Decarboxylation rate

The first quantitative study of the reaction was carried out with anthracene-9-carboxylic acid (which possesses the necessary steric requirement by virtue of the peri-hydrogen atoms, and is very reactive at the 9 position towards electrophilic substitution). Schenkel632 found that the decarboxylation rate was increased in the presence of acid, and the first-order rate coefficients (believed to be in sec-1) are given in Table 205. It was subsequently concluded that in the absence of acid,... 

The rate of decarboxylation of benzoylacetic acid is also accelerated in mixed dioxane-water (Hay and Tate, 1970) and acetonitrile-water (Straub and Bender, 1972) solutions. In no case, however, has an acceleration greater than 2.5 been observed. In aprotic solvents, such as benzene, decarboxylation rates are depressed (Swain et al., 1961). 

At alkaline conditions (pH=l 1) no phenol, hydroquinone were detected. This is possibly due to the fact that the rate of phenol oxidation increases under alkaline conditions with optimum pH between 9.5 and 13 [17] and so once it is formed, it is readily oxidised. The absence of phenol and increased concentration of p-hydroxybenzoic acid could be also explained by reduced decarboxylation rates under conditions of high pH, which would result in the oxidation of p-hydroxybenzaldehyde to form p-hydroxybenzoic acid. 

Most importantly, the careful kinetic analysis of the rise and decay of the transient species in equation (69) shows that the decarboxylation of Ph2C(OH)CO occurs within a few picoseconds (kc c = (2-8) x 1011 s-1). The observation of such ultrafast (decarboxylation) rate constants, which nearly approach those of barrier-free unimolecular reactions, suggests that the advances in time-resolved spectroscopy can be exploited to probe the transition state for C—C bond cleavages via charge-transfer photolysis. 

A. P. Zeng, J. Modak, and W. D. Deckwer, Nonlinear dynamics of eucaryotic pyruvate dehydrogenase multienzyme complex Decarboxylation rate, oscillations, and multiplicity. Biotechnol. Prog. 18(6), 1265 1276 (2002). 

Kemp et al., 1978). The rate is slowest in an aqueous solution and is enhanced in aprotic and/or dipolar solvents. The rate augmentation of 106—108 is attainable in dipolar aprotic solvents such as dimethyl sulfoxide and hexamethylphosphoramide (HMPA). Interestingly, the decarboxylation rate of 4-hydroxybenzisoxazole-3-carboxylate [53], a substance which contains its own protic environment, is very slow and hardly subject to a solvent effect (1.3 x 10-6 s-1 in water and 8.9 x 10-6 s-1 in dimethylformamide Kemp et al., 1975). The result is consistent with the fact that hydrogen-bonding with solvent molecules suppresses the decarboxylation. 

The presence of a highly hydrophobic microenvironment is also reflected in the decarboxylation rate of [52] (see Table 4). The rate constant of decarboxylation in the presence of [7] (2 x 10 4 M) is at least 10 times larger than that in the presence of CTAB (1 x 10-3 M). 

Another mechanistically useful nucleophile is acetate ion and related carboxylates. Acetate ion is difficult to oxidize (Eberson, 1963) and reacts with radical cations in a bond-forming reaction (Eberson and Nyberg, 1976). The oxidation product, the acetoxyl radical, has properties which make trapping it very unlikely in that its decarboxylation rate constant is 1.3 X 109 s 1 (Hillborn... 

Table X summarizes the kinetic parameters derived from the decarboxylation rates of nitrobenzisoxazole carboxylate catalyzed by each of four different lauryl polyethylenimines.

Furthermore, the polymer is an effective catalyst at concentrations of the order of 10 6 M, corresponding to catalytic site concentrations of about 10 4 M. This means that the decarboxylation rate can be enhanced more than 1000 times by addition of 5x10 6 M polymer to the aqueous solution. In contrast, for micellar systems the maximum enhancement reported is at best 100-fold.49... 

Values of the Acid Dissociation Constant (K) and Ring-Opening Rate Constant (k2) of [L4Co(02COH)]2 + and the Decarboxylation Rate Constant (k2) of the Corresponding Monodentate Bicarbonato Complexes (at 25.0 °C,... 

These equations are in line with Eq. (30), such that kx denotes the ring-opening rate constant of the protonated carbonato complex and / 2 is the decarboxylation rate constant of the ring-opened bicarbonato complex. Values of these rate constants and the acid dissociation constants of some protonated carbonato complexes of cobalt(TII) (see Table III) reflect the ligand dependence with respect to charge variations, steric constraint, and donor properties of the non-labile ligands. 

Orotidine 5 -phosphate undergoes an unusual decarboxylation (Fig. 25-14, step/), which apparently is not assisted by any coenzyme or metal ion but is enhanced over the spontaneous decarboxylation rate 1017-fold. No covalent bond formation with the enzyme has been detected.268 It has been suggested that the enzyme stabilizes a dipolar ionic tautomer of the substrate. Decarboxylation to form an intermediate ylid would be assisted by the adjacent positive charge.269,270 Alternatively, a concerted mechanism may be assisted by a nearby lysine side chain.270a d Hereditary absence of this decarboxylase is one cause of orotic aciduria. Treatment with uridine is of some value.271... 

It was reported that the decarboxylation rate of l-hydroxytetrazole-5-carboxylic acid 363 is considerably higher than that of isomeric 2-hydroxytetrazole-5-carboxylic acid 360 <1999TL6093>. 

Our final reaction is an instructive example of specific anion solvation in a non-SN reaction. The decarboxylation rate of 6-nitrobenzisoxazole-3-carboxylate as the tetra-... 

Benzoic acid and most mono-substituted benzoic acids are stable with respect to decarboxylation in aqueous solution, even at a temperature of 100 °C. However, decarboxylation may occur with a measurable rate if either strong electron-withdrawing or strong electron-releasing substituents are present in the aromatic acid. The decarboxylation rate of 2,4,6-trinitrobenzoic acid is increased by addition of base to the aqueous solution, and it attains a maximum value when the substrate is completely transformed to the anion [236]. A carbon-13 isotope effect of ft, 2/ft, 3 = 1.036 (50 °C) has been observed [237]. There is no D20 solvent isotope effect [238]. These findings indicate that the mechanism of decarboxylation of 2,4,6-trinitrobenzoic acid is a unimolecular electrophilic substitution (SE1), viz. 

Matters are different with aromatic acids carrying electron-releasing substituents as their decarboxylation rates are increased by addition of dilute strong acid. Consequently, another mechanism must be operating. 

The pH dependence of the decarboxylation rate of some aromatic amino acids has been studied in dilute solutions of strong acids and in acetate buffers, at a constant ionic strength of ju = 0.1 N. The results can be fitted into a rate equation with two terms, viz. 

It is to be expected that the solvent should exert a strong influence on the decarboxylation rate via stabilization of (zwitter)ionic structures (II and IV). As mentioned above [226], in water no decarboxylation was observed from the radical zwitterion of 4-methoxybenzoic acid (i.e. A < 10 s ). 

With arylacetoxyl radicals, relatively slower decarboxylation rate constants have been measured (Table 11), pointing in this case towards higher activation barriers. Two main factors have been suggested to account for such behavior ... 

Experimental studies show that decarboxylation rates for acetic acid are extremely sensitive to temperature and the types of available catalytic surfaces. Rate constants for acetic acid decarboxylation at 100 °C differ by more than 14 orders of magnitude between experiments conducted in stainless steel and in catalytically less active titanium (Kharaka et al., 1983 Drummond and Palmer, 1986). Naturally occurring mineral surfaces provide rather weak catalysts (Bell, 1991). Decarboxylation rates calculated from field data indicate half-life values of 10-60 Myr at 100 °C (Kharaka, 1986 Lundegard and Kharaka, 1994). 

The discovery, in these waters, of high concentrations (up to 1 X 10" mg L ) of mono- and dicarboxylic acid anions, phenols, and other reactive organic species has led to numerous field and laboratory studies to determine their distribution and importance in inorganic and organic interactions. The observed concentrations are minimum values, because the organics are degraded by bacteria and by thermal decarboxylation reactions. Decarboxylation rates estimated from... 

Fio. 8. Specific activity of proteinoid on pyruvic acid ae a function of the pH. The decarboxylation rate is expressed as cpm of COa. A, proteinoid B, amino acid mixture composed as proteinoid C, pyruvic acid in aqueous solution without any additives. Taken from Hardebeck et al. 19). 

From decarboxylation rates of pyridazinylacetic acids, it is concluded that decarboxylation takes place preferentially by the zwitterionic mecha-nism. ° Pyridazine catalyzes ester hydrolysis. ... 

The decarboxylation rate of 3-carboxybenzisoxazoles depends on the substituent and strongly on the solvent, which is evidenced by the use of the empirical Kamlet-Taft-Abraham solvatochromic parameter set in a multilinear correlation analysis based on LSER, along with a theoretical computational set of molecular parameters <94JCS(P2)1641>. The effect of temperature and evaluation of thermodynamic parameters for the polarographic behavior of 3-methyl-4-(2 substituted benz-eneazo)-2-isoxazol-5-ones are reported <94Mi 303-02). 

Benzylic esters have been studied in considerable detail often as a continuation of the pioneering work by Zimmerman and co-workers (Scheme 2) in 1963 [44]. There are several reasons for this. First, the synthesis of compounds with the structural variables required to probe specific mechanistic questions is often straightforward. Second, products are usually formed from both ion pairs and radical pairs and, therefore, the structural variables that control this partitioning can be systematically studied. Third, the radical pair (ARCH2-O CO)— R) incorporates a built-in radical clock, the decarboxylation of the acyloxy radical, which serves as a useful probe for the reactivity of the radical pair. If the carbon of the acyloxy radical is sp hybridized, this decarboxylation rate is on the 1- to 1000-ps time scale, depending on R, so that decarboxylation will often occur within the solvent cage before diffusional escape. The topic of benzylic ester photochemistry has been recently reviewed twice by Pincock [5,98] and therefore only a brief summary will be given here. 

Decarboxylation. Most of the amino acids commonly found in proteins decompose by a slow, irreversible decarboxylation. The kinetics of decarboxylation of several amino acids have been studied by Abelson (45, 46), Conway and Libby (27), and Vallentyne (47, 48). The decarboxylation rates were determined by heating unbuffered aqueous solutions of the amino acids at elevated temperatures (between 100° and... 

In less than one minute, half of the 2,4-dihydroxybenzoic acid is decomposed already at 160 °C in the microreactor setup [39, 40]. This demonstrates the need to find optimal process parameters for temperature and residence time to achieve efficient high-p,T operation, which is achieved by detailed parameter variation (e.g., as statistical analysis. Design of Experiments, DoE). In this way, the best operation point was foimd (200 °C 40 bar 2000 mL h 16 s). There is also the question of substrate selectivity, e.g., the isomeric product 2,6-dihydroxybenzoic acid exhibits lower decarboxylation rates. 

While experimental studies of organic acid decarboxylation have established some of the controls, the relevance experimentally determined rate constants for natural systems, where potential catalysts of many types abound, is questionable. Field calibrations clearly are advantageous from the standpoint of eliminating the effects of kinetic artifacts, but other Kmitations exist. In trying to relate the effects of time and temperature on decarboxylation rates in natural systems, the effects that variations in the type and abundance of organic matter can have on the production of organic acids, and therefore on their primary concentration, must be minimized. 

The following sequence of dipositive metal ions shows a decreasing effect on the rate of decarboxylation of oxaloacetic acid Cu(II), Zn(II), Co(II), Ni(II), Mn(II), Cu(II) (91). The rate constants for these decarboxylations approximately parallel the formation constants of the corresponding metal oxalates. A similar result was found in the decarboxylation of acetonedicarboxylic acid in the presence of certain transition metal ions the decarboxylation rates paralleled the formation constants of the metal malonates (170). These parallelisms indicate that the effectiveness of a metal ion in these decarboxylation reactions depends on its ability to chelate with the oxalate ion and the malonate ion, which resemble the transition states of the oxaloacetic and acetonedicarboxylic acids, respectively. 

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