Rate and equilibrium constants for hydrolysis and isomerization of (E)- and (Z)-p-methoxybenzaldehyde oximes

The interconversion and hydrolysis of (E)- and (Z)-oximes of p-methoxybenzaldehyde in aqueous solutions of perchloric acid have been studied in the acid concentration range from pH 3 to 1 1 M. Kinetic measurements confirm that isomedzation and hydrolysis proceed through a common tetrahedral intermediate To. At low acid concentrations, attack of water on the protonated oxime is rate-determining in the hydrolysis reaction and no separate isomerization is observable. However, as the acid concentration increases the attack of water becomes faster than loss hydroxylamine from To and E to Z isomerization is observable as a faster reaction in competition with hydrolysis. From kinetic and equilibrium measurements a comprehensive set of rate and equilibrium constants for protonation, hydrolysis and hydration of (E)- and (Z)-oximes is derived. For the neutral oximes K-T= [E]/[Z] = 8 and for the protonated oximes pK(a) = -0.55 and 0.80 for the E and Z isomers, respectively. By combining these values with measurements of oxime formation from p-methoxybenzaldehyde and hydroxylamine, including K-add=[T-0]/ [aldehyde][NH2OH] = [T-0]/[oxime] =1.8, values of pK(R) = -3.65 and -5.90 and K-H2O = 6.3 x 10(-5) and 7.9 x 10(-6) for (E)- and (Z)-oximes may also be derived. The values of pK(a), pK(R) and K-H2O for the oximes are compared with corresponding values for p-methoxybenzaldehyde and its hydrate (equivalent toT(0)). Copyright (C) 2004 John Wiley Sons, Ltd.