R. O'ferrall, D. O'Brien
Jun 1, 2004
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0
Influential Citations
16
Citations
Journal
Journal of Physical Organic Chemistry
Abstract
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 11M. Kinetic measurements confirm that isomerization and hydrolysis proceed through a common tetrahedral intermediate T 0 . 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 a d d =[T 0 ]/ [aldehyde][NH 2 OH] = [T 0 ]/[oxime] = 1.8, values of pK R = -3.65 and -5.90 and K H 2 O = 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 H 2 O for the oximes are compared with corresponding values forp-methoxybenzaldehyde and its hydrate (= To).