give hydroxyl aldehyde, aldehyde with the same carbon number as the reactant alkanes, and aldehydes with one carbon less than the original alkane, respectively. The rate constants of for the isomerization reaction, reaction with O2, and the unimolecular... 

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. 

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... 

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 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]. 

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... 

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]. 

Rate constants for alkoxy radical isomerizations can be combined with rate constants for alkoxy radical decomposition and reaction with O2 to predict the relative importance of the three pathways (Atkinson 1994). Alkoxy radicals can also react with NO and N02, but under ambient tropospheric conditions these reactions are generally of negligible importance. 

Other classes of radicals have also been examined with TRIR methods. The rate constant for P-scission of alkoxy radicals (Scheme 2.6) was found to be enhanced in polar solvents by monitoring the rate of production of the product carbonyl compound [74]. The decarboxlyation of peroxyesters [75], the reactivity of azidyl radicals [76], and chlorine atom abstraction reactions [77] have also been investigated. 

Free-radical additions to alkynes generate vinyl radicals (equation 1) and information about the structure of these intermediates has been obtained either by spectroscopic or chemical means. Vinylic intermediates are generally cr-type radicals (1), in which the unpaired electron is in an orbital with substantial s character. The degree of bending and the inversion barrier depends on the a-substituent. That is, for vinyl (R=H) the rate constant for the inversion lies between 3 x 10 and 3 x 10 s at -180 °C, whereas 1-methylvinyl inverts somewhat more slowly. Electronegative substituents, such as alkoxy, increase the barrier of inversion. ... 

The only kinetic data for the self-reactions of CFCI2CH2O2 and CF2CICH2O2 are the k b8 values measured via pulsed radiolysis by Wallington and Nielsen [93]. The presence of a-hydrogen atoms implies the availability of the molecular channel represented by reaction (30) however, the branching ratio is unknown. The a-hydrogens also imply that there will be a competition between dissociation of the alkoxy radical and its reaction with Oj. Since the extent of this is unknown, the actual self-reaction rate constant cannot be obtained from k b,-... 

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