Wyniki wyszukiwania - BazTech

Ograniczanie wyników

Znaleziono wyników: 1

Liczba wyników na stronie

Wyniki wyszukiwania

Wyszukiwano:
w słowach kluczowych: kinetyka rozpadu wiązań

Sortuj według:

Ogranicz wyniki do:

Kinetyka rozrywania wiązania w aromatycznych anionorodnikach oraz ich tworzenia

Jaworski J.S.

Wiadomości Chemiczne

1998

[Z] 52, 5-6

367-382

The theoretical model proposed by Savéant [10] for the kinetics of the cleavage of aromatic anion radicals containing potential leaving groups and of their formation from aryl radicals and nucleophiles (essential steps of the SRN1 mechanism, shown in the Scheme 1) is presented. In that model the bond cleavage in anion radicals is viewed as an intramolecular dissociative one electron transfer and the reverse reaction as an associative single-electron transfer. It leads to the quadratic relationship of the activation barrier DG= and the reaction driving force DG0 (eq. (9)), similar to the classic equation of the Marcus theory for the outer-sphere electron transfer. Experimental consequences of the Savéant model are also reviewed. They include the relationship between the cleavage rate constants k4 and the formal potentials of the radical anion formation E0RX?RX for the series of aryl chlorides and bromided in DMF (Fig. 3), solvent effects on the k4 values (Chapter 4) and the effect of substituents (Chapter 5). In particula, it was shown that the solvent effect on the thermodynamic contribution to the activation free-energy causes the increase of the cleavage rate constant for chloroanthracene radical anions with the solvent acceptor number (Fig. 4). However, for the dissociationof the C-Cl bond in radical anions of 4-chlorobenzophenone the solvent effect on the intrinsic activation barrier DG0=, given by eqs (13) and (14), is dominant and the intrinsic rate constant depends (Fig.5) on the solvent Pekar factor (1/eop -1/e0). The use of the Hammett equation to the cleavage rate constants is also discussed; it works in the case when the thermodynamic contribution to the activation barrier (eq. (12)) strongly depends on a substituent (Fig.6). All the reviewed experimental data on the kinetics of the bond cleavage and the formation of radical anions can be rationalized on the basis of the Savéant model.