Hydrogen abstraction: relative rates

The relative rates of reaction of ethane toluene and ethylbenzene with bromine atoms have been measured The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one Arrange these hydrocarbons in order of decreasing reactivity... 

For a given hydrogen donor S—H, replacement by S—D leads to a decreased rate of reduction, relative to nonproductive decay to the ground state." This decreased rate is consistent with a primary isotope effect in the hydrogen abstraction step,... 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

The mechanism of alkyl hydrogen exchange was not clarified, but a possible mechanism was postulated. Partial hydride abstraction by a Lewis acid site may have occured forming a carbocation-like species followed by exchange of a proton at a R-carbon. Such a mechanism predicts exchange to occur preferentially at methyl groups adjacent to the most stable carbocations (benzylic > 3° > 2° > 1°). This is consistent with the observed relative rates of epimerization of steranes during thermal maturation of sediments (83). 

The rate constants of 1-AdS with EtsSiH and f-BuMePhSiH are relatively fast taking into consideration the endothermicity of these reactions. The sila-nethiyl radicals Ph3SiS and t-BuMePhSiS abstract hydrogen with similar rate constants, whereas the more hindered (t-BuO)3SiS and i-Pr3SiS radicals show an enhancement in their reactivity, which suggests that entropic effects may play an important role. 

Table VIII. Absolute (and Relative) Rate Constants for Hydrogen Atom Abstraction from Hydrocarbons at 30°C.

The copolymer composition equation was first applied to co-oxidations in mixtures of aldehydes (25, 39) and later to numerous pairs of hydrocarbons and their derivatives (1, 2, 3, 4, 8, 27, 31, 32, 33). For oxidations of mixtures of A and B, attack by a peroxy radical first gives (by addition or hydrogen abstraction) A and B radicals in the presence of sufficient oxygen all these are then converted to A02 and B02 peroxy radicals. From the relative rates of reaction, A[A]/A[B], of A and B at two or more average feeds [A] / [B], in long kinetic chains, the copolymer composition equation... 

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

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