Oxidation potential as a measure of the reactivity of anionic nucleophiles. Behaviour of different classes of nucleophiles

Abstract

Anodic peak potentials for 42 anionic nucleophiles are reported along with the rate constants for the reactions of the anions with benzyl chloride. By comparing these rate constants with those of the reactions of outer-sphere electron-transfer reagents (radical anions), it has been demonstrated that the reactions of the anions with benzyl chloride are typical single-step S_N2 reactions as opposed to a twostep process comprising dissociative single electron transfer (SET) to benzyl chloride followed by radical combination. The data suggest that very electron-rich nucleophilic anions with potentials of –1.5 to –2.2 V (and more negative) with respect to the ferrocenium/ferrocene couple might participate in a SET reaction. The 42 anions studied included representatives of four classes of nucleophiles, viz. sulfur-, carbon-, oxygen- and nitrogen-centred anions. The previously observed correlation of rate constants and anodic peak potential was again found for these anions with the most reactive species having the most negative peak potentials. Correlations by class indicate that the sensitivity of the rate constants to changes in peak potential is greatest (and equivalent) for the sulfur-, carbon- and oxygen-anions and for representatives with the same peak potential, the rate constants by class follow the order sulfur > carbon > oxygen. The nitrogen nucleophiles show a significantly lower sensitivity of rate constant to changes in peak potential than do the other three classes.