Carbon kinetics

Kim, M. et al, Hydrogen production by catalytic decomposition of methane over activated carbons Kinetic study, Int.. Hydrogen Energ., 29,187, 2004. 

J. M. Hawkins, A. Meyer, Optically Active Carbon -Kinetic Resolution of C76 by Asymmetric Osmylation , Science 1993,260,1918-1920. 

As mentioned earlier, products of disproportionation often accompany the rearrangement products. This reaction is also acid-catalysed and it is a reasonable assumption that reaction proceeds via the protonated species. Experiments with the 4,4 -diiodohydrazobenzene (19) showed that there were significant nitrogen and para-carbon kinetic isotope effects23. This implies that disproportionation must take place after C—C bonding has occurred, i.e. that the intermediate must be the quinonoid form 20 (and cannot, for example, be a jr-complex), which is then believed to react with another reactant molecule to give the disproportionation products (Scheme 4). 

It may be concluded that for reactions where the proton is less or more than one-half transferred in the transition state, i.e. the A—H and H—B force constants are unequal, the primary hydrogen-deuterium kinetic isotope effect will be less than the maximum of seven. The maximum isotope effect will be observed only when the proton is exactly half-way between A and B in the transition state. This relationship is also found for carbon kinetic isotope effects where the isotopically labelled carbon is transferred between two atoms in the reaction10,11. This makes interpreting carbon isotope effects difficult. 

Disproportionation (equation 13) is one of the side reactions that can occur in benzidine rearrangements. Shine and coworkers measured the nitrogen and carbon kinetic isotope effects for the disproportionation reaction of 4,4 -diiodohydrazobenzene, which only yielded disproportionation products, at 25 °C in 70% aqueous dioxane that was 0.376 M in perchloric acid29. The reaction was first order in hydrazobenzene and it has been assumed that an intermediate was involved in the disproportionation reaction. This intermediate must be one of a radical ion30 (equations 14 and 15), a jr-complex31 (equation 16) or a quinonoid structure32 (equation 17). 

Rhee and Shine39 used an impressive combination of nitrogen and carbon kinetic isotope effects to demonstrate that a quinonoidal-type intermediate is formed in the rate-determining step of the acid-catalyzed disproportionation reaction of 4,4 -dichlorohydrazobenzene (equation 26). When the reaction was carried out at 0°C in 60% aqueous dioxane that was 0.5 M in perchloric acid and 0.5 M in lithium perchlorate, extensive product analyses indicated that the major pathway was the disproportionation reaction. In fact, the disproportionation reaction accounted for approximately 72% of the product (compounds 6 and 7) while approximately 13% went to the ortho-semidine (8) and approximately 15% was consumed in the para-semidine (9) rearrangement. 

It is worth noting that Yamataka and coworkers47 also found large (near the theoretical maximum) alpha carbon kinetic isotope effects for the Menshutkin reactions between 3,5-disubstituted pyridines and methyl iodide (equation 38, Table 6). 

The third equation in Equation 11.47 represents a kinetic isotope effect of the first isotopomer pair measured in the presence of the second (which IE has perturbed the commitment). In order to make the changes in apparent commitment (cf/H2k3) sufficiently pronounced, deuterium is usually selected as the second isotope (H2). The first, (HI), on the other hand, is usually a heavy-atom (e.g. 13C, lsO, etc.). Most frequently this approach has been used for carbon kinetic isotope effects in which case Equation 11.47 becomes ... 

As a first example we discuss a mechanism in which the formation of the enzyme-intermediate complex, El, is sensitive to hydrogen isotopic substitution, while the next step characterized by rate constants ks and k6 exhibits a carbon kinetic isotope effect. Expressions for the three kinetic isotope effects that can be determined experimentally are ... 

Carbon kinetic isotope effects on enzyme-catalyzed decarboxylations are among the most intensively studied enzyme reactions. This is because of the central role that carbon dioxide plays in plant metabolism and also because precise kinetic measurements are relatively easy to obtain since the carbon dioxide liberated in the reaction can be immediately analyzed using isotope ratio mass spectrometry. 

The practical usefulness of Equations 11.46 through 11.53 has been demonstrated for the malic enzyme catalyzed conversion of L-malate to pyruvate (Equation 11.72). Table 11.1 lists experimentally determined isotope effects for this reaction. Comparison of carbon kinetic isotope effects for protio and deutero-malate substituted at position 2 (the carbon that undergoes sp3 to sp2 transition) rules out the possibility that the hydride transfer and the decarboxylation events are concerted. This conclusion follows from Equation 11.48 which, for a concerted reaction, predicts that 13(V/K) should be smaller than 13(V/K)D, which is opposite to the order observed experimentally. 

Isotope effects on both the carbon and hydrogen of the breaking C-H bond have been measured. However, for this reaction both forward and reverse commitments are sizable so the three equations corresponding to Equation 11.48 have four unknowns the forward and reverse commitments and two intrinsic isotope effects. Measurements of the secondary deuterium kinetic isotope effect (at position 4 of nicotinamide ring of NADP+) and the carbon kinetic isotope effect with the secondary position deuterated introduce two additional equations, but only one more unknown ... 

Another way of perturbing the commitments is solvent deuteration. Change of the carbon kinetic isotope effect in the case of E. coli enzyme indicates that the proton transfer precedes the decarboxylation step ... 

In this study, benzaldehyde and benzaldehyde-methyllithium adduct were fully optimized at HF/6-31G and their vibrational frequencies were calculated. The authors used MeLi instead of lithium pinacolone enolate, since it was assumed that the equilibrium IBs are not much different for the MeLi addition and lithium enolate addition. Dehalogena-tion and enone-isomerization probe experiments detected no evidence of a single electron transfer to occur during the course of the reaction. The primary carbonyl carbon kinetic isotope effects and chemical probe experiments led them to conclude that the reaction of lithium pinacolone enolate with benzaldehyde proceeds via a polar mechanism. 

Distinction between PL and ET mechanisms is not straightforward. Various experimental methods have been used so far to demonstrate the ET process, including spectroscopic detection of radical intermediates detection of products indicative of radical intermediates " and measurement of secondary deuterium " and carbonyl carbon kinetic isotope effects (KlEs) "" . The combination of several experimental methods, including KIE, substituent effect and probe experiments, was shown to be useful in distinguishing the ET process from the PL process for the addition reactions of the Grignard and other organometallic reagents . 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

A new study has been reported of the hydrolysis of the p-tolyldiazonium cation using a combination of carbon kinetic isotope effects, theoretical calculations, and dynamics trajectories. Overall, the results are in best accord with a process at the boundary between. S N 1 and. S N2Ar mechanisms.1 There has been a kinetic study of the thermolysis of o-carboxybenzenediazonium fluoride to give o-fluorobenzoic acid in liquid hydrogen fluoride under pressure.2... 

Recently, we have modeled9 intrinsic carbon kinetic isotope effects on the ornithine decarboxylase-catalyzed decarboxylations. Decarboxylations occur from the pyridoxal 5 -phosphate (PLP) - substrate complexes. These reactions provide a good model case since a number of 13C kinetic isotope effects for the wild-type enzyme and its mutants, as well as for physiological and slow substrates, have been reported.10 Using AM1/CHARMM/MD calculations on nearly 18000-atom models... 

Kobya, M. 2004. Removal of chromium (VI) from aqueous solutions by adsorption onto hazelnut shell activated carbon Kinetics and equilibrium studies. Bioresource Technology, 91 317-21. 

As mentioned, three types of kinetics are involved in the removal of nitrogen nitrification kinetics, denitrification kinetics, and carbon kinetics. Carbon kinetics refer to the kinetics of the heterotrophic aerobic reactions. By analogy with the nitrification or denitrification kinetics,... 

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effects for the reaction of lithium pinacolone enolate (112) with benzaldehyde (equation 31) were analyzed by Yamataka, Mishima and coworkers ° and the results were compared with those for other lithium reagents such as MeLi, PhLi and AllLi. Ab initio (HF/6-31-I-G ) calculations were carried out to estimate the equilibrium isotope effect (EIE) on the addition to benzaldehyde. In general, a carbonyl addition reaction (equation 32) proceeds by way of either a direct one-step polar nucleophilic attack (PL) or a two-step process involving electron transfer (ET) and a radical ion intermediate. The carbonyl-carbon KIE was of primary nature for the PL or the radical coupling (RC) rate-determining ET mechanism, while it was considered to be less important for the ET rate-determining mechanism. The reaction of 112 with benzaldehyde gave a small positive KIE = 1.019),... 

Crowley J. N., Saueressig G., Bergamaschi P., Fischer H., and Harriss G. W. (1999) Carbon kinetic isotope effect in the reaction CH4 -I- Cl a relative rate study using FTIR spectroscopy. Chem. Phys. Lett. 303, 268-274. 

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