CARBANIONS, ELECTRON-TRANSFER VS PROTON CAPTURE .6. DETERMINATION OF THE EFFECT OF ION-PAIRING ON THE ELECTRON-TRANSFER EQUILIBRIUM BETWEEN FLUORADENIDE ION AND NITROBENZENE

Fluoradenide ion in methanol reacts with nitrobenzene in an endoergic electron-transfer equilibrium which is followed by a rate-determining, second-order loss of nitrobenzenide ion. The rate constant for the second process has been determined by directly measuring the decay rate of photochemically produced nitrobenzenide using ESR spectrometry. Combination of KA measurements with rate constants for loss of fluoradenide and loss of nitrobenzenide allows calculation of the effect of reaction variables on the electron-transfer equilibrium constant. This method has been used to show that ion pairing, which can be detected spectroscopically for the carbanion, stabilizes this carbanion more than nitrobenzenide ion, driving the equilibrium toward carbanion. A similar effect is observed for the proton-transfer equilibrium. The phenomenon is explained in terms of competition between ion pairing and hydrogen bonding as a mode of carbanion stabilization in this protic, polar solvent. © 1979, American Chemical Society. All rights reserved.