Kinetics of SNAr reactions of 1‐phenoxy‐nitrobenzenes with aliphatic amines in toluene: ring substituent and solvent effects on reaction pathways
Rate constants are reported for the reactions of a series of 4-substituted-1-phenoxy-2,6-dinitrobenzenes 1 and 6-substituted-1-phenoxy-2,4-dinitrobenzenes 2 activated by CF3, COOCH3, CN, NO2 groups or by ring-nitrogen with n-butylamine, pyrrolidine or piperidine in toluene. The results are compared with those reported previously for reactions of the same substrates and nucleophiles in acetonitrile [Crampton et al., Eur. J. Org. Chem., 2006, 1222–1230]. Plots of the overall second-order rate constant, kA versus [Am] for the reactions with n-butylamine and pyrrolidine followed a similar kinetic pattern to that obtained for the same reactions in acetonitrile whereas for reactions with piperidine, an upward (convex) curvature is observed, an indication that a small third order term in amine may be present. Overall reactivity is lower in toluene than in acetonitrile. Mechanisms for the overall substitution process have been rationalised in terms of the proton transfer mechanism and cyclic transition mechanism, for reactions involving third-order kinetics.