The rates of hydrolysis of the following orthoesters, 6 or 7-substituted 1, 1-diethoxy 3, 4- dihydrobenzo-2-pyrans (R= 6-MeO, H, 7-MeO, 7-NO2 and 6-NO2) were determined at 27° C in water/dioxan (2: 1 by volume). The catalytic rate constants as well as the Hammett ρ constants show that the rate-determining step is the formation of a cyclic carboxonium ion intermediate and constitute a kinetic piece of evidence for the determination of the nature ...
