Acidic hydrolysis reaction rates compared

With such Co(III) complexes, rate enhancement of 10 have been observed relative to alkaline hydrolysis of the amide function. Such acceleration of hydrolysis reactions is comparable to rates obtained with carboxypeptidase A for its substrates. Notice that in process A, which is believed to be favored over process B with Co(III) ion, the carbonyl group becomes polarized and susceptible to external water attack much more so than in the noncomplex form. Thus the metal ion plays again the role of a super acid. In other words,... 

R = Me, Et, and PhCH2, respectively Fig. 8.1). In 80% human plasma at pH 7.4 and 37°, these model prodrugs were hydrolyzed with tm values of 3.5, 16, and 2.6 min, respectively [59]. Such rates of enzymatic hydrolysis are comparable to those of various carbamoylmethyl esters of benzoic acid (Table 8.2). It is important to note that the direct liberation of benzoic acid by Reaction a (Fig. 8.1) was severalfold faster than the competitive Reaction b. Reaction c was very slow in human plasma (tm > 100 h). In HO -catalyzed hydrolysis, the opposite regioselectivity was seen, with the terminal ester bridge being cleaved markedly faster than the central one. No data appears to be available on chemical hydrolysis at neutral pH. 

Although there are fewer studies of the heterogeneous chemisty of BrONOz and HOBr than of the corresponding chlorine compounds, it is clear from the laboratory studies that have been done that analogous chemistry occurs, and at least as fast as for the chlorine compounds. Table 12.8 shows some of the most important reactions and typical values of the reaction probabilities. On ice, the hydrolyses of CIONO, and BrONOz proceed at comparable rates (Tables 12.5 and 12.8). However, toward midlatitudes the particles are largely concentrated sulfuric acid-water mixtures, and on this surface the C10N02 hydrolysis reaction probability... 

When comparing the hydrolysis rate constants of a series of carboxylic acid esters (Table 13.8), it can be seen that the values for the acid-catalyzed reactions are all of the same magnitude, whereas the rate constants for the base-catalyzed reactions vary by several orders of magnitude. Explain these findings. 

One of the characteristics of the acid-catalyzed hydrolysis of esters, that is shared by ester formation also, is that substituent effects on the rate coefficients are small, and not simply related to a values (see below, p. 131). The data in Table 14 show that this is also true for the, sO-exchange reaction of substituted benzoic acids. This is borne out by the relative constancy of the ratio khyJkexch for the different substituted acids it was not possible to obtain a meaningful p value from the data of Table 14, because of the small number of points and the large amount of scatter evident on the Hammett plot. Mesitoic acid is highly unreactive, compared with the m- and p-substi-tuted esters used, as is its methyl ester towards alkaline hydrolysis138, and presumably reacts by the seriously hindered Aac2 route. 

A re-investigation of the mechanism of the hydrolysis of acetic anhydrides in water was made by Bunton and Fendler4. They found that the hydrolysis of trimethylacetic anhydride follows an A-2 mechanism both in water and in aqueous dioxan and is slower than that of acetic anhydride. If acetic anhydride follows an A-l mechanism in water, both anhydrides should have the same mechanism and both should have similar reactivities in aqueous acids. If the A-2 mechanism is followed, steric effects should make trimethylacetic anhydride the less reactive compound. The entropy of activation of the hydrolysis catalysed by perchloric acid was re-estimated, taking into account the electrolyte effect of the perchlorate ions upon the rate of spontaneous hydrolysis, and a new value for A5 of—35 eu (for the spontaneous hydrolysis) and — 10 eu at 2 M HC104 for the acid-catalysed hydrolysis was obtained, compared with the value of + 2.2eu obtained previously. The new value is in the order of magnitude of A 5 for A-2 reactions but smaller than that observed for trimethylacetic anhydride under similar acidic conditions (— 26 eu). Plots... 

Lipases are the most common enzymes used in non conventional media like organic solvents and supercritical carbon dioxide. Lipases usually hydrolyse fats into fatty acids and glycerol. The special property of lipases is their ability to act at the interface between water and oil. In these experiments lipase (EC 3.1.1.34) from Rhizopus arrhizus (Boehringer Mannheim) was used to investigate the effects of lipase under hydrostatic pressure. The analysed reaction was the hydrolysis of p-Nitrophenyllaureate at different concentrations at 35 °C. The dependance of the kinetic constants between 1 bar and 3000 bar is presented in table 2. Like the thermophilic GDH at 1000 bar lipase is activated under pressure as well. The initial reaction rate increases by a factor of 1.5 at 1000 bar compared to the initial reaction rate at ambient... 

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