医薬品の分解とその安定化(第17報)ペンタゾシンの酸触媒水和反応の動力学的検討

概要

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The kinetics of the acid-catalyzed hydration of pentazocine (I) was examined in aqueous hydrochloric acid solutions from 3.9×10^<-2> to 5.0M. I undergoes hydration in aqueous acidic media to form an alcohol derivative corresponding to 1,2,3,4,5,6-hexa-hydro-6,11-dimethyl-3-(3-hydroxy-3-methylbutyl)-2,6-methano-3-benzazocin-8-ol(II), and its reaction rate follows a pseudo-first-order reaction. Values of log k_<obs> (hr^<-1> are far from directly proportional to acid concentration (log [HCl]) and instead nearly proportional to the acidity function (H_0). Further, participation of water molecule in the transition state (as rate-determining step) was examined by Bunnett w-value, and a linear relation-ship was found between log k_<obs>+H_0 and log αH_2O with the slope, w, of -1.28. These results suggest that the hydration transition state is I plus a proton, i.e., free carbonium ion. Accordingly, the hydration mechanism is free from firmly bound water molecules in the transition state. Arrhenius parameters were determined by plotting the hydration rate constants against temperature, and activation energy and activation entropy were calculated.
1976-11-25