Micellar media hydrolysis

Amoxicillin and ampicillin Pharmaceutical formulation Hydrolysis in micellar medium and catalyzed by Cu(ll)... 

Munoz, M., Rodriguez, A., Graciani, M. del M., Moya, M.L. Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic, zwitterionic, nonionic, and mixed micellar solutions. Int. J. Chem. Kinet. 2002, 34(1), 445-451. 

The lower polarity of the mieellar Stern region (2) is also widely thought to make a signifieant eontribution to the rate retarding effects of micelles in fact, the transition state of the type of hydrolysis reactions commonly used to probe the micellar pseudophase is thought to be more polar than the reactants, indicating that a more polar medium would be favorable for reaction to occur. 

Take as an example the experimentally most studied reaction in micellar catalysis, ester hydrolysis in a basic medium... 

Three papers from Ghosh s group deal with the hydrolysis of benzohydroxamic acids in acidic and alkaline conditions. A pre-equihbrium protonation followed by a slow A-2 type nucleophilic attack by water is seen as the mechanism of the acid-catalysed hydrolysis of j -chlorophenylbenzohydroxamic acid (107 R = / -ClCgEU) by mineral acids (HCl, HCIO4) in 20% aqueous dioxane. An A-2 mechanism was also supported for the reaction of (107 R = Me) imder comparable conditions. The alkaline hydrolysis imder micellar conditions of (107 R = Ph) and a series of para-substituted derivatives has been investigated in the presence of cationic and anionic micelles in 5% dioxane-water medium at 55 °C. Cationic surfactants exerted a catalytic effect and anionic surfactants were inhibitory. The rate-surfactant profiles were analysed in terms of the pseudophase and Piszkiewicz models. The detection of N2O in the products of the oxidation of hydroxamic acids suggests the intermediacy of nitroxyl, HNO, in the process. Scheme 9 may represent the pathway followed. 

Micelles concentrate reactants from the surrounding medium and provide microenvironments favorable to reaction. Rate enhancement arises from electrostatic and hydrophobic interactions between reactants and micelles. Rates may be strongly dependent on the struture of the reactant. For example, the hydrolysis of methyl ort/iobenzoate is catalyzed by micellar sodium dodecylsulfate, whereas the hydrolysis of methyl ortho-formate (which has less hydrophobic character and less affinity for the micelle core) is not ". The more pronounced the hydrophobic nature of the reactant, the more rapid is the micellar catalysis. ... 

Examples of micellar systems are given below where the surfactant provides the medium for catalysis but does not directly participate in the reaction. For instance, JV-acetyl histidine can bring about the hydrolysis of p-nitro-phenyl esters via an acyl-imidazole intermediate. 

Fats are digested in three different and coordinated processes emulsification, hydrolysis of the substrate, and micellar solubilization of the hydrolysis products in the aqueous medium of the intestinal contents (Friedman and Nylund, 1980). Many determinants are therefore involved in these steps. It has been shown that intestinal fat absorption is influenced by several factors, including ... 

Lipases with a lid demonstrate a more complex behavior in aqueous medium. They react preferentially with micellar substrates (5 ). Figure 14.1 shows the mechanism for simultaneous hydrolysis of S and S. The velocity of the reaction is expressed as the sum of Michaelis and Hill equations where a steep iuCTease of activity is observed after formation of5 (5 A ). 

First-order rate constants (ko,J for hydrolysis of ethyl cyclohexanone-2-carboxylate (ECHC) decrease monotonically with increase in the concentration of nonionic surfactants, C12E9 and CigE2o, at 0.050 M HCl. These observed data fit to Equation 3.2 (Chapter 3) with = 13.95 x 10 sec and calculated respective values of and are (3.05 0.16) x 10 see- and 310 5 A/- for C,2E9 micelles and (3.24 + 0.15) x 10 sec" and 480 4 M for C,6E2o micelles. The experimentally determined value of ko s (= 3.05 x 1(T sec- ) at 0.083 M HCl and 83% v/v propanol in mixed water-propanol solvent is similar to the k values. The rate of hydrolysis of ECHC is sensitive to both uncatalyzed and specific-acid-catalyzed reaction steps and, therefore, close similarity between k i values at 0.050-M HCl and ko,j value at 0.083-M HCl and 83% v/v propanol may not refiect the nonionic micellar rate-retarding effect as due to only micellar reaction medium effect. 

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