Polarographic Reduction of Benzoate in Acet,pnitrile

The polarographic wave observed for acetonitrile solutions of benzoic acid is due to reduction of the acid proton: the apparent diffusion current constant for the wave increases with concentration of benzoic acid as expected for the reduction of the acidic proton of a slightly dissociated acid. No wave was observed for ethyl benzoate. The situation in respect to the electrochemical reduction of benzoic acid and the benzoate functionality at the dropping mercury electrode is not entirely clear. The reduction wave observed for benzoic acid in unbuffered aqueous solution would seem to be due to the reduction of hydrogen ion; the acid is not reduced in buffered aqueous media [1]. On the other hand, there seems to be some uncertainty in the literature (d. below) due to the work of Stackelberg and Stracke [2], who apparently presented evidence for the reduction of benzoic acid in 75"/0 dioxane, which was 0.05 Min Et 4NBr. In the latter medium, benzoic acid itself gave two waves (Table 1), tetraethylammonium benzoate also gave two waves, but ethyl benzoate gave only one wave. Stackelberg and Stracke wanted to assign the first benzoic acid wave to a hydrogen-ion reduction but were deterred by the presence of the wave in the tetraethylammonium salt. The possible presence of free benzoic acid in the salt and ester or its formation during test solution preparation was apparently not considered. In earlier work, Wawzonek and co-workers [3] had referred to the reduction of ethyl benzoate in 50"/0 dioxane solution (0.1 Min n-Bu4NI) (Et/2 ;= -2.18 v . vs , S. C. E.; i dI C = 7.62) and had implied that benzoic acid might be reduced (E l / 2 = -1.87 v . vs , S.C.E.; idlC = 2.12). The implied polarographic reduction of the benzoate functionality, apparently based on the work of Stackelberg and Stracke, is indicated in several standard works, e.g., references [4], [5] and [6]. The first • Present address: Ritsumeikan University, Kyoto, Japan RECEIVED: August 7, 1966