First-order rate constant esters

Some authors use O] instead of cr as the substituent constant in such correlations.) An example is provided by the aminolysis of phenyl esters in dioxane the substrates RCOOPh were reacted with -butylamine, and the observed first-order rate constants were related to amine concentration by = k2 [amine] kj [amine]. The rate constants kz and k could be correlated by means of Eq. (7-54), the reaction constants being p = +2.14, b = + 1.03 (for A 2) and p = -1-3.03,8 = -1-1.08 (for ks). Thus, the two reactions are about equally sensitive to steric effects, whereas the amine-catalyzed reaction is more susceptible to electronic effects than is the uncatalyzed reaction. 

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

The corresponding methyl ester similarly underwent the facile rearrangement. A solvent polarity study on the rearrangement rate of the allyl vinyl ether was conducted in solvent systems ranging from pure methanol to water at 60°C.156 The first-order rate constant for the rearrangement of the allyl vinyl ether in water was 18 x 10-5 s 1, compared with 0.79 x 10-5 s 1 in pure methanol. 

Worked Example 8.7 Methyl ethanoate is hydrolysed when dissolved in excess hydrochloric acid at 298 K. The ester s concentration was 0.01 mol dm-3 at the start of the reaction, but 8.09 x 10 2 after 21 min. What is the value of the first-order rate constant k ... 

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

Other 4-nitrophenyl esters have also been reported to be substrates of various hydrolases. For example, 4-nitrophenyl hexanoate (7.19) was hydrolyzed by bovine serum albumin [39], The affinity of the substrate for the macromolecule was found to be high (Km/n = 0.040 mM, where n is the number of sites), but the reaction itself was slow ( = 5 10-3 s-1, where k2 is the first-order rate constant of the formation of the phenol product from the enzyme-substrate complex). Another ester, 4-nitrophenyl pivalate (7.20), was hydrolyzed by cytoplasmic aldehyde dehydrogenase at a maximum velocity ca. 1/3 and an affinity ca. 1/20 those of the acetate [40], However, the rate-limiting steps were different for the two substrates, namely acylation of the enzyme for the pivalate, and acyl-enzyme hydrolysis for the acetate (see Chapt. 3). 

First-Order Rate Constants for Amine Acylation by p-Nitrophenyl Esters"- ... 

Non-linear kinetics have been reported for aminolysis of sulfamate esters RNHSO2ONP (Np=/7-N02C6H4) in chloroform. The first-order rate constants obs for reaction with imidazoles (primarily) under pseudo-first-order conditions display saturation curvature with increasing amine concentration, according to the expression... 

We determined the first-order rate constants for the disappearance of diazo ester 34 with the several rhodium] 11) catalysts. The rate of disappearance of the diazo ester 34 upon exposure to each of the catalysts was monitored at 27 1°C, by following the UV absorbance at 2=265 run. The decrease in absorbance of the starting material was plotted versus time. The approximately linear portion of this direct plot, from 80% to 20% of the absorbance, was used to calculate the first-order rate constant for the disappearance of the diazo ester (Tab. 16.4). 

The observed first-order rate constants, Kobs (s ) for the reaction of the rhodium] 11) complexes with diazo ester 34 varied over a range of > 10, in which the pivalate catalyst (entry 3) was almost two orders of magnitude faster than any of the other catalysts studied. The carboxamidate catalysts (entries 8-10) were slower than all the carboxy-lates, while the bridged phosphine catalyst (entry 7) behaved like most of the other car-boxy late s. 

Tab. 16.4 Observed first-order rate constants for the insertion to elimination ratios for the reaction of the rhodium(II) catalysts with diazo ester 32.

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...

In 1984 Gassman and Granrud, and Novak and co-workers, published their results on reactions of similar esters of hydroxamic acids. Gassman and Granrud showed that the rearrangement of the methanesulfonate esters 45a-g in CDCI3 (Scheme 23) occurred in a first-order fashion and that kobs correlated with with a slope,, of -9.2. Novak and co-workers showed that the pH-independent first-order rate constants for hydrolysis in 5 vol%... 

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 ... 

Table II lists first-order rate constants, corrected for hydrolysis of ester in buffer alone. Propylamine served as a reference amine in its presence k (in min-1) for aminolysis decreased progressively from 0.98 x 10-2 to 0.51 x 10-2 to 0.05 x 10-2 as the length of the acyl group increased from 2 to 12 carbons (see Table II). The sharp drop for nitrophenyl laurate may be the result of micelle formation23 even at concentrations of 6x 10-6 M...

With both substrates, cleavage of salicyl esters is markedly accelerated in the presence of polyethylenimines. The first-order rate constant k2 in... 

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 ). 

In Fig. 11, at high concentrations of ethylene carbonate, the rate constants ks[EC] and kR[EC] for insertion into the EBTHI zirconaaziridine 17q are much greater than kSSR and ksss and insertion occurs more rapidly than the equilibrium can be maintained. The product ratio reflects the equilibrium of 17q, where Keq is 17.2 (Eq. 32) [21]. Beak has called this limit a dynamic thermodynamic resolution pathway [66]. In contrast, at the lowest concentration of ethylene carbonate in Fig. 10, the first-order rate constants kSSR and ksss for diastereomer interconversion are comparable to the effective first-order rate constants for insertion. As Keq is known to be 17.2, ks/kR can be calculated the 53% ee of (S)-amino acid ester 19q (Scheme 9) implies that kslkR<0.19 (Eq. 33) and that the rate constant for insertion kR[EC] into the minor diastereomer is at least five times faster than ks[EC] into the major diastereomer. 

The small inverse dependence of the first-order rate constant on solvent polarity is in agreement with a concerted electrocyclic ring cleavage through an isopolar activated complex to vinylketene, which is converted into the corresponding ester in alcoholic solvents [158]. 

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,... 

Thus, the surface serves to accumulate both the ester (by complex formation and electrostatic attraction) and the nucleophile (by electrostatic attraction alone), facilitating reaction. In fact, the overall first-order rate constant for hydrolysis (Ar ) reflects this concentration effect Atj, reaches its highest value at the pH where the product [>Al-MPT][OH"ld is at a maximum. No other role of the oxide surface in promoting hydrolysis need be postulated. 

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... 

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