THE MECHANISM
of anodic substitution reaction such as acetoxylation, cyanation etc of aromatic compounds has been widely elucidated.lA
Reviews on the problem are available.6>s Eberson first systematically proposed that an organic substrate is anodically oxidized with a two-electron withdrawal to form a carbonium ion, to which a nucleophile such as acetate anion attaches to complete the acetoxylation reaction. This mechanism, with slight modifications, is the usual result of researches in this field.
We show here that the first encounter of two reactants, aromatic substrate and nucleophile, occurs at the stage of the formation of the aromatic radical cation but not after the formation of the carbonium ion.
The demonstration was conducted by using p-dimethoxybenzene and 9-phenylanthracene, both of which are anodically oxidized in acetonitrile in two one-electron steps. As the substituting reagent, tetra-ethylammonium bromide and cyanide were used. The reaction medium was acetonitrile containing O-3 M tetrabutylammonium perchlorate and the electrode material was bright platinum. Figure shows the cyclic voltammogram of p-dimethibxybenzene with (curve A) and without. (curve B) the presence of cyanide anion. The cathodic counterpart of the first anodic current peak appears when the solution contains no cyanide but disappears when a sufficient amount of cyanide is present. This indicates that cyanide ion interacts with the cation radical of p-dimethoxybenzene.
A similar result was observed with a mixed solution of 9phenylanthracene and cyanide anion.
In order to confirm that coupling of the nucleophile does occur on to the aromatic cation radical, we have attempted the use of a rotating ring-disk electrode (RRDE).' This provides many advantages in elucidating reaction mechanisms.
Figure represents the usual anodic polarogram of 9-phenylanthracene observed at the disk of RRDE (curve A) and the simultaneous current flowing through the ring (curve B). In obtaining this set of curves, the potential of the disk electrode was changed continually and the ring potential was fixed at O-75 V(sce). A hump-like cathodic current at the ring appears in the potential range of the first anodic wave, indicating that the cation radical of 9-phenylanthracene is relatively stable and is transferred to the ring electrode. On the contrary, the product at the second anodic wave is quite labile and quickly disappears before reaching at the ring. An entirely similar behaviour can be seen with p-dimethoxybenzene (Fig. ).
The present discussion mainly concerns the result shown in Fig. , which was obtained by sweeping the potential of the ring electrode in the cathodic direction and keeping the disk potential at a desired value. The curves in Fig. , which are usual anodic polarograms for 9-phenylanthracene observed with and without bromide anion, are shown for comparison. Evidently, protons liberated from the organic