Hydrolysis kinetics temperature effects

The first objective has been accomplished by the development of an HPLC procedure as reported by Spalik et al. ( 5) and GC/NPD procedures developed by Lemley and Zhong ( ). The second and third objectives are being accomplished by fundamental solution studies and reactive ion exchange experiments conducted in parallel. Lemley and Zhong (7) determined recently the solution kinetics data for base hydrolysis of aldicarb and its oxidative metabolites at ppm concentrations and for acid hydrolysis of aldicarb sulfone. They have since ( ) reported similar results for ppb solutions of aldicarb and its metabolites. In addition, the effect on base hydrolysis of temperature and chlorination was studied and the effect of using actual well water as compared to distilled water was determined. Similar base hydrolysis data for carbofuran, methomyl and oxamyl will be presented in this work. 

More elaborate and ambitious studies on the dissolution reactions of silica were conducted by Xiao and Lasaga (1994, 1996). Their objective was to provide full descriptions of the reaction pathway of quartz dissolution in acidic and basic solutions, from the adsorption of H2O or OH on a site, the formation of possible reaction intermediates and transition states, to the hydrolysis of the Si-O-Si bonds. Also, their aim was to extract kinetic properties such as changes in activation energy, kinetic isotope effects, catalytic and temperature effects, and the overall rate law form. The reaction mechanisms investigated were... 

Another source of error can come from the calculated temperature of the solar module. A very simple model was used that relies only on irradiance, ambient temperature, and an assumed maximum temperature. In this case, we assumed that the maximum temperature would be 80 °C in Miami. The results obtained from adjusting the model to reach other maximum temperatures are shown in Fig. 3.9. Surprisingly, the effect of the temperature model is very small if the kinetics are second order in RH. In the case of a low Ea hydrolysis, as for PC, there is no effect at all. With a high Ea hydrolysis, as for PET, the correlation changes by 40 % if the maximum temperature were 95 °C instead of 80 °C. The differences are much greater if the hydrolysis kinetics are assumed to be first order in RH 50 % for PC and 140 % for PET with the same temperature change. Thus, the accuracy of the temperature model is not critical in the case of second-order kinetics but is quite important if the kinetics were first order in RH. 

The hydrolysis of pyridine-diphenylborane follows second-order kinetics, first-order both in water and amine-borane 175). An observed kinetic isotope effect is small and temperature-independent for substitution of deuterium for protium on boron, whereas, the effect is large and temperature-dependent for a similar isotopic substitution in water 175, 176), A mechanism consistent with the isotope effect involves an initial attack of water on the boron-hydrogen bond... 

In order to verify this assumption, we studied the temperature dependence of the kinetic isotope effect during hydrolysis of some specific substrates, i.e. amino acid esters, by serine proteases like a-chymotrypsin[468,469] and 3-trypsin[470]. It is well known that a two-stage hydrolysis mechanism takes place for this class of enzymes with the formation, as an intermediate product, of an acyl derivative of the amino acid serine (Ser-195), a constituent of the polypeptide chain of the protein. The side group (-CH2OH) of serine is located in the region of the active center[471-474] ... 

PETP flakes produced from used soft drinks bottles were subjected to alkaline hydrolysis in aqueous sodium hydroxide. A phase transfer catalyst (trioctylmethylammonium bromide) was used to enable the depolymerisation reaction to take place at room temperature and under mild conditions. The effects of temperature, alkali concentration, PETP particle size, PETP concentration and catalyst to PETP ratio on the reaction kinetics were studied. The disodium terephthalate produced was treated with sulphuric to give terephthalic acid of high purity. A simple theoretical model was developed to describe the hydrolysis rate. 17 refs. 

The polyethylenimines are also effective in the cleavage of nitrophenyl-sulfate esters and nitrophenylphosphate esters. These have not yet been studied as extensively as the acyl esters, but interesting kinetic accelerations are already apparent. Nitrocatechol sulfate, for example, is very stable in aqueous solution at ambient temperature. In fact, even in the presence of 2 M imidazole no hydrolysis can be detected at room temperature. At 95°C in the presence of 2 M imidazole cleavage is barely perceptible. In contrast, a modified polyethylenimine with attached imidazole groups cleaves the sulfate ester at 20°C.34 Some kinetic parameters are compared in Table VI. It is obvious that accelerations of many orders of magnitude are effected by the polymer. 

Many examples are present in the scientific Uterature underlining the effort in producing kinetic data [9—11]. The Edwards historical study that started the investigation on the mechanism of the hydrolysis of aspirin required hundreds of kinetic experiments [12,13]. Several examples are reported by Carstensen [1] in his review on the subject where, beside the large space dedicated to the determination of the pH-rate profile, the effect of temperature, ionic strength, buffer concentration, and dielectic constant on the stability of drugs was treated. 

Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

The problem of norbomyl cation stabilities vs. solvolysis rate discrepancies in the norbomyl system has been addressed in an important paper.159 The classical and non-classical norbomyl cations do not resemble the 2-endo- and 2-exo -norbomyl solvolysis transition states very closely. The authors conclude that Brown was wrong, but that Winstein was not entirely right either.159 A substituent in the benzene ring has little effect upon the kinetics of the acid-catalysed hydrolysis of 2-exo-norbomyl phenyl ether.160 The FTIR spectra of matrix-isolated 2-methylbenzonorbomen-2-yl cations have been examined at —196 °C the structure can best be represented as (108), rather like a phenonium cation, but at higher temperatures a transition takes place to a structure that is more nearly represented as (109), with some 71-bridging.161 The stereoselectivities of some 7-methyl-7-norbom(en)yl cations have been investigated (110) has a classical structure and reacts in a stereo-random manner, whereas (111) is... 

The substrate concentration dependence of the reaction rates was investigated kinetically to analyze the substrate binding effect. Figure 4 shows the relationships between the hydrolysis rate of amylose in the presence of the random copolymer catalyst and the concentration of the substrate at some reaction temperatures. The reaction rate clearly showed the saturation phenomenon at each reaction temperature. If the reaction proceeds via complex formation between catalyst and substrate, the elementary reaction could be described in the most simplified form as... 

In a review of nucleophile isotope effects in chemistry, the hydrolysis of formates was discussed.10 The effect of dioxane on the acid-catalysed hydrolysis of ethyl formate was studied by carrying out the reaction in 0-80% (v/v) dioxane at different temperatures ranging from 20 to 40 °C. It was proposed that up to 1.5 mol of water are associated with the activated complex.11 Kinetic studies of the alkaline hydrolysis of ethyl decanoate12 in DMF-H2O solutions and of ethyl isovalerate13 in aqueous acetone were reported. 

Kinetic studies have been reported of the reactions of a series of 2-substituted-5-nitrothiophenes (substituent = Br, OMe, OPh, OC6H4-4-NO2) with secondary amines in room-temperature ionic liquids. The kinetic behaviour is similar to that of the corresponding reactions in methanol so that most reactions do not show base catalysis. The observation that reactivity is higher in the ionic liquids than in methanol (or benzene) is attributed to relatively poor solvation of the reagents by the ionic liquids. As in conventional solvents, 2-bromo-3-nitrothiophene shows higher reactivity than 2-bromo-5-nitrothiophene.42 Solvent effects on the kinetics of the alkaline hydrolysis of 2-phenylthio-3,5-dinitropyridine in aqueous organic solvents have been analysed.43... 

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