P-nitrophenyl hexanoate

Additional support for a discrete binding site is derived from the observation that potassium iodide depresses the rate of appearance of phenol from p-nitrophenyl hexanoate in the presence of 16. In contrast, potassium iodide modestly accelerates the reaction of 17 with p-nitrophenyl hexanoate, in... 

Fig. 8 Correlation of the transition state binding (pKjs) of alcohols catalysing the cleavage of p-nitrophenyl hexanoate by /3-CD with their binding in the initial state ternary complexes (pK,) [see (26)]. Data from Table A5.15.

The latexes prepared in the group of Ford [68,69] consisted of hydrophobic monomers and a cross-linker, i.e., styrene, methacrylate monomers with various substitutes in the ester moiety, divinylbenzene, and hydrophilic monomers bearing charged groups, viz., styrylmethyl(trimethyl)ammonium and styrylmethyl(tributyl)ammonium cations. The latexes were catalytically active in reactions of decarboxylation of 6-nitrobenzioxazole-3-carboxylate (Scheme 6) and p-nitrophenyl hexanoate hydrolysis (Scheme 7, n = 6). 

Cordes and co-workers 191 found that the alkaline hydrolysis of p-nitrophenyl hexanoate is subject to catalysis by polyvinylpyridine-based polysoaps. For example, k bs is increased from 0.1 mm to 1.4 mm in the presence of 5 x 10 7 M 38% polysoap (23) (the same material used in the Strauss work). With 5 x 10-7 M polymer having a 15 % dodecyl content, the rate is increased only 3 times above background. The simplest rationale for the kinetics invokes both hydrophobic and electrostatic forces. Thus, dodecyl chains on the polymer hydrophobically bind p-nitrophenyl hexanoate to the polymer surface. Since the polymer possesses a high density of cationic nitrogens, hydroxide ions also accumulate at the polymer surface where they catalyze the hydrolysis of bound ester. Addition of nitrate ion to the aqueous reaction... 

Cordes, etal also employed poly-4-vinylpyridine alkylated with lauryl bromide and ethyl bromide (PVP-Ljj, 27) for the hydrolysis of p-nitrophenyl hexanoate (23,... 

Fig. 3-3. First-order rate constant (kobs) hydrolysis of p-nitrophenyl hexanoate pk>tted as a function of the concentration of polysoaps (PVP-LiJ), pH 9.95-10.1.

The addition of PVMI was expected to enhance the hydroxide-ion catalyzed hydrolysis of the anionic ester, since both the hydroxide ions and the negatively charged ester are attracted to the polycation so that the rate of their mutual collision is increased. For any given polyelectrolyte concentration the increase in the rate of hydrolysis should be independent of pH. And this is what has actually been found at pH values greater than 9. At lower pH values, however, a completely unexpected behavior resulted, i. e. the polycation was found to increase the rate of hydrolysis of NABS by the largest factor in that pH range (pH s 6) in which direct water attack on the ester makes the dominant contribution to the overall reaction rate (Fig. 8). The influence of hydrophobic forces appears to be ruled out in this case since PVMI has no effect on the solvolytic rate of the neutral ester p-nitrophenyl acetate and p-nitrophenyl hexanoate. Thus, the causes of the above-mentioned phenomenon are obscure this very fact adds, in the author s opinion, further interest to the study of reactions in polyelectrolyte solutions. The examination of such factors as the enthalpy and entropy of activation may be of particular relevance for a deeper insight into these complex reaction systems. 

True catalysis of esterolysis, (as opposed to rapid acyl transfer and formation of a stable intermediate) requires a second rapid step in which the acyl group is transferred to water or to an oxygen nucleophile. This has presented micellar histidines and imidazoles with a second role, in experiments first carried out separately by Tagaki, Moss, and Tonellato. In the simplest experimenta mixture of (49) with myristoylhistidine (72) reacts with p-nitrophenyl acetate first by nucleophilic attack of imidazole and then transfer of the acyl group to (49) in a rapid step. In the bifunctional surfactant (73) this process was shown to be intermolecular (but presumably intramicellar). The acylimidazole can be observed spectroscopically when the substrate is p-nitrophenyl hexanoate. Similar conclusions are recorded in a study by Tonellato, who additionally... 

Romsted, L.R., Cordes, E.H. Secondary valence force catalysis. Vn. Catalysis of hydrolysis of p-nitrophenyl hexanoate by micelle-forming cationic detergents. J. Am. Chem. Soc. 1968, 90(16), 4404-4409. 

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