Kinetic Study of the Acid-Promoted Hydrolysis of Some Representative 2-Fluoro Nitrogen Heterocycles

Article abstract of DOI:10.1021/jo00335a008

The acid-promoted hydrolysis of the 2-fluoro derivatives of pyridine, the four isomeric picolines, quinoline, pyrimidine, 4-methylpyrimidine, and 4,6-dimethylpyrimidine have been studied in hydrochloric acid over the concentration range of 0.05-8.0 F HCl.At each acid concentration, the reactions followed pseudo-first-order kinetics, and at low concentrations of acid, the rate of reaction increased linearly with h₀.However, at higher acid concentrations negative deviations from linearity were observed for all the substrates and rate maxima for all but the pyrimidines.These results were correlated with the decline in water activity by means of the Bunnett ω and ω* relationships, as well as the Bunnett-Olsen LFER.The slopes of these correlations were suggestive of a proton transfer role for water in the reactions of the less activated 2-fluoropyridines and of 2-fluoroquinoline, while the correlations indicate a nucleophilic role for water in the reactions of the more highly activated pyrimidines.Entropies of activation, calculated both from the pseudo-first-order rate constants, and from the values of k₂* obtained from the intercepts of the LFER plot, were significantly more negative for the pyridine and quinoline systems for the pyrimidines.The above results are interpreted as consistent with nucleophilic attack by water in the rate-determining step for the reaction of the pyrimidines, while for the less activated substrates nucleophilic attack may be assisted by proton transfer to additional water molecules.