Quinoline, radical anion, dimerization

With radical ions, the dimerization equilibrium is strongly influenced by the solvation and association of radical ions with counter ions. It has been shown that the free ions dimerize much more slowly than do the respective contact ion pairs e.g., the quinoline radical anion does not dimerize in the powerfully solvating hexamethylphosphoramide, but it does dimerize rapidly in tetra-hydrofuran (160). Thus, two equilibria should be distinguished (160), viz. 

The equilibrium between radical-anion and dimer for pyridine and quinoline has been examined in a number of aprotic solvents. Radical-anions of pyridine dimer-ise rapidly in liquid ammonia in tire presence of alkali metal ions [15] In hex-amethylphosphoramide with alkali metal counter ions, the monomer is detectable in an equlibrium concentration [16], The monomeric species can be stabilised by substituents and 2- or 4-cyanopyridines give radical-anions which persist in liquid ammonia while 3-cyanopyridine radical-anion dimerises with a rate constant of 2 x 10 [17], Quinoline radical-anion is stable in hexamelhylphosphoramide [16] but in liquid ammonia it dimerises irreversibly [18]. 

Crooks, R. M. and Bard, A. J. (1988b) Electrochemistry in near-critical and supercritical fluids. Part V. The dimerization of quinoline and acridine radical anions in ammonia from 70°C to 150°C. J. Electroanal. Chem. 240, 253-279. 

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