Relative Rates of Analogous Radical Reactions

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

Decomposition of 14 in styrene produced very similar products. About the same amount of fert-butylperacetate was formed in both styrene and cumene indicating that the scission reaction of 15 is much faster than its rate of addition to the styrene double bond. This is surprising since the analogous addition of tert-butoxy radical to styrene is very fest relative to the analogous scission reaction to form acetone and methyl radicals. Again, this can be explained by the relative stability of alkyl radicals. 

For radical-radical reactions, the full mode coupling and anharmonicity effects for the relative and overall rotational motions must be explicitly accounted for. We have derived a direct variable reaction coordinate transition state theory approach that appears to 3deld accurate rate coefficients for a number of alkyl radical reactions.This approach is analogous to that embodied in Eq. (4.10) for the long-range transition state, but includes variational optimizations of the form of the reaction coordinate and does not make the large orbital moment of inertia assumption. A detailed description of this approach was provided in some of our recent articles. 

The absolute rate constants for a variety of cyclizafions have been measured. In particular, the rates of decarbonylafion of a variety of alkoxycarbonyl radicals have been obtained by LFP studies on PTOC oxalates." From these data, rate constants for the reduction of alkoxycarbonyl radicals with BusSnH and their 5-exo cyclizafions were determined. Whereas cyclizations were slightly faster than the analogous alkyl radical 5-exo cyclizations, their reactions with BusSnH were 10 times slower, indicating that cyclization processes should be synthetically useful. The rate constants for the cyclization of a number of variously substituted a-amide radicals have been determined together with their relative reactivities towards reduction using BusSnH (Scheme 16). Cyclizations of secondary-based radicals were found to be similar to the corresponding alkyl-substituted radicals. In addition, the rate constants were subject to minor electronic... 

Therefore, in analogy with methane system, in the course of experiments as Cl-atom reacts with chloro or dichloro-methane, there is a potential for formation of HO-radicals. Hence in the initial stages of the reaction at lower rates of methane conversion we expect that HO will primarily attack benzene. As the reaction proceeds, oxidation products of CH4 and benzene increase in concentration and compete for the HO radicals. Thus, we expect to observe a curvature in relative rate plot of benzene at longer irradiation times, as shown in Figure 13.6. In FTIR studies, for the case of experiments in nitrogen diluent, curvature was observed but substantially lower concentrations of benzene were consumed. It is... 

There is an interesting exception to this observation. As noted above, aldehyde oxidations tend to be very fast and to have relatively long kinetic chain lengths. Most chain terminations occur via bimolecular reactions of acylperoxy radicals (eq. (7a)) these reactions result in carbon dioxide generation and are inefficient. If one adds manganese catalyst, some of the acylperoxy radicals will be reduced to peroxy acid and Mn " will be produced. Mn can carry the chain via an analog of reaction (18), but does not participate in chain termination reactions. As a result, kinetic chain lengths and rates tend to increase. 

The photochemistry of 4-chloroanilines in methanol, dioxane-water and diox-ane-methanol solvents has been investigated for more than thirty years by Latowski185,186. Large quantum yields of HC1 formation (hci) have been observed for the photolysis of 91a in protic solvents (e.g. Hci = 0.78 in methanol at 254 nm). However, the values of 4>hx are relatively small for 4-bromoaniline (HBt = 0.19), 4-iodoaniline (cbm = 0.29), 2-chloroaniline (hci < 0.02) and 3-chloroaniline (hci = 0.02) under the same condition. N-Acetylation of 91a to 4-chloroacetanilide also inhibits the photolytic process. In conjunction with the solvent- and concentration-dependent photolysis rates of 91a, these results indicate an electron-transfer mechanism for the photochemical reaction electron transfer occurred from an excited 91a to an unexcited 91a molecule, followed by ionization reactions. However, recent analysis of photoproducts from 91a in water/methanol mixtures has shown that benzidine (92) is a major product along with aniline (equation 29)187. As a result, a carbene mechanism that leads to the formation of aniline radicals was put forward in analogy to the photochemistry of 4-halophenols188,189. For example, the photolysis of 91a in aqueous solution first results in the transient species carbene 93 followed by the formation of the aniline radical 94 that was observed as the primary product (Scheme 13)190. In addition to la and 92, other identified secondary products include 4-aminodiphenylamine, 2-aminodiphenylamine, hydrazobenzene, 4-chloronitrosobenzene and 4-chloronitrobenzene, but they are all in low yields191. 

Following the pattern of H-abstraction reactions of the lower alkyl radicals, methyl to propyl, the analogous reactions of butyl radicals have been studied relative to the radical combination reactions. Unfortunately, the values of the rate coefficients of these combination reactions are by no means certain. Recent measurements of the combination reaction of... 

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. 

To establish the effect of structure on the ratio of recombination and disproportionation, the knowledge of reactions of low-molecular analogs may be use. It may be deduced from the ratios of rate constants of disproportionation and combination determined for different types of alkyl radicals that primary alkyl radicals enter combination preferably while tertiary radicals dispoiporti[Pg.153]

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