Rate and Mechanism of Hydrolysis

The rates and mechanisms of hydrolysis and transesterification of phenyl benzoate in aqueous ethanolic KOH solution were determined by non-linear least-squares regression.4 Kinetic studies of the transesterification of a series of 4-nitrophenyl 3- and 4-substituted benzoates by 4-chlorophenol in DMF in the presence of potassium carbonate at various temperatures were reported.5... 

The hydrolysis of (-)-a-pinene to trfl 5 -(-)-sobrerol was found to occur by an 5 1 mechanism in refluxing water but by an 5 2 mechanism in warm or room temperature water. " The change in mechanism is attributed primarily to the change in the p T of water from 14.0 at 25 °C to 12.3 at 100 °C and consequent significant increases in H+ ion (the catalyst) concentration. The effect of temperature, organic co-solvents, and the concentration of the solute on the rate and mechanism of hydrolysis was also investigated. Chiral benzyl and allylic alcohols also hydrolyse by an 5 1 mechanism in refluxing water. 

Hydrolysis and Other Reactions and Features.- The effects of substituents in the aromatic rings of 3-D-glucopyranosldes on their rates and mechanisms of hydrolysis in concentrated potassium hydroxide (12.75 M) have been studied,as have the reactions of some dlsaccharldes with alkaline hydrogen peroxide. Five different... 

How deeply one wishes to query the mechanism depends on the detail sought. In one sense, the quest is never done a finer and finer resolution of the mechanism may be obtained with further study. For example, the rates and mechanisms of electron transfer reactions have been studied experimentally and theoretically since the 1950s. but the research continues unabated as issues of ever finer detail and broader import are examined. The same can be said of other reactions—nucleophilic substitution, hydrolysis, etc. 

The existence of acylium ions in the reactions of acyl halides with Lewis acids has been amply demonstrated88,89, but they have not so far been directly observed in hydrolysis reactions. To achieve direct observation of an acylium ion, it must, at best, be generated into a non-reactive environment if its rate of formation is slow, or be detectable in small concentrations if it is destroyed rapidly. The limiting factors are, therefore, the rate and mechanism of heterolysis and the stability and concentration of the acylium ion formed. In this... 

S. Elliot, E. Lu, F.S. Rowland (1989). Rates and mechanisms for hydrolysis of carbonyl sulfide in natural waters. Environ. Sci. Technol., 23,458-461. 

An important question regarding peptides and proteins is concerned with the equilibria among several conformational states. It has been suggested that enzyme function may be linked to the occurrence of particular conformations in solution.24 377 A mechanism recently proposed for the hydrolysis of oligosa-charides by the enzyme lysozyme, for example, is based on the observation of specific substrate and enzyme sidechain conformations in a molecular dynamics simulation of a lysozyme-substrate complex.378 Also, local conformational equilibria and the barriers between conformations are important in determining the rates and mechanisms of folding and rebinding processes. 

The rate and mechanism of this hydrolysis was performed by Naiditch and Yost (13). These authors found that hydrolysis is catalyzed by acid as well as water, but the effect of water is much less than that of acid. The rate equation can be expressed... 

The reaction rate and mechanism of the acid catalysed decomposition of Ni4(OH)4 have been studied by stopped-flow measurements. Potentiometric titrations of 0.46 M nickel(ll) perchlorate solutions at 25°C and 1.5 M ionic strength, maintained by Na-CIO4, were also performed to study the hydrolysis under the conditions used for the kinetic measurements. The authors suggested the presence of the Ni (OH)4 species, only, and log y 4 4= -(27.03 + 0.06) was obtained. This value compares favourably with the corresponding value reported in [65BUR7L1L]. The reported data indicate a slight tendency for a decrease in the calculated log,o values with decreasing pH, which may be due to the presence a minor hydroxo complex. Therefore, the reported experimental data were re-evaluated for the purpose of this review, but no further species can be identified with any confidence. 

Three-co-ordinate sulphur. The most inorganic example has been the study of the kinetics and mechanism of hydrolysis of dithionites. Kinetics of attack of iodide and of thiourea at the 55 -dimethylsulphinium cation (M62S-NH2+) in water and in aqueous dimethyl sulphoxide indicate pre-equilibrium protonation followed by a rate-determining associative process. The overall activation entropy is negative. Nucleophilic substitution, by vinyl-lithium, at the triphenylsulphonium cation (PhaS ) is also associative. Indeed the species (30) is an intermediate rather than a transition state. Comparison of the stereochemical courses of reactions at tetrahedral phosphorus and at three-co-ordinate sulphur, which is... 

Most organic reactions occur in solution and, naturally, any comprehensive analysis of the mechanism, kinetics, and thermodynamics of a given reaction must take account of effects of the medium, in other words, of the solvent effects. The number of publications is very great devoted to experimental research into the multifarious effects a solvent has on the rate and mechanism of organic reactions which were first detected about one hundred years ago by Menshutkin. How essential the effect of a solvent can be shows, for example, a comparison between the reaction of alkali hydrolysis of alkylhalogenides and esters in water and in the gas phase. The rate of the gas-phase reaction is by 20 orders of magnitude faster than in solution (see Sect. 5.1.1.4). 

R. Breslow, D. E. McClure, R. S. Brown, and J. Eisenach (1975), Very fast zinc catalyzed hydrolysis of an anhydride. A model for the rate and mechanism of carboxypeptidase A catalysis. /. Amer. Chem. Soc. 97,194-195. 

FIGURE 27 19 Proposed mechanism of hydrolysis of a peptide catalyzed by carboxypeptidase A The peptide is bound at the active site by an ionic bond between its C terminal ammo acid and the positively charged side chain of arginine 145 Coordination of Zn to oxygen makes the carbon of the carbonyl group more positive and increases the rate of nucleophilic attack by water... 

The differenee in reaction rates of the amino alcohols to isobutyraldehyde and the secondary amine in strong acidic solutions is determined by the reactivity as well as the concentration of the intermediate zwitterions [Fig. 2, Eq. (10)]. Since several of the equilibrium constants of the foregoing reactions are unknown, an estimate of the relative concentrations of these dipolar species is difficult. As far as the reactivity is concerned, the rate of decomposition is expected to be higher, according as the basicity of the secondary amines is lower, since the necessary driving force to expel the amine will increase with increasing basicity of the secondary amine. The kinetics and mechanism of the hydrolysis of enamines demonstrate that not only resonance in the starting material is an important factor [e.g., if... 

The mechanism of hydrolysis of o-carboxyaryl phosphates, whose dianions also hydrolyze much faster then, e.g., the phenyl phosphate monoanion 79,80) (maximum rate at about pH 4.8 and 25 °C 81)), was long a point of mechanistic contention. Thorough investigations81 led to proposal of a fast initial transprotonation... 

While much is now known about the function, structure, and mechanism of PLCj,-, there remain numerous unanswered questions. For example, it is well documented that PLCSc preferentially hydrolyzes micellular substrates and monomeric substrates with longer acyl side chains. However, the basis for the discontinuity in the rate of hydrolysis that is observed at the critical micelle concentration of the substrate is unknown as is the reason for the increase in the Km for soluble substrates as the length of the acyl side chains decrease. What is... 

The acid-catalysed hydrolysis of the acylal, 1-phenoxyethyl propionate (13), has been studied using the PM3 method in the gas phase. The kinetics and mechanism of the hydrolysis of tetrahydro-2-furyl and tetrahydropyran-2-yl alkanoates (14) in water and water-20% ethanol have been reported. In acidic and neutral media, kinetics, activation parameters, isotope-exchange studies, substituent effects, solvent effects and the lack of buffer catalysis pointed clearly to an Aai-1 mechanism with formation of the tetrahydro-2-furyl or tetrahydropyran-2-yl carbocation as the rate-limiting step (Scheme 1). There is no evidence of a base-promoted Bac2 mechanism up to pH 12. ... 

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