KINETIC STUDY OF HYDROLYSIS OF BENZOATES.PART XXV.Ortho SUBSTITUENT EFFECT IN ALKALINE HYDROLYSIS OF PHENYL ESTERS OF SUBSTITUTED BENZOIC ACIDS IN WATER

摘要

The second-order rate constants k_2 for alkaline hydrolysis of phenyl esters of meta-,para-and oortho-substituted benzoic acids,X-C_6H_4CO_2C_6H_5 (X=H,3-Cl,3-NO_2,3-CH_3,4-NO_2,4-Cl,4-F,4-CH_3,4-OCH_3,4-NH_2,2-NO_2,2-CN,2-F,2-Cl,2-Br,2-I,2-CH_3,2-OCH_3,2-CF_3,2-NH_2),and of substituted phenyl esters of benzoic acid,C_6H_5CO_2C_6H_4-X (X=2-I,2-CF_3,2-C(CH_3)_3,4-Cl,4-CH_3,4-OCH_3,4-NH_2),have been measured spectrophotometrically in water at 25 deg C.The substituent effect in alkaline hydrolysis of phenyl esters of para-substituted benzoic acids,similar to that for ethyl esters of para-substituted benzoic acids,was found to be precisely described by the Hammett relationship (rho=1.7 in water).The log k value for alkaline hydrolysis of phenyl and ethyl esters of meta-,para- and ortho-substituted benzoic acids,X-C_6H_4CO_2R,was nicely correlated with log K_(m,p,ortho)=log k_(deg)+(rho)_(m,p~(sigma))+(rho_I)_(ortho)~(sigma_I)+ (rho_R~(deg))_(ortho)sigma_R~(deg)+delta_(ortho)E_s~B where sigma,sigma_I,sigma_R~(deg) are the Hammett polar,Taft inductive and Taft resonance (sigma_R~(deg)=sigma~(deg)-sigma_I) substituent constants,respectively.E_s~B is the steric scale for ortho sub-stituents calculated on the basis of the log it values for the acid hydrolysis of ortho-substituted phenyl benzoates in water owing to the ortho substituent in the phenyl of phenyl benzoates.In water,the main factors responsible for changes in the ortho substituent effect in alkaline hydrolysis of phenyl and ethyl esters of ortho-substituted benzoic acids,X-C_6H_4CO_2R,were found to be the inductive and steric factors while the role of the resonance term was negligible ((rho_R~(deg))_(ortho) ca.0.3).In alkaline hydrolysis of substituted benzoates in neat water,the ortho inductive effect appeared to be 1.5 times and steric influence 2.7 times higher than the corresponding influences from the ortho position in the phenyl of phenyl benzoates.The contributions of the steric effects in alkaline hydrolysis of esters of ortho-substituted benzoic acids was found to be approximately the same as in acid hydrolysis of esters of ortho-substituted benzoic and acid esterification of ortho-substituted benzoic acids.