Transition states hydrogen abstraction

S. E. Bradforth, D. W. Arnold, R. B. Metz, A. Weaver, and D. M. Neumark, Spectroscopy of the transition state Hydrogen abstraction reactions of fluorine, J. Phys. Chem. 95 8066... 

According to the Hammond postulate, the transition state for abstraction by chlorine resembles the reactant because this is an exothermic reaction. In contrast, the transition state for abstraction by bromine resembles the product because it is an endothermic reaction (see Figure 21.2). In the case of abstraction by chlorine the carbon-hydrogen bond is only slightly broken in the transition state, and the stability... 

By the Hammond postulate, the transition state for abstraction of a hydrogen by a chlorine atom resembles the reactants and has only a small amount of radical character. Therefore, the transition state leading to a tertiary radical is only slightly more stable than the transition state leading to a primary radical. 

The conformation is a stable, six-membered, hydrogen-bridged, cyclic transition state. Therefore, abstraction of this hydrogen occurs before the other reactions can occur. Under reduced pressure, decarbonylation occurs due to the lower collision rate a lower collision rate means that the excitation energy is not as readily lost, and decarbonylation is greatly favored. 

The alkyl radical-forming step is exothermic for chlorination, endothermic for bromination. Applying Hammond s postulate to these elementary steps, we conclude that alkyl radical character is more highly developed in the transition state for abstraction of hydrogen by a bromine atom than by a chlorine atom. Thus, bromination is more sensitive to the stability of the free-radical intermediate than chlorination and more selective. 

In these transition states the carbon-hydrogen bonds are only about half-broken, so transition states for abstraction of different hydrogens don t differ much... 

The transition states for abstraction of hydrogen from an alkane by chlorine and bromine radicals are very different from each other. In the transition states for abstractions by bromine, the radical on carbon is far more developed than it is in the abstractions by chlorine, which means that differences in the stabilities of different radicals will be far more important in determining the major product of the bromine abstractions than in the chlorine abstractions (Fig. 11.50). 

This proposal, however, has been criticized on the basis of transition state theory (74). Hydroperoxy radicals produced in reaction 23 or 24 readily participate in chain-terminating reactions (eq. 17) and are only weak hydrogen abstractors. When they succeed in abstracting hydrogen, they generate hydrogen peroxide ... 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Substitution, addition, and group transfer reactions can occur intramolecularly. Intramolecular substitution reactions that involve hydrogen abstraction have some important synthetic applications, since they permit functionalization of carbon atoms relatively remote from the initial reaction site. ° The preference for a six-membered cyclic transition state in the hydrogen abstraction step imparts position selectivity to the process ... 

The selectivity observed in most intramolecular functionalizations depends on the preference for a six-membered transition state in the hydrogen-atom abstraction step. Appropriate molecules can be constmcted in which steric or conformational effects dictate a preference for selective abstraction of a hydrogen that is more remote from the reactive radical. 

The yield of 18-oximino compounds from 20jS-nitrites varies from 15 to 36% whereas with a 20oc-nitrite a 60-65% yield is reported. This difference has been ascribed to steric hindrance in the transition state for hydrogen abstraction in the case of the 20j3 derivative. Similar differences... 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

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