Kinetic Evidence for the Occurence of the Oxydianionic Tetrahedral Intermediates in the Hydrolyses of Methyl Salicylate and methyl o-Methoxybenzoate in Highly Alkaline Medium

Article abstract of DOI:10.1021/jo00140a022

The kinetics of hydrolyses of methyl salicylate and methyl o-methoxybenzoate have been studied at various hydroxide ion concentrations ranging from 0.01 to 4.40 M for methyl salicylate and from 0.005 to 0.200 M formethyl o-methoxybenzoate at 35 deg C.The observed rate constants are independent of -> within the range 0.01-0.04 M and vary according to equation k_obsd = A + B-> + C->² within the range 0.04-3.60 M for hydrolysis of methyl salicylate where A = k₃k₁/(k₁ + k₃),B = k₂k₄/(k_-2 + k₄), and C = k₂k₅K/(k_-2 + k₄).The observed rate constants for hydrolysis of methyl o-methoxybenzoate follow the equation k_obsd = B¹-> + C¹->² where B¹ = k₁^'k₂^'/(k_-1^' + k₂^') and C¹ = k₁^'k₃^'K^'/(k_-1^' + k₂^').The pH-independent observed rate constants are 1E4 and 1E5 times larger than the corresponding values for water-catalyzed cleavages of methyl benzoate and methyl o-methoxybenzoate, respectively.This rate enhancement has been attributed to the probable intramolecular general-base-catalyzed neutral hydrolysis of methyl salicylate.The ratios B/B¹ and C/C¹ have been found to be 0.0214 and 0.0004, respectively.The apperance of C and C¹ terms in the kinetic equations has been attributed to the existence of the oxydianionic tetrahedral intermediates in the reaction mechanisms.The temperature dependence of hydrolysis of methyl salicylate has also been studied at two differentOH^- concentrations.The intramolecular general-base-catalyzed rate enhancement has been found to be due toa favorable ΔS^*.The hydrolytic cleavage of methyl salicylate has been found to be sensitive to the ionic strenght.The probable mechanisms for hydrolyses of both esters are considered.