ORTHO ESTER HYDROLYSIS - CONCERTED NATURE OF THE DIALKOXYCARBONIUM ION FORMING STAGE

Hates of hydrolysis of a group of 2-alkoxy-2-phenvl-l,;}-dioxolanes catalyzed by a series of monohydrogenphosphonate anions, RPO3H-, in aqueous solution were measured under conditions where loss of the exocyclic group is rate determining. These data give Brønsted relations whose exponents decrease with decreasing basicity of the exocyclic oxygen atom [Rfexocyclic alkoxy group) = OCH3, 0.90; OCH2CH2OCHa, 0.85; OCH2CH2CL 0.80;OCH2C≡CH, 0.70; OCH2CHCl2,0.69], which, according to an argument based upon “tilting” potential energy surfaces. requires the proton transfer and C-O bond breaking that occur in this reaction stage to be concerted rather than stepwise. © 1979, American Chemical Society. All rights reserved.