Nucleophilic character of the alkyl radicals. 19. 1a Absolute rate constants in the homolytic alkylation of protonated heteroaromatic bases by n-butyl and tert-butyl radicals

The rate constants for the homolytic alkylation of protonated heteroaromatic bases (quinoline, pyridine, and 4-cyano-, 4-acetyl-, 4-methyl-, and 4-methoxypyridine) by n-butyl and tert-butyl radicals were measured at 57 °C by competition of the aromatic addition with the alkyl radical oxidation by Cu2+ salts (for which the rates are known). With the more activated substrates (quinoline and 4-cyano- and 4-acetylpyridine) the tert-butyl radical is significantly more reactive than the n-butyl radical, clearly showing that polar effects are more important than steric and enthalpic effects in determining the reaction rates. The reversibility of the alkylation by the tert-butyl radical is discussed. © 1980, American Chemical Society. All rights reserved.