Radical substitution reactions rates

Rate Constants of Free Radical Substitution Reactions of Flydrogen Atom, Alkyl, and Stannyl Radicals with Peroxides R" + R1OOR1 > ROR1 + R10 ... 

Experimental data on the substitution reactions of free radicals with peroxides were analyzed by the IPM method [64]. The calculated parameters are collected in Table 6.27. The activation energies and the rate constants of radical substitution reactions calculated by the IPM method are presented in Table 6.28. 

The /-butyl group must always be in the equatorial position, and so this determines the position of the smaller bromo substituent. In the cis isomer, the bromo substituent is in the axial position and so is capable of acting as a neighbouring species in the radical substitution reaction. This promotes substitution on the adjacent carbon, and also increases the rate of reaction. In the tram isomer, the bromine is in the equatorial position, and so cannot assist the substitution reaction in this way. 

The rate-determining step of the radical substitution reaction is hydrogen atom abstraction to form a radical. The relative rates of radical formation are benzylic allyl > 3° > 2° > 1° > vinyl methyl. To determine the relative amounts of products obtained from the radical halo-genation of an alkane, both probability and the relative rate at which a particular hydrogen is abstracted must be taken into account. The reactivity-selectivity principle states that the more reactive a species is, the less selective it will be. A... 

The differences in the relative rates of radical formation by a bromine radical are so great that the reactivity factor is vastly more important than the probability factor in determining the relative amounts of products obtained in a radical substitution reaction. 

The rate-determining step of a radical substitution reaction is removal of a hydrogen atom to form an alkyl radical. 

In the luminol-H202-HRP chemiluminescence system, some of the aniline derivatives were functionalized as enhancers, while some were inhibitors of the fluorescence [26,27]. A variety of meta-, para-, and ortlzo-substituted aniline were studied. The structures of some of these aniline derivatives are listed in Scheme 5. The reaction constants with HRP-I and HRP-II were measured, and the Hammett constants (a) of their substituents were determined. The results indicated that the aniline substituents with a less than - 0.27 exhibited inhibition behavior. The aniline derivatives acted as enhancers when a was between - 0.27 and + 0.18. A slight enhancement or inhibition was observed if a is greater than -l- 0.18. The factors of enhancement were determined by the potential of reduction of its aniline radicals, the reaction rates of the aniline with HRP-I and HRP-II, and the maximum concentration of the aniline radicals allowed during the oxidation [28]. 

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

Direct esr evidence for the intermediacy of radical-cations was obtained on flowing solutions of Co(III) acetate and a variety of substituted benzenes and polynuclear aromatics together in glacial acetic acid or trifluoroacetic acid solution . A p value of —2.4 was reported for a series of toluenes but addition of chloride ions, which greatly accelerated the reaction rate, resulted in p falling to —1.35. Only trace quantities of -CH2OAC adducts were obtained and benzyl acetate is the chief product from toluene, in conformity with the equation given above. 

The most rapid reaction is N—N-dimerization (the rates of reactions A, B, C are related as 1 0.15 0.02 [94], Naphthylaminyl radicals recombine with the formation of N—C-dimers only [95], probably because voluminous naphthalene rings sterically hinder N—N-dimerization. A correlation between the rate constant of hyperfine splitting on the nitrogen atom of the aminyl radical and the rate constant of recombination of substituted ( (YC6H4)2N ) diphenyl-aminyl radicals was observed [95],... 

The dependence of relative rates in radical addition reactions on the nucleophilicity of the attacking radical has also been demonstrated by Minisci and coworkers (Table 7)17. The evaluation of relative rate constants was in this case based on the product analysis in reactions, in which substituted alkyl radicals were first generated by oxidative decomposition of diacyl peroxides, then added to a mixture of two alkenes, one of them the diene. The final products were obtained by oxidation of the intermediate allyl radicals to cations which were trapped with methanol. The data for the acrylonitrile-butadiene... 

Based on the data collected in this section, one must conclude that the addition of radicals to dienes is certainly rapid enough to compete against the typical chain-breaking processes and that especially the addition of electrophilic radicals to polyenes appears to bear significant potential. Terminally substituted polyenes are likely to be unsuitable for radical addition reactions due to their lower addition rates and to undesirable side reactions. 

Rate constants for reactions of Bu3SnH with some a-substituted carbon-centered radicals have been determined. These values were obtained by initially calibrating a substituted radical clock on an absolute kinetic scale and then using the clock in competition kinetic studies with Bu3SnH. Radical clocks 24 and 25 were calibrated by kinetic ESR spectroscopy,88 whereas rate constants for clocks 26-31 were measured directly by LFP.19,89 90 For one case, reaction of Bu3SnH with radical 29, a rate constant was measured directly by LFP using the cyclization of 29 as the probe reaction.19... 

The tertiary a-ester (26) and a-cyano (27) radicals react about an order of magnitude less rapidly with Bu3SnH than do tertiary alkyl radicals. On the basis of the results with secondary radicals 28-31, the kinetic effect is unlikely to be due to electronics. The radical clocks 26 and 27 also cyclize considerably less rapidly than a secondary radical counterpart (26 with R = H) or their tertiary alkyl radical analogue (i.e., 26 with R = X = CH3), and the slow cyclization rates for 26 and 27 were ascribed to an enforced planarity in ester- and cyano-substituted radicals that, in the case of tertiary species, results in a steric interaction in the transition states for cyclization.89 It is possible that a steric effect due to an enforced planar tertiary radical center also is involved in the kinetic effect on the tin hydride reaction rate constants. 

Kinetic experiments have been performed on a copper-catalyzed substitution reaction of an alkyl halide, and the reaction rate was found to be first order in the copper salt, the halide, and the Grignard reagent [121]. This was not the case for a silver-catalyzed substitution reaction with a primary bromide, in which the reaction was found to be zero order in Grignard reagents [122]. A radical mechanism might be operative in the case of the silver-catalyzed reaction, whereas a nucleophilic substitution mechanism is suggested in the copper-catalyzed reaction [122]. The same behavior was also observed in the stoichiometric conjugate addition (Sect. 10.2.1) [30]. 

---