Pseudo first-order rate constant esters

Table 9. Pseudo-first-order rate constants for the release of p-nitrophenol in the reactions of optically active esters in a CTAB micelle...

Pseudo first-order rate constants (105 A /s-1) for p-nitrophenol release from amino-acid ester hydrobromides (RC00C6H[Pg.412]

The reactions obeyed pseudo-first-order kinetics consistent with a rapid reversible protonation of the substrate, S, at the ester carbonyl followed by a rate-determining decomposition to acetic acid and nitrenium ion according to Scheme 19. In accordance with equation 13, the pseudo-first-order rate constant, k, was shown to be proportional to acid concentration and inversely proportional to the activity of the water/acetonitrile solvent . 

Fig. 10 Concentration dependence of the pseudo first-order rate constants in a 100 mM phosphate buffer at pH 7.5 and 37 °C for a niunber of surface active betaine esters with hydrophobic tails of different sizes. For comparison, the rate constant for a non-siuface active compound (ethyl betainate) is included, (o) Oleyl betainate, ( ) tetradecyl be-tainate, ( ) dodecyl betainate, ( ) decyl betainate, (0) ethyl betainate. All compounds have chloride counterions...

Bromination of Beta Keto Esters. The work of K. A. Pedersen on the bromination of beta keto esters in the presence of copper(II) and other divalent metal ions provides several examples of reactions proceeding via complexes (29), The complexes provide an alternative and much more rapid route for the bromination reaction. These reactions are accelerated by bases which take up a proton from the beta keto esters. For such substrates, e.g. ethyl acetoacetate, the general expression for the pseudo first order rate constant in the presence of copper has the form ... 

To compare the catalytic effectiveness of our polymer with that reported for other substances that accentuate nitrophenyl ester cleavage, we26 have carried out a series of experiments (at pH 7.3) in which the residue molar concentration of polymer imidazole groups was substantially in excess of the concentration of substrate, p-nitrophenol acylate. Pseudo-first-order rate constants k[ were determined at each of a number of polymer concentrations. Under these conditions k[ was found to be linear with [P-Im]0, the initial residue concentration of methylene-imidazole groups ... 

The volume of OH- added gives a measure of how much ester has been hydrolysed at various times throughout the reaction, and enables the pseudo-first order rate constant /clsl to be found, from which the second order kobs can be calculated. 

The pseudo-first-order rate constant for the hydrolysis of the 14C-labelled methyl ester 199 at 135 °C with HBr was 6 x 10 2 min 1, tV2 - 11.2 min. A linear plot between -In (A - At) and time (min) has been obtained. (At is the radioactivity of the product at a given time, while the radioactivity recorded after 2 h hydrolysis has been taken as A ). 

Albery and Bell found that, if is identified with the difference in pseudo first-order rate constants in HCIO4 and HCl divided by the concentrations of acid and chloride ions, a plot of log against the square of the mean ionic activity coefficient of HCl, /, gives a straight line. This is nicely consistent with the above formulation if f jf can be expected to be fairly independent of acid concentration. In fact, this is quite plausible over the range of acid concentration considered, since both diazoacetic ester and its transition state for reaction with chloride are dipolar species of comparable size (1 and 2). 

Hydrolysis by all three mechanisms can occur simultaneously and independently for some classes of compounds such as esters (Fig. 2-32) therefore, it is possible to define an overall conditional pseudo-first-order rate constant kj,... 

A, A, and A represented the absorbancies at initial time, at completion of the reaction, and at the time of measurement, respectively, k was considered to be a pseudo first-order rate constant defined by the expression -dc/dt = k C(C/C ), where C/Cq represents the relative reactivity of the ester groups on a pol3nner chain as a function of extent of reaction. A reasonably good fit to this was obtained for reactions conducted in the absence of catalysts as well as in those catalyzed by N-methyl-imidazole and poly-(N-vinylimidazole). 

Alkyl halides are even less reactive than acyl halides, as indicated by the compilation of reaction rates of thiolate anions with various types of alkyl halides (282). Nevertheless, potentially useful affinity labels have been synthesized with alkyl halide substituents and have been shown to specifically inactivate several enzymes, albeit slowly the low reactivity of the alkyl halides may minimize nonspecific reaction. Adenosine 5 -(2-bromoethyl)phosphate has been characterized and reported to inactivate NAD -dependent isocitrate dehydrogenase (283). The 2 - and 3 -(2-bromoethyl)-AMP labels have also been synthesized, and model reactions of the bromoethyl-AMPs with cysteine, lysine, histidine, and tyrosine have been studied (284). More recently, esters of adenosine 5 -monophosphate have been prepared with ethyl, propyl, or hexyl moieties and bromo or chloro substituents at the w position (285). Yeast alcohol dehydrogenase exhibited enhanced inactivation by the hexyl derivative, but inactivation rates of other dehydrogenases were unremarkable. Two iodopropyl derivatives of cAMP have been described, namely, 1, A -(3-iodopropyleno)adenosine 3, 5 -cyclic monophosphate and 3 -0-(2-iodo-3-hydroxypropyl)adenosine 3, 5 -cyclic monophosphate the latter inactivates cAMP phosphodiesterase from human platelets, with a pseudo-first-order rate constant of 0.147 hr" (286). 

Pseudo-first-order rate constants (10 k/sec) for >-nitrophenol release [72] from amino acid p-nitrophenyl ester hydrobromides, RC02C6H4-/ -N02-Br ... 

Pseudo-first-order rate constants and relative rates for the release [75] of / nitrophenol from amino acid and dipeptide ester hydrobromide substrates in the presence of l,l-l,l,l,i,-(60) and l,l-l,l,l,l-(60) plus excess of KBr at 20°C... 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

By plotting the log (% residual ester) vs. time, a straight line was obtained corresponding to the pseudo first order rate constant. Concurrently by multiplying the slope of the line by —2.303, the rate constant, k, was obtained. 

Sufllce it to state that in contrast to 22a the imidazolyl-substituted cyclophanes 22b and 22 c indeed catalyze the hydrolysis of aliphatic carboxylic activated esters, 22 c being a little more effective. The efficiency as represented by the ratio of pseudo first order rate constants of the catalyzed versus the uncatalyzed reaction k Jk a displayes a sharp maximum with the ester of tetradecanoic acid [22c] 100), whereas... 

Kinetic Measurement. The hydrolysis of p- and m-nitrophenyl acetate was followed by measuring the absorbance at 400 nm with a JASCO UVIDEC-1 spectrophotometer. The reaction was initiated by addition of 15 yl of a stock solution of the ester in acetonitrile to 3.0 ml of Tris-HCl buffering solution. The pH of the solution was 9.10. The final concentration of nitrophenyl ester was 2.5xl0" M. The reaction temperature was controlled at 30.0 0.5°C. Plots of log(Aar-A) Vs. time for the reaction in the absence and the presence of 1, 2 and y-cyclodextrin gave straight lines. The pseudo-first-order rate constants were calculated from the plots. The rate of hydrolysis was measured to at least 20% completion of the reaction. The rate constants reported are averages of the values in three or four runs which agreed within 5%. After the kinetic measurement, it was determined by analytical HPLC that the tosyl moiety attached at the CD was not decomposed. 

These CD-based polymers were engaged in hydrolysis reaction of various p-nitrophenol esters derivatives. The reaction was monitored by UV at 400 mn. Among the CD-based polymers, only poly- 3-CD-A showed a high hydrolysis activity. The pseudo first-order rate constant k measured for the ester hydrolysis catalyzed by poly- 3-CD-A is compared to the rate constant measured using the native 3-CD. The rate enhancements resulting from the use of poly- 3-CD-A are presented in Table 2.1. 

FIG. 8 Hydrolysis of betaine esters at pH 7.5. k is the pseudo-first-order rate constant and c is the concentration of betaine ester. 

The pH-rate profile of the methyl ester of aspirin has three regions, with slopes -1, zero and +1, and the molecule does not contain ionisable groups. By analogy with Figure 13.4, the pseudo-first-order rate constant must have the form... 

A search of the literature on the reactions of primary and secondary amines with esters, amides, and imides reveals that generally SB-catalyzed aminolysis term in the rate law appears only when the rate law contains an additional GB-catalyzed term. It is also quite conunon that the rate law contains only a GB-catalyzed term. But it is almost rare or nonexistent that a rate law does not contain a GB-catalyzed term but contains an SB-catalyzed term. These general observations can be easily understood in terms of a stepwise mechanism similar to Scheme 2.32 with HNu = RNH2. However, a recent report on the kinetic studies of the cleavage of A-phthaloylglysine (46) in the buffers of hydrazine shows that pseudo-first-order rate constants (k bs) for the cleavage of 46 follow Equation 2.40 ... 

Fife et al. have reported intramolecular GB-assisted ester hydrolysis. The pseudo-first-order rate constants (k bj.) are pH-independent from pH 8 to pH 4. The triflnoroethyl, phenyl, and p-nitrophenyl esters of 2-aminobenzoic acid hydrolyze with similar rate constants in the pH-independent reactions, and the rates of these water reactions are twofold slower in D2O than in HjO. The most likely mechanism involves the intramolecular GB assistance by the neighboring amino group. The rate enhanc ents in the pH-independent reaction in comparison with the pH-indepen-dent hydrolysis of the corresponding para-substituted esters are 50- to 100-fold. 

Most of the studies on the effects of metallomicelles on the rate of hydrolysis of esters involve so-called activated esters in which nucleophilic attack is the rate-determining step. The effects of copper-containing metallomicelles (formed from both copper(II)-hydrophobic ligand complex as well as from hydrophobic ligand [L = Af,Af,M-trimethyl-Ai -tetradecylethylenediamine] containing free ions) on the rate of hydrolysis of amides 19 and 20 as well as activated esters 21, 22, and 23 have been stndied at pH 7.0 and 31°C. The apparent rate enhancements (kj i) of Cn +(L) metallomicelles on the rate of hydrolysis of 19 to 23, 2-nitrophenyl acetate (2-NPA), and 4-nitrophenyl acetate (4-NPA) under various reaction conditions are summarized in Table 6.4." The actual rate enhancements due to metallomicelles, Cu +(L), under various reaction conditions are not possible to estimate because of the lack of pseudo-first-order rate constants (kobs) for hydrolysis of 19 to 23, 2-NPA, and 4-NPA in the presence of Cu -A7V,A 7V -tetramethylethylenediamine complex. However, the values of k,, for 19 and 20 are almost same at 0.002-M Cu +(L) and 0.002 M Cu +(L) + 0.001-M Triton. The presence of 0.001-Af CTABr comicelles has no effect on k, for 20 but decreases kj i from 46 to 29 for 19 (Table 6.4), which may be attributed to larger cationie mixed micellar affinity of anionic 19 than that of neutral 20. 

Pseudo-first-order rate constants for the alkaline hydrolysis of these esters in the presence of a series of varying concentrations of caffeine and theophylline-7-acetate obey Equation 7.48 with [M XJ replaced by [caffeine] or [theophylline-7-acetate]. 

Pseudo-first-order rate constants (k<,bj.) for aminolysis of amide, imide, and ester groups in buffers of anunes follow Equation 7.52... 

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