Aromatic substitution. Part XVIII. Kinetics of reactions between some halogeno-pyridines and -picolines and their N-oxides with methoxide ion in methanol and in dimethyl sulphoxide. Effect of alkyl groups on rates on orientation in nucleophilic aromatic substitution

Abstract

The rates and activation parameters for the reactions of 2-fluoro-, 2-chloro-, 2-bromo-, 2-chloro-3-methyl-, 2-chloro-5-methyl-, 2-bromo-3-methyl-, 2-bromo-5-methyl-, 2,3-dichloro-, 2,5-dichloro-, 2-chloro-3-nitro-, and 2-chloro-5-nitro-pyridine, and of 2-bromo-, 2-bromo-3-methyl-, and 2-bromo-5-methyl-pyridine N-oxide with potassium methoxide in methanol have been determined and compared. The kinetics of the reaction of 2-bromo-, 2-bromo-3-methyl-, and 2-bromo-5-methyl-pyridine and methoxide ion in dimethyl sulphoxide containing small amounts of methanol have also been determined; the rates were very sensitive to the methanol concentration. The rates were in the order 2-halogeno- > 2-halogeno-3-methyl- > 2-halogeno-5-methyl-, and were dependent upon Eact when the halogen was Br but upon ΔS‡ when it was Cl. The effects of the β-substituent in the pyridine nucleus upon the ortho : para ratios are discussed in terms of their effects upon the energies and entropies of activation.