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.
抄録全体を表示