Kinetics isomerization

Isomerization. Maleic acid is isomerized to fumaric acid by thermal treatment and a variety of catalytic species. Isomerization occurs above the 130 to 140°C melting point range for maleic acid but below 230°C, at which point fumaric acid is dehydrated to maleic anhydride. Derivatives of maleic acid can also be isomerized. Kinetic data are available for both the uncatalyzed (73) and thiourea catalyzed (74) isomerizations of the cis to trans diacids. These data suggest that neither carbonium ion nor succinate intermediates are involved in the isomerization. Rather, conjugate addition imparts sufficient single bond character to afford rotation about the central C—C bond of the diacid (75). 

Luchkevich et al. (1986, Table 6) demonstrated that for the three isomeric nitro-benzenediazonium ions and their (Z)-diazohxydroxides the acidity constants can be determined by ultraviolet spectrophotometry, by potentiometry, from the kinetics of reaction with hydroxide ions, from the (Z) (E) isomerization kinetics, and from the kinetics of azo coupling reactions. These independent methods gave surprisingly consistent results. ... 

Disproportionation process those containing Ce or Eu, have an increased catalyst activity and lifetime Isomerization, kinetically controlled in 46... 

As pointed out (see Section 5.1.4) alkyl group migration (i.e., isomerization) in these carbocationic intermediates can take place with ease, even under conditions where the product dialkylbenzenes themselves do not undergo further isomerization. Kinetically controlled alkylation free of thermodynamically controlled alkyl group migration therefore cannot be judged on absence of product alkylbenzene isomerization. 

C. Further Considerations of the Role of Hydrogen in Isomerization Kinetics 61... 

Rearrangements of six-coordinate tris(diketonate) metal complexes have been the subject of numerous intense and careful investigations and the great body of literature has been incisively reviewed.77,283 A principal conclusion is that in no case has a unique rearrangement been unambiguously established and indeed, one may not exist. The difficulty arises from the consideration that the various five-coordinate transition states (or intermediates) which result from bond rupture processes can have quite similar energies. Thus, combinations of mechanisms can obtain which lead to extremely complicated DNMR spectra or isomerization kinetics. 

Azobenzene has also been added to the internal surface of mesoporous films and powders through postsynthesis grafting.58,60-62 The effect of oligomer size on azobenzene isomerization kinetics was studied for branched oligomer-modified azobenzene derivatives in solution and covalently grafted to nanoporous silica 61 the size of the dendrimer had little effect on the thermal cis to trans isomerization rate in solution... 

Absolute rate constants and Arrhenius parameters have been determined for the thermal E,Z-isomerization of the stable disilene derivatives 92-96 in deuteriated aromatic solvents or THF-ds solution by XH or 29Si NMR spectroscopy133-136. With 1,2-dialkyl- and 1,2-diamino-l,2-dimesityldisilenes such as 92a-94, the (E)-isomers are considerably more stable than the (Z)-isomers, and so rate constants for E,Z-isomerization were determined after first generating mixtures enriched in the (Z)-isomer by UV-irradiation of the (El-isomer, and then monitoring the recovery of the solution to its equilibrium composition. On the other hand, little difference in thermodynamic stability is observed between the (Eland (Z)-isomers of tetraaryldisilenes such as 95a,b, and E,Z-isomerization kinetics were hence determined starting from solutions prepared from the individual, pure (or almost... 

An interesting approach is a method that uses the modulation of the degree of circular polarization (e.g., between pure 1-cpl and linearly polarized light) to determine fast isomerization kinetics [71]. However, preliminary results on [Co(ox)(phen)2] and rhodamin 6G 14 were not followed by the announced full papers, and the arcticle is rarely cited. In principle the concept may be used to determine interconversion and the CD spectra of rotamers (like 14) which are the reactants in asymmetric synthesis (see Sec. II.C.). 

Several X-ray structures have proved that under various conditions (e.g. condensation under alkaline or slightly acidic conditions) exclusively the C4-symmetrical isomer is formed,212,213 in agreement with semiempirical calculations. Most probably this is due to its stabilization via four intramolecular O-H-O hydrogen bonds (thermodynamic aspect), while a rapid isomerization (kinetic aspect, see Figure 21) remedies wrong structures formed during the synthesis. 

An interesting application of the molecular dynamics technique on single chains is found in the work of Mattice et al. One paper by these authors is cited here because it is relevant to both RIS and DRIS studies and deals with the isomerization kinetics of alkane chains. The authors have computed the trajectories for linear polyethylene chains of sizes C,o to Cioo- The simulation was fully atomistic, with bond lengths, bond angles, and rotational states all being variable. Analysis of the results shows that for very short times, correlations between rotational isomeric transitions at bonds i and i 2 exist, which is something a Brownian dynamics simulation had shown earlier. 

Hanson KM, Simon JD. The photochemical isomerization kinetics of urocanic acid and their effects upon the in vitro and in vivo photoisomerization spectra. Photochem Pho-tobiol 1997 66(6) 817-820. 

The study of n-pentane isomerization kinetics on H-mordenite enabled us to bring out some peculiarities of this catalyst. In the absence of hydrogen, the reaction proceeded at a low rate. This probably results from hydrogen rearrangement. For example, in the presence of nitrogen at 280°C and 30 atm, the amounts of isopentane and cracking products were 5% and 5%. 

Liu, Z.-E, Hashimoto, K., and Fujishima, A. (1991). Thermal cis-trans isomerization kinetics of azo compounds in the assembled mono-layer film an electrochemical approach. Chem. Phys. Lett. 185, 501-504. 

Keywords Allyl alcohol, Allylamine, Aza-allyl, Azomethine, BINAP, BIPHEMP, Citronellal, Cobalt, DIOP, Enol, Enamine, Imine, Isomerization, Kinetic resolution. Nitrogen-triggered, Rhodium, Ruthenium... 

Figure 10 shows the isomerization kinetics for a —20/1 mixture of 2-methyl-1-pentene with S02 after a dose of 0.8 Mrad. No significant isomerization occurred upon irradiation of pure 2-methyl-1-pentene, even after 40 Mrad of absorbed dose. However, after the small dose of 0.8 Mrad 2M1P (in the presence of S02) underwent isomerization to 2M2P as seen in Figure 10. The kinetics are similar to that observed following PMPS radiolysis (compare Figure 3). The equi-molar mixtures prepared under vacuum also showed accelerated isomerization after irradiation, see Figure 11. During preparation of the 1 1 mixtures upto 20% isomerization occurred prior to any irradiation. This effect was not observed for the 20/1 mixture (Figure 10) and its cause is not known at present.

Confirmation of the reaction mechanism is provided by kinetic data dependent upon the same pK for the N- l coordinated ruthenium(III) complex (see Figure 6) Owing to the instability of l-[(Ado)(NH3)3Ru(II)l and severe restrictions required of the oxidizing partner isomerization kinetic rates were derived from cyclic voltammetric data using the method of Nicholson and Shain " " after forming the N-1 coordinated Ru(II) complex at the electrode surface by electrolytic reduction of the N- 6 bound Ru(III) species. Since the specific rates estimated by this method were independent of concentration the rate law is taken to be first order in the complex. 

Measurement of Xylose Isomerization Kinetics and Equilibrium All experiments were carried out at 34 °C in a volume of 25 ml in 50-ml shake flasks agitated at 130 rpm in an incubated shaker. Each experiment was conducted in duplicate. All experiments used 60 g/ 1 xylose, and unless otherwise noted, 5.2 g/1 of enzyme pellets (0.13 g) was used for each experiment. Buffered solutions used in making the isomerization media were 0.01 M Tris buffer (pHed to 7.5 using 0.01 M NaOH) and 0.05 M sodium citrate buffer (pHed to 4.5 using citric acid). The pH was measured at the beginning of the experiments but was not monitored throughout. We have observed a small drift in pH of less than 1 unit over the course of 48 hrs. Even with this drift, the pH of the bulk solution stayed within the range suitable for fermentation and well-below the pH optimum of XI (pH 7.5). In experiments with co-inunobilized pellets, urea concentration was 0,0.01, or 0.1 M. 

Although our system has not been optimized to achieve the most favorable internal pH for XI activity, we are able to demonstrate significant XI activity in our co-immobilized enzyme pellets at a bulk pH of 4.5 with 0.01 M urea. As the overall production rate of ethanol is limited by the total concentration of xylulose available to the yeast [9], it is important to determine if we can modify our experimental conditions to favorably enhance the isomerization and the xylose/xylulose proportions. Hence, the next experiments focus on addition of borate to the reaction medium in an effort to enhance the isomerization kinetics and favorably shift the equilibrium. Results for both unaltered and co-immobilized enzyme pellets are presented. 

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