Decarboxylation, kinetic parameters

The rates of decarboxylation of a range of 3-substituted mesitoic acids in 82.1 wt. % phosphoric and 83.0 wt. % sulphuric acids have been measured636 over a range of temperatures as indicated in Table 209, which gives the first-order rate coefficients together with the log A values and the energies, enthalpies, and entropies of activation calculated at 80 °C for sulphuric acid, and 119 °C for phosphoric acid these kinetic parameters are somewhat different from those... 

Kinetic Parameters for the Cycloheptaamylose-Catalyzed Decarboxylation of Phenylcyanoacetic acid anions at pH 8.6 and 60.4° ... 

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]. 

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

TABLE X. Kinetic Parameters for Catalyzed Decarboxylation of Nitrobenzisoxazole Carboxylate by Modified Polyethylenimines3... 

Oxidation of oxalic acid with dimethyl-V,V-dichlorohydantoin and dichloroisocya-nuric acid is of first order with respect to the oxidant. The order with respect to the reductant is fractional. The reactions are catalysed by Mn(II). Suitable mechanisms are proposed.129 A mechanism involving synchronous oxidative decarboxylation has been suggested for the oxidation of a-amino acids with l,3-dichloro-5,5-dimethylhydantoin.130 Kinetic parameters have been determined and a mechanism has been proposed for the oxidation of thiadiazole and oxadiazole with trichloroiso-cyanuric acid.131 Oxidation of two phenoxazine dyes, Nile Blue and Meldola Blue, with acidic chlorite and hypochlorous acid is of first order with respect to each of the reductant and chlorite anion. The rate constants and activation parameters for the oxidation have been determined.132... 

There have been k12/k13 KIEs measured on C02 release (1.033) by this enzyme which indicate that the ratio kjk5 is not very different from unity, i.e. that transimination and decarboxylation are both partially rate-limiting147. Based on a comparison of a variety of KIEs, as well as steady-state kinetic parameters Vmax and VmaJKM for the pyruvyl-dependent and pyridoxal-dependent decarboxylases, no obvious reasons could be found why nature would preferentially select one pathway over the other. 

Table 44 Kinetic Parameters for the Decarboxylation of c/s-[Rh(en)2(OC02)X)n + (from ref, 627)...

Table 45 Kinetic Parameters for the Decarboxylations of trarw-[Rh(en)2(X)(0C02H)]"+...

Table 9. Kinetic parameters of the decarboxylation of 34 in water at 25 C in the presence of hosts 36, 24, and 25 at [host] > FM , = normalized figure of merit k Ko/Ar ...

The thermal decarboxylation of tropone at temperatures between 420 and 490 °C has been examined using a flow apparatus. The kinetic parameters obtained are consistent with a one-step concerted decarbonylation via a norcaradienone species. ... 

Interpretation of kinetic patterns for decarboxylation of the d5-[Co(py)2(C03)2] anion is clouded by a slight uncertainty as to whether both carbonate ligands are indeed bidentate. Kinetic parameters, including the solvent isotope effect, for loss of the first carbonate ligand (Table 5.15) are consistent with decarboxylation of a bidentate group, but the activation parameters do fall within the narrow... 

Table 5.15. Kinetic Parameters for Decarboxylation of Cobalt(IU)-Tetraazaligand-Carbonate Complexes...

Kinetic parameters for formation and decarboxylation of carbonato-rhod-ium(III) complexes are given in Tables 5.16 and 5.17. In acid solution, the cis-[Rh(en)2(C03)] cation loses carbon dioxide slowly. Rate constants and activation parameters (A// and A5 ) are reported for the two paths indicated by ... 

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-... 

Table 5.16. Kinetic Parameters for Decarboxylation of Carbonato-Rhodium(III) Complexes (Aqueous Solution 298.2 K). Subscripts 1 and 2 Referring to the ki and lc2 (Acid-Independent and Acid-Dependent) Terms of Equation (41)... 

Kinetics of decarboxylation of x-carbonato complexes have been studied for the 1,4,7-triazacyclononane (LLL) complexes (45), with M = Rh or Cr. Both components of the right-hand side of the expression kobs = ki + A 2[H ] are important for the chromium(III) compound, but only the acid-catalyzed path is significant for the rhodium(III) compound. Rate constants and activation parameters are extremely similar for the k2[H ] paths for rhodium(III) and for chromium(III), consistent with the expected carbon-oxygen bond rupture in the rate-determining step. The kinetic parameters here are also, of course, very... 

Propionic and butyric acids have structures similar to acetic acid and are expected to display much the same chemical and kinetic behavior. Virtually no experimental studies of their decarboxylation kinetics in aqueous solutions have been reported, although it is expected that these reactions are also catalyzed heterogeneously. The rate constant for decarboxylation of a 1.1 mol kg solution of n-butyric acid is 4.20 x 10 s (titanium oxide surface at 359 °C), whereas the comparable value for acetic acid is 3.89 x 10 s (Palmer and Drummond 1986). However, there are no corresponding data on the activation parameters for these acids so that the expected similarity of the linear isokinetic plots for these and acetic acid has not yet been tested. 

Table 3 Kinetic Parameters of Wild and Mutant Enzymes for the Decarboxylation of Phenylmalonic Acid...

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... 

The kinetics and mechanisms of gas-phase elimination of ethyl 1-piperidinecarboxyl-ate, ethyl pipecolinate, and ethyl 1-methylpipecolinate has been determined in a static reaction system.9 The reactions proved to be homogeneous, unimolecular, and obey a first-order rate law. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of these elimination reactions has been suggested on the basis of the kinetic and thermodynamic parameters. 

The effect of solvent upon the activation parameters for decarboxylation has been studied extensively. All of the reactions reported here show first-order kinetics however, the species which undergoes decarboxylation is usually unknown. Whether this species is the free acid, the carboxylate anion or a zwitterion is usually not resolved. The activation parameters may not reflect a particular reaction path, but instead the sum of decarboxylation pathways. In a number of instances linear AH vs. AS plots are realized for a given acid in a variety of solvents. The equation for such a relationship is given by... 

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. 

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