Variation of N-Substituents in ,-Bipyridyl

N -Disubstituted Bipyridyls and Dipyridyl Ethenes 2.1 Variation of N-Substituents in 4,4 -Bipyridyl 

The redox system i (R = alkyl Iqx Viologenes was the first to be interpreted correctly (E. Weitz ). It is especially well suited for studying the effects of N-substituents because steric effects are virtually absent. In spite of the great importance of some of these quaternary salts as universal herbicides (R = CH3, paraquat only potentials Ej were known for a long period of time, since the reductions SEM/RED in aqueous medium are mostly irreversible In aprotic media, however, as in acetonitrile and DMF, E2 and Ei are ideally reversibleThis holds true for most of the investi ted substituents as can be seen from Table 1. 

The different substituents in la - lx act in the expected way with R = CH3 (If) as reference system electron donating substituents make reduction of 1 more difficult whereas electron attracting substituents hinder oxidation. For this reason la - If, Ih - Ip and Ir are synthesized on the level OX whereas Ig, Iq and is - lx can be isolated only as form RED . 

Substituent effects on redox potentials can be rationalized by free energy relationships in many cases Although 1 can be related to aromatic compounds, a linear correlation with Hammett s a-constants fails. As can be seen from the substituents -N(CH3)2 Ik) and -OCH3 11) the electron attracting effect is the dominating one and not the resonance effect. Therefore Taft s a -constants describe the substituent effects more correctly. According to (1) 

(1) unknown a can be estimated as to be seen from Fig. 1 (a of SiMes uncertain because of irreversible potentials). The deviation of the charged substi- 

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