Phenyl salicylate hydrolysis of ionized

Khan, M.N., Arifin, Z. Effects of cationic micelles on the intramolecular general base-catalyzed hydrolysis of ionized phenyl salicylate at different temperatures. J. Chem. Res. (S) 1995, 132-133. 

The presence of B(OH>3 increases the rate of hydrolysis of ionized phenyl salicylate (PS ) by nearly 10 -fold compared to the rate of hydrolysis of phenyl benzoate under essentially similar conditions (Equation 2.34). However, the hydrolysis of PS crtn also occur with measurable rate in the absence of B(OH)3. Nearly 10 -fold rate enhancement due to the presence of boric acid is attributed to the boric-acid-induced intramolecular reaction involving transition state TS,6. An alternative and kinetically indistinguishable mechanism involving transition state TSiy has been ruled out on the basis of the absence of enhanced nucleophilic reactivity of tertiary and secondary amines toward phenyl salicylate in the presence of borate buffer. 

Pseudo-first-order rate constants, k bs, for hydrolysis of ionized phenyl salicylate in the presence of different concentrations of CTABr were used to calculate k Kg and Kg (considered to be unknown parameters) from Equation 3.2 and such calculated values of k Kg and Kg are 0.61 + 0.24 Af- sec and (6.3 1.1) X 10 M respectively. These values of k Kg and Kg yielded k as 9.6 X 10 sec , which is exactly the same as the one calculated from Equation 3.2 considering k and Kg as unknown parameters. The quality of the data fit of a set of observed data to Equation 3.2 remained unchanged with the change in the choice of unknown parameters from k, and Kg to k Kg and Kg. This analysis thus rules out the inherent perception of a possible compensatory effect between the calculated values of k and Kg when they, rather than k, Kg and Kg, are considered unknown parameters in using Equation 3.2 for data analysis. 

The kinetics of the alkaline hydrolysis of 2-methylpentyl salicylate (24) have been studied in various aqueous propanol and r-butanol mixtures and in mixtures of water and ethane-l,2-diol. ° Further smdies of the aminolysis of ionized phenyl salicylate (25) have been reported, in which it was observed that, in mixed acetonitrile-water solvents, glycine, 1,2-diaminoethane and 3-aminopropanol all reacted as did simple amines, via an intramolecular general-base-catalysed process. ... 

The aminolysis and methanolysis of ionized phenyl salicylate (189) have been examined under micellar conditions. The effect of CTABr on the rates of aminolysis of (189) by -butylamine, piperidine, and pyrrolidine is to bring about a rate decrease (up to 17-fold with pyrrolidine). The results are interpreted in terms of binding constants for the amines with CTABr and the pseudo-phase model.The effects of mixed surfactants SDS and CTABr on the methanolysis of (189) and the alkaline hydrolysis of phenyl benzoate suggest that micellar aggregates are involved in the processes.The effects of NaOH and KBr on the intramolecular general base-catalysed methanolysis of (189) in the presence of CTABr has been investigated. Pseudo-first-order rate constants were not affected by either additive but other changes were noted. The effect of mixed MeCN-water solvents on the same reaction has also been probed. 

The effect of cetyltrimethylammonium bromide on the rate constants of hydrolysis of ethyl glycinate hydrochloride has been reported. " Cationic and non-ionic micelles inhibited and anionic micelles accelerated the acid hydrolysis of A-p-tolyl-benzohydroxamic acid (189). The rates of acid hydrolysis of acetohydroxamic acid, MeCONHOH, benzohydroxamic acid, PhCONHOH, and A-phenylbenzohydroxamic acid, PhCON(OH)Ph, were increased by perfiuorooctanoic acid and by sodium 1-dodecanesulfonate and sodium dodecyl sulfate (SDS). " The effects of micelles of SDS upon the rates of reaction of ionized phenyl salicylate with Bu"NH2, piperidine and pyrroiidine have been reported. 

The rates of hydrolysis of phenyl and methyl salicylates became independent of [HO ] within the [HOi range of 0.002 to 0.070 M in the absence of micelles. The values of ionization constants, K, for phenyl salicylate and methyl salicylate in water solvent are 5.67 x 10 ° M and 2.48 x 10 ° M, respectively. Thus, within the [HO ] range of - 0.002 to 0.070 M, both phenyl and methyl salicylates exist in fully ionized forms. The pH-independent rate of hydrolysis of salicylate esters involve ionized salicylate ester and H2O as the reactants. Intramolecular general base catalysis has been shown to occur in the aminolysis of ionized phenyl salicylate in both aqueous pseudophase and micellar pseudophase of SDS. - The brief reaction scheme for hydrolysis or alkanolysis of salicylate ester or any substrate containing an easily ionizable proton in the presence of micelles, D , may be given in terms of PP model of micelle as depicted in 8cheme 4.2... 

The rate of hydrolysis of phenyl salicylate remains independent of [HO] within its range 0.005 to 0.060 M at 30°C. The [HO]-independent hydrolysis of phenyl salicylate is expected to involve either H2O and PS" (ionized phenyl salicylate) or kinetically indistinguishable HO and PSH (nonionized phenyl salicylate) as the reactants. The occurrence of these kinetically indistinguishable reaction steps has been resolved satisfactorily and it has been concluded that pH-independent hydrolysis involves H2O and PS as the reactants. The rates of pH-independent hydrolysis and alkanolysis of phenyl salicylate have been shown to be increased by lO -fold owing to the intramolecular GB (2-0 group) assistance through an intramolecular intimate ion-pair (IIP). 

The rate of hydrolysis of phenyl salicylate in aqueous solvent was found to be [HO-]-independent within the [HO ] range -0.002 to 0.070 and under such conditions, the hydrolysis reactions of salicylate esters involved ionized salicylate ester and H2O as the reactants. A brief mechanism for [HO"]-independent hydrolysis of PS" is shown in Scheme 4.1, in which IIP and HP represent intramolecular intimate ion-pair (highly reactive intermediate) and internally hydrogen-bonded highly reactive intermediate, respectively. Although micelles accelerate or decelerate the rate of a reaction, it is very rare that they change the mechanism of the reaction with the change in the reaction environment from aqueous pseudophase to the micellar pseudophase." It is, therefore, reasonable to assume that the mechanism of hydrolysis of PS- remains the same in both the aqueous and micellar pseudophases. 

---