Effect of structure on reactivity in oxime formation of benzaldehydes

Second-order rate constants for substituted benzaldehyde oxime formation increase linearly with the activity of hydrated protons over the pH range ca 2-7. Under these conditions, first-order rate constants show saturation behavior with increasing hydroxylamine concentration, establishing carbinolamine dehydration as the rate-determining step. Equilibrium constants for the formation of the neutral carbinolamine are correlated by the sigma(+) substituent constants; rho = 1.26. Under more acidic conditions, second-order rate constants increase less rapidly than the activity of hydrated protons, indicative of a transition to pH-independent carbinolamine formation, presumably the uncatalyzed addition of amine to aldehyde. The corresponding value of rho for this process is 1.21. This value, together with that for the equilibrium constants (see above), suggests that C-N bond formation is nearly complete in the transition state. The rate constants for acid-catalyzed carbinolamine dehydration are correlated by the a substituent constants; rho = -0.85. Copyright (C) 1999 John Wiley & Sons, Ltd.