ABSTRACT
The latter is involved, for example, in the reduction of aromatics to their dihydro derivatives, of quinones to hydroquinones, carbonyls to alcohols. A priori, there are six plausible different possibilities of transferring two electrons and two protons, depending on the exact order of the steps. These six paths involve the ;articipation of seven different intermediates: A 7 , A 2-, AH7 , AH· , AH+, AH~+ ,and AH2. It is thus seen that the discussion of the possible mechanisrn(s) may rapidly become an overwhelming task, particularly when more steps are involved, as in the 4e + 4H+ sequence in Eq. (126). Thus in order to discuss the exact route followed, a convenient representation of the different possible schemes is highly desirable. Such a representation of all possible paths is given by "square-schemes" diagrams [l O I], such as that shown in Scheme 8 for the A/ AH2 reduction. The horizontal
transformations correspond to electron exchanges, whereas the vertical transformations involve a proton transfer.
