ELECTROSTATIC CATALYSIS .2. A COMPARISON OF SPONTANEOUS AND ALKALINE HYDROLYTIC RATE CONSTANTS FOR ALPHA-SUBSTITUTED O-NITROPHENYL ESTERS

The pH-log khydr profiles for the hydrolysis of a series of α-substituted o-nitrophenyl acetate esters [XCOO-o-NP where X = CH3CH2, CH3, PhCH2, CH3CH2SCH2, (CH3)3N+CH2, PhOCH2, BrCH2, ClCH2, C5H5-N+CH2, and Cl2CH] have been determined in water at 30°, ionic strength = 1.0, between pH 1 and 12.53. The values of khydr, at all pH values are quantitatively provided by summation of rates for spontaneous general base catalyzed hydrolysis (KH) and hydroxide ion catalyzed hydrolysis (Koh[HO-]). For the esters in which the α-substituent group equals CH3, CH3CH2, and PhCH2 a specific acid catalyzed term (kHαH) must be included to provide Khydr at low values of pH. A plot of log KH vs. log koH for all esters, including the positive charged species, was found to be linear and to follow the equation log koh = 0.84 log kH + 8.0. The fact that esters containing formal positive charges do not show positive deviations from the plot of log k,oh vs. log kH is indicative that electrostatic facilitation for the nucleophilic displacement of o-nitrophenoxide by hydroxide ion is unimportant. © 1969, American Chemical Society. All rights reserved.