Hydrolysis rate, effect

Steric and inductive effects determine the rate of formation of the pentacovalent siUcon reaction complex. In alkaline hydrolysis, replacement of a hydrogen by alkyl groups, which have lower electronegativity and greater steric requirements, leads to slower hydrolysis rates. Replacement of alkyl groups with bulkier alkyl substituents has the same effect. Reaction rates decrease according to ... 

Hydrolysis of TEOS in various solvents is such that for a particular system increases directiy with the concentration of H" or H O" in acidic media and with the concentration of OH in basic media. The dominant factor in controlling the hydrolysis rate is pH (21). However, the nature of the acid plays an important role, so that a small addition of HCl induces a 1500-fold increase in whereas acetic acid has Httie effect. Hydrolysis is also temperature-dependent. The reaction rate increases 10-fold when the temperature is varied from 20 to 45°C. Nmr experiments show that varies in different solvents as foUows acetonitrile > methanol > dimethylformamide > dioxane > formamide, where the k in acetonitrile is about 20 times larger than the k in formamide. The nature of the alkoxy groups on the siHcon atom also influences the rate constant. The longer and the bulkier the alkoxide group, the lower the (3). 

GB is unstable in the presence of water. Maximum stability in aqueous solutions occurs from pH 4.0—6.5 with the hydrolysis rate increasing as the pH increases. The half-life in distilled water at 25°C is ca 36 h, but hydrolysis is accelerated in the presence of acids or bases. Because bases are far more effective in this respect than acids, caustic solutions are useful for decontamination. 

Steiic effects of another kind become important in highly branched substrates, in which ionization is facilitated by relief of steric crowding in going from the tetrahedral groimd state to the transition state for ionization. The ratio of the hydrolysis rates in 8OV0 aqueous acetone of t-butyl /F-nitrobenzoate and 2,3,3-trimethyl-2-butyl p-nitrobenzoate is 1 4.4. 

The hydrolysis of aspirin [Eq. (6-70), R = CH3] is a classic example demonstrating a sigmoid pH-rate effect. Figure 6-13 shows this curve for trimethyl-... 

In the acidic and alkaline hydrolysis rates of the same ester, the steric and resonance effects. re the same. 

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. 

This aromatic alcohol has been an effective preservative and still is used in several ophthalmic products. Over the years it has proved to be a relatively safe preservative for ophthalmic products [138] and has produced minimal effects in various tests [99,136,139]. In addition to its relatively slower rate of activity, it imposes a number of limitations on the formulation and packaging. It possesses adequate stability when stored at room temperature in an acidic solution, usually about pH 5 or below. If autoclaved for 20-30 minutes at a pH of 5, it will decompose about 30%. The hydrolytic decomposition of chlorobutanol produces hydrochloric acid (HC1), resulting in a decreasing pH as a function of time. As a result, the hydrolysis rate also decreases. Chlorobutanol is generally used at a concentration of 0.5%. Its maximum water solubility is only about 0.7% at room temperature, which may be lowered by active or excipients, and is slow to dissolve. Heat can be used to increase dissolution rate but will also cause some decomposition and loss from sublimation. Concentrations as low as 0.125% have shown antimicrobial activity under the proper conditions. 

Different effects of formaldehyde on the hydrolysis of urea are reported. On the one hand, Garrido and colleagues,3 applying anoxic conditions, observed that an inhibitory effect started at 50 mg/L formaldehyde and the levels of inhibition were 50% and 90% for concentrations of formaldehyde of 100 mg/L and 300 mg/L, respectively. Similar effects were found by Campos and colleagues,33 working with an anoxic USB, who observed that formaldehyde concentrations in the reactor of 250 to 300 mg/L caused an inhibition of around 53%. This inhibition on the ureolytic activity was also reported by Walker.36 On the other hand, Eiroa and colleagues37 carried out batch assays at different initial urea concentrations from 90 to 370mg/L N-urea in the presence of 430 mg/L formaldehyde. They observed that a complete hydrolysis was achieved and initial urea hydrolysis rates remained constant. 

The reaction of cyclohexaamylose with a series of p-carboxyphenyl esters is an example of a decelerating effect which may be clearly attributed to nonproductive binding. Rate effects imposed by cyclohexaamylose on the hydrolyses of three such esters are summarized in Table IX. As the hydrophobicity of the ester function is increased by alkyl substitution, the hydrolysis is inhibited the stability of the inclusion complex, on the other... 

The rate effects imposed by this derivative, however, are dependent on the structure of the substrate. For example, the hydrolysis of 8-acetoxy-5-quinoline-sulfonate (AQS), a large substrate which cannot be included within the cyclohexaamylose cavity, is not enhanced by this derivative. Moreover, in contrast to the effects of unmodified cycloamyloses on the hydrolyses of nitrophenyl acetates, the rate accelerations imposed by this... 

Dissolve the toxin to be conjugated in 0.1M sodium phosphate, 0.15 MNaCl, pH 7.5, at a concentration of lOmg/ml. Some protocols use as an SPDP reaction buffer, 50mM sodium borate, 0.3 M NaCl, 0.5 percent n-butanol, pH 9.0. Both buffer systems work well for the NHS ester modification reaction, although the pH 9 buffer is at the higher end of effective derivatization with active esters, since the hydrolysis rate is dramatically increased at this level of alkalinity. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

In the discussions of micellar effects thus far there has been essentially no discussion of the possible effect of micellar charge upon reactivity in the micellar pseudophase. This is an interesting point because in most of the original discussions of micellar rate effects it was assumed that rate constants in micelles were affected by the presence of polar or ionic head groups. It is impracticable to seek an answer to this question for spontaneous reactions of anionic substrates because they bind weakly if at all to anionic micelles (p. 245). The problem can be examined for spontaneous unimolecular and water-catalysed reactions of non-ionic substrates in cationic and anionic micelles, and there appears to be a significant relation between reaction mechanism and the effect of micellar charge upon the rate of the spontaneous hydrolysis of micellar-bound substrates. 

Deacylation or hydrolysis of chiral carbamates, carbonates and alkanoates Micelles and comicelles of N-hexadecyl-N-methylephedrinium bromide or N -myristoyl-histidine with CTABr. Rate effects and enantioselectivities examined Fomasier and Tonellato, 1984... 

Fig. 7.—Effect of the volume of the substituent at carbon atom six on the hydrolysis rate of vanillin /S-D-glucopyranoside (1.04 X 10 M cone.) by /3-glucosidase.41...

The effectiveness of micellar control on the rate and stereochemical course of hydrolysis at a saturated carbon atom was found to be fairly striking. Chiral 1-methylheptyl trifluoromethanesulfonate [45] undergoes hydrolysis via alkyl-oxygen bond fission, and the hydrolysis rate was only 1/300 (for CTAB) or 1/350 (for SDS) as fast as the rate in pure water (Okamoto et al., 1975). Interestingly, the 2-octanol formed shows net inversion (70%) in a nonmicellar... 

In natural waters, the base-catalyzed hydrolysis rate of a weakly HS-as-sociated pollutant (e.g., Parathion) was not significantly affected by HS, while for more strongly associated pollutants (e.g., DDT) the effect of HS was clearly potentially significant in this reaction. 

M. Narisada, T. Yoshida, M. Ohtani, K. Ezumi, M. Takasuka, Synthesis and Substituent Effects on Antibacterial Activity, Alkaline Hydrolysis Rates, and Infrared Absorption Frequencies of Some Cephem Analogues Related to Latamoxef (Moxalactam) , J. Med. Chem. 1983, 26, 1577-1582. 

M. Narisada, J. Nishikawa, F. Watanabe, Y. Terui, Synthesis and 3 -Substituent Effects of Some 7a-Methoxy-l-oxacephems on Antibacterial Activity and Alkaline Hydrolysis Rates , J. Med. Chem. 1987, 30, 514-522. 

The exceptional reactivity of aflatoxin B1 exo-8,9-epoxide raises the question of its potential detoxification by EHs. Despite the short half-life, the epoxide does react with DNA (toxification) and glutathione 5-transferases (detoxification), but a role for EH appeared dubious [207], Rat liver or recombinant rat EH has since been shown to provide a modest enhancement of up to 22% in the hydrolysis rate of aflatoxin B1 exo-S,9-epoxide, and to decrease somewhat the genotoxicity of aflatoxin B1 when the ratio of EH to cytochrome P450 is high (ca. 50-fold). Purified human EH provided no such enhancement in hydrolysis, nor did it have a clear effect on genotoxicity. Thus, little evidence exists to support a role for EH in the detoxification of aflatoxin B1 [208],... 

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