Effect of structure on reactivity in oxime formation of benzaldehydes

✍ Scribed by M. Calzadilla; A. Malpica; T. Cordova

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 58 KB
• Volume: 12
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199909)12:9<708::aid-poc180>3.0.co;2-t

✦ Synopsis

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 ratedetermining step. Equilibrium constants for the formation of the neutral carbinolamine are correlated by the s substituent constants; & = 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 r 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 s substituent constants; & = À0.85.