Equilibria in Liquid-Phase Electron-Transfer Reactions

3 EQUILIBRIA IN LIQUID-PHASE ELECTRON-TRANSFER REACTIONS 

This estimation has been made for a reaction between the anion-radical of tetracyanoquinodi-methide (TCNQ) and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ lida and Akamatu 1967) (TCNQ) + DDQ = TCNQ + (DDQ). The equilibrium constant of the reaction was determined spectrometrically in AN. Having K and y2(DCNQ) as the known values, y2(TCNQ) was determined to be 38 mV. The experimental (polarographic) value was 40 mV. As seen, the calculated and experimental values turned out to be close. 

equilibrium constants of homogeneous electron-transfer reactions between (A) and B are evidently connected with a difference in reduction potentials of A and B. This connection reflects a dehnite physical phenomenon. Namely, if two redox systems are in the same solution, they react with each other until a unitary electric potential is reached. For the transfer of only one electron at room temperature, the equation log K = 2.3 [ i/2(A) - 1/2(6)] 0.059 can be employed. 

In all of the examples considered, 2 of the acceptor was much more negative than that of a donor. In liquid phase, one-electron transfer from a donor to an acceptor can, however, proceed even with an unfavorable difference between these potentials if the system contains the third component—the so-called mediator. The mediator is a substance capable of accepting an electron from a donor and sending it instantly to an acceptor. 

As an example, the reaction of tetrachloromethane with iV,iV,iV, iV -tetramethyl-l,4-phen-ylenediamine (TMPDA) can be discussed. The presence of p-benzoquinone (Q) in the system provokes electron transfer (Sosonkin et al. 1983). Because benzoquinone itself and tetramethyl-p-phenylenediamine interact faintly, the effect is evidently a result of redox catalysis. The following equations reflect such kind of catalysis  

---