Decarboxylation data kinetics

The effect of various metal ions on the rate of acid dicarboxylation has been explored. Since the topic is beyond the scope of this chapter, only a brief comment will be made. Metal ions which enter into complexation equilibrium with nitro-acetate ion retard the rate of decarboxylation The kinetic data were... 

Biocatalytk decarboxylation is a imique reaction, in the sense that it can be considered to be a protonation reaction to a carbanion equivalent intermediate in aqueous medimn. Thus, if optically active compoimds can be prepared via this type of reaction, it would be a very characteristic biotransformation, as compared to ordinary organic reactions. An enzyme isolated from a specific strain of Alcaligenes bronchisepticus catalyzes the asymmetric decarboxylation of a-aryl-a-methyhnalonic acid to give optically active a-arylpropionic acids. The effect of additives revealed that this enzyme requires no biotin, no co-enzyme A, and no ATP, as ordinary decarboxylases and transcarboxylases do. Studies on inhibitors of this enzyme and spectroscopic analysis made it clear that the Cys residue plays an essential role in the present reaction. The imique reaction mechanism based on these results and kinetic data in its support are presented. 

Similarly, vesicular reactivity is dependent on bilayer fluidity and Arrhenius (or Eyring) plots for the decarboxylation of 6-NBIC show a break around Tm. " For the Kemp elimination in different bilayers, it was found that the bilayer for which kinetic data had been gathered below its was least effective as a catalyst. Ester hydrolysis has also been found to be faster above r. For the decarboxylation of 6-NBIC, the increase in catalytic efficiency was attributed to different aggregate surface dynamics based on the observation that vesicles above showed intermediate activation parameters between vesicles below and micelles. One could, of course, discuss causality here considering the fact that many of the bilayer... 

Decarboxylase reaction Kinetic constants The optimum pH of the decarboxylase reaction was determined with the natural substrates of both enzymes, pyruvate (PDC) and benzoylformate (BFD). Both enzymes show a pH optimum at pH 6.0-6.5 for the decarboxylation reaction [4, 5] and investigation of the kinetic parameters gave hyperbolic v/[S] plots. The kinetic constants are given in Table 2.2.3.1. The catalytic activity of both enzymes increases with the temperature up to about 60 °C. From these data activation energies of 34 kj moT (PDC) and 38 kJ mol (BFD) were calculated using the Arrhenius equation [4, 6-8]. 

Diacyl peroxides are another important source of free-radicals and, consequently, are also commonly used as initiators of free-radical reactions. There is a vast amount of data pertaining to the kinetics and mechanism of decomposition of these compounds in conventional solvents there are a number of side reactions, both radical and ionic in nature, that complicate the kinetics of their decomposition. Generally, these compounds decompose by initial 0-0 bond cleavage that generates carboxyl radicals (RC02 ), which subsequently decarboxylate yielding R (Scheme 4.7)... 

The KIEs for scission of the C—C bond to form the enamine (by measuring 45C02/44C02 ratios) of both the model amine and the acetoacetate decarboxylase catalyzed reactions were studied by O Leary and Baughn125. In the primary amine catalyzed reaction the KIE exhibited pH-dependence consistent with the kinetic data above. In the enzyme catalyzed reaction a pH independent k12/k13 KIE of 1.018 was reported, consistent with the idea that a nucleophilic attack by the lysine amino group and decarboxylation are both partially rate-limiting. 

All three enzymes have the same cofactor requirements ferrous ion, a-ketoglutarate, molecular oxygen, and ascorbate (vitamin C). The reducing equivalents required for the hydroxylation reaction are provided by the decarboxylation of equimolar amounts of a-ketoglutarate to succinate and carbon dioxide. One atom of the O2 molecule is incorporated into succinate while the other is incorporated into the hydroxyl group. The data on enzyme kinetics and mechanism of reaction are consistent with the ordered binding of Fe " ", a-ketoglutarate, O2, and the peptide... 

Shown in Fig. 2 are examples of two kinetic traces of transient absorbance difference which reflect formation and decarboxylation of carbonyloxy radicals after photo-induced decomposition of DINPO and TBNC. A detailed understanding of the kinetics is obtained from modeling the data. Within the experimental time resolution (150 fs), peroxide primary dissociation produces carbonyloxy radical intermediates, which decay either directly from an electronically excited state within about 500 fs or in a statistical unimolecular reaction on a ps to ps time-scale in the electronic ground state (see Fig. 3). In the case of DINPO photodissociation at 266 nm, the excited state of the 1-naphthylcarbonyloxy radical is too high energetically to be populated to any relevant extent.The reaction on the ground state PES can be treated by statistical unimolecular rate theory. 

Fig. 3 a Proposed mechanism of ODCase-catalyzed decarboxylation of OMP by 02 protonation. Both the protonation and decarboxylation steps would be expected to be slightly sensitive to isotopic substitution at Nl. b Model reactions used to assess the feasibility of the 02 protonation mechanism, or any mechanism with a pre-decarboxylation step that is isotopically sensitive at Nl, and the measured Nl equilibrium and kinetic isotope effects, a Data from [31]. b Data from [30]. c Model reactions for which Nl equilibrium and kinetic isotope effects were determined using computational approaches, and the computed values [32]... 

The decarboxylation of 2-substituted pyridinecarboxylic acids was studied in ethylene glycol at constant pressure and in sulfuric acid and in ammonium bisulfate. The reactions obeyed apparent first order kinetics. The determined parameters of activation are given in Table X-1. The data were again... 

Experimental data of stearic acid decarboxylation in a laboratory-scale fixed bed reactor for formation of heptadecane were evaluated studied with the aid of mathematical modeling. Reaction kinetics, catalyst deactivation, and axial dispersion were the central elements of the model. The effect of internal mass transfer resistance in catalyst pores was found negligible due to the slow reaction rates. The model was used for an extensive sensitivity study and parameter estimation. With optimized parameters, the model was able to describe the experimentally observed trends adequately. A reactor scale-up study was made by selecting the reactor geometry (diameter and length of the reactor, size and the shape of the catalyst particles) and operating conditions (superficial liquid velocity, temperature, and pressure) in such a way that nonideal flow and mass and heat transfer phenomena in pilot scale were avoided. 

A kinetic study of the decarboxylative debromination of /3-bromo-acids has confirmed its mechanistic assignment to the general class of heterolytic fragmentations. On the basis of data presented, the complete set of products from pyrolysis of a given saturated monoacid calcium salt can be predicted unsaturated and diacid salts give much more diverse and numerous products. Equilibrium constants have been determined in a number of solvents for the ring-chain tautomerism of substituted cw-S-benzoyl and c/j-3-acyl acrylic acids. 

Rate constants and activation parameters are also given for loss of carbon dioxide from the intermediate d5-[Rh(en)2(C03H)(OH2)]. Here the rhodium(III) system has two advantages over its cobalt(III) counterpart, for not only is it possible to obtain kinetic data for the monodentate bicarbonato intermediate but also there is no concomitant isomerization to complicate the carbon dioxide loss kinetics in the rhodium(III) case. In the reverse direction, kinetic parameters were obtained for the reaction of c/5-[Rh(en)2(OH2)(OH)] and of d5-[Rh(en)2(OH)2] with carbon dioxide-(bi)carbonate. The products are cw-[Rh(en)2(C03)(0H2)] and d5-[Rh(en)2(C03)(0H)] there is no mention of a bidentate carbonato product. Kinetics of decarboxylation of the new complexes trans-[Rh(en)2(C03)(0H2)] and rra 5-[Rh(en)2(C03)2]" have been reported, with values for rate constants and activation parameters given. The bis(carbonato) complex loses both carbonate ligands at low pHs, but only one carbonate when pH > 6.5. The kinetics of the reverse carbonate-formation reactions, from trans-... 

The Hammick reaction (p. 163) provides some evidence for this view. Kinetic data for the decarboxylation of picolinic and methylpicolinic acids are given in Table 6,3. Methyl groups have the expected effect, but the results and those for reaction in a range of solvents do not permit choice between the zwitterion (10) and the form (11) as the entity undergoing decarboxylation" . 

The model results discussed above express only the effects of time and temperature on organic acid generation from kerogen, and the effect of porosity on organic acid concentrations. They do not account for the effects of time and temperature on organic acid decarboxylation since decarboxylation kinetics under natural burial conditions are poorly known. Therefore, the results are best applied to Neogene source rocks where the field data discussed earlier indicate that the effect of thermal decarboxylation or decomposition on total acetate concentration is small, and where temperatures do not exceed approximately 150 °C. 

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