Nucleophilic heteroaromatic substitution. Part XXXIV. Fluorine versus chlorine mobility in reactions with methanolic methoxide ion and with piperidine in various solvents

The rate constants and the activation parameters for the title reactions of some 2-halogenopyridines, 2- and 4-halogenoquinolines, and 2-halogenoquinoxalines are reported. The observed leaving-group effects on both rate constants, kF > kCl, and activation enthalpies, ΔHF‡ < ΔHCl‡, though analogous to those known for the similar reactions of nitro-activated aromatic compounds, reveal new features that characterise aza-activation. These include relatively low kF/kCl values for the piperidinodehalogenations in piperidine solution and a significant positional effect on the relative halogen mobility for the reactions with methoxide ion. The reaction mechanisms are discussed in the light of these observations and other previously reported facts. The results with the γ-fluoro-derivatives were made possible by the syntheses of previously unreported 4-fluoro-2,8-dimethyl- and 4-fluoro-2-methoxy-quinoline.