Polarographic reduction of aldehydes and ketones XX. Electroreduction of benzocyclenones

Abstract

Reduction of indanone (I), tetralone (II) and benzosuberone (III) was studied by d. c. and pulse polarography, cyclic voltammetry and controlled potential electrolysis with a dropping mercury electrode in aqueous solutions containing 2% ethanol at various pH values. In acidic media the protonated form, at higher pH values the free carbonyl form is reduced. The reduction of the unprotonated form is accompanied by protonation of the radical anion formed. The behavior of all three ketones I–III in this pH region is similar and differs only in the half-wave potential values and the rates of protonation of the radical anion. The reduction of the protonated form is affected both by differences in potentials of the ketones I–III and by differences in the rates of protonation, but most significantly by the half-wave potential of the radical formed in the first electron uptake. For indanone (I) this reduction is so positive that only one two-electron step is observed. For benzosuberone (III) the half-wave potential of this reduction is sufficiently more negative than the first electron uptake and two one-electron separated waves result. For tetralone (II) the reduction of the radical occurs at so negative a potential that it is overlapped by the hydrogen evolution current. Only one one-electron wave is thus observed. The effect of ring size on potentials and protonation reactions is discussed from the point of view of ring strain, adsorbability and radical stability.