Transition states solvent effects

Ladanyi B M and Hynes J T 1986 Transition state solvent effects on atom transfer rates in solution J. Am. Ohem. Soc. 108 585-93... 

Separation of Initial and Transition State Solvent Effects... 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

INITIAL STATE AND TRANSITION STATE SOLVENT EFFECTS REACTIONS IN PROTIC AND DIPOLAR APROTIC MEDIA... 

INITIAL STATE AND TRANSITION STATE SOLVENT EFFECTS... 

Cleavage of Pb(C2H5)4 by I2 in polar solvents is slower than that of Pb(CH3>4 [39, 43, 47] and that of Pb(C3H7-n)4 [43]. In nonpolar solvents such as CCI4 and CqHq, a reverse order applies k(Pb(CH3)4)relative reactivities of Pb(C2H5)4 and Pb(CH3)4 are due to transition state solvent effects [43, 52]. Reaction of aqueous iodine solution with Pb(C2H5)4 is reported to be considerably faster than with Pb(CH3)4 [34]. 

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