Isotope effects, solvent

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

The initially observed perturbation (or equilibrium isotope effect) will disappear as isotopic scrambling (or mixing) subsequently occurs. This is illustrated above for the malic enzyme note the initial displacement of the equilibrium toward malate, followed by readjustment of the system to its flnal equilibrium position. The magnitude of the initial perturbation provides information on the occurrence of kinetic isotope effects and the nature of the rate-limiting step in an enzymatic process. See also Kinetic Isotope Effect Solvent Isotope Effect... 

An elementary reaction or step (in a chemical process) for which the rate constants are altered by an isotopic substitution in substrate, product, or solvent. See Kinetic Isotope Effect Solvent Isotope Effect... 

KINETIC ISOTOPE EFFECT EQUILIBRIUM PERTURBATION METHOD KINETIC ISOTOPE EFFECT SOLVENT ISOTOPE EFFECT EQUILIBRIUM THERMODYNAMIOS (Measurable Quantities)... 

KINETIC ISOTOPE EFFECT EQUILIBRIUM ISOTOPE EFFECT SOLVENT ISOTOPE EFFECT HEAVY ATOM ISOTOPE EFFECT INTRAMOLECULAR KINETIC ISOTOPE EFFECT... 

ISOTOPICALLY SENSITIVE STEP KINETIC ISOTOPE EFFECT SOLVENT ISOTOPE EFFECT Isotopic fractionation factor, FRACTIONATION FACTOR ISOTOPIC PERTURBATION... 

CHEMICAL KINETICS SOLVENT-TRAPPED INTERMEDIATES SOLVENT ISOTOPE EFFECT KINETIC ISOTOPE EFFECT Solvent parameter,... 

Solvent isotope effects. Much mechanistic information can be obtained about reactions involving proton transfer from solvent kinetic isotope effects, particularly in solvents of mixed isotopic composition. For practical reasons work is essentially confined to H/D effects, especially those in water. Unlike ordinary primary hydrogen isotope effects, solvent isotope effects have to take into account a host of exchangeable sites, subject to equilibrium as well as kinetic isotope effects. A key concept is that of the fractionation factor, (p, which is the deuterium occupancy of a site in a 1 1 H2O/D2O mixture more formally it is defined by equation l.l ... 

Linkage of catalysis and regulation in enzyme action — carbon isotope effects, solvent isotope effects, and proton inventories for the unregulated pyruvate decarboxylase of Zymomonas mohilis, J. Am. Chem. Soc. 117, 7317-7322. 

It remains to be established which of the two dinuclear moieties of 1 is responsible for catalytic and/or O2 binding and/or proton/electron abstraction from hydrocarbons. Work in progress, which includes measuring kinetic isotope effects, solvent effects, etc., is aimed at elucidating the oxidation mechanism. 

Kinetic isotope effects primary and secondary deuterium kinetic isotope effects. Heavy atom isotope effects. Solvent isotope effects. SnI and Sn2 mechanisms. 

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... 

The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and the kinetic evidence of a solvent isotope effect are consistent with rate-limiting protonation with simple alkenes such as 2-metlQ lpropene and 2,3-dimethyl-2-butene. 

The kinetic features of this reaction, including the solvent isotope effect, are consistent with a rate-determining protonation to form a vinyl cation. ... 

The second-order rate constants for hydration and the kinetic solvent isotope effect for hydration of several 2-substituted 1,3-butadienes ate given below. Discuss the information these data provide about the hydration mechanism. 

Solvent isotope effects are usually in the range / h20+ = 2-3. These values reflect the greater equilibrium acidity of deuterated acids (Section 4.5) and indicate that the initial protonation is a fast preequilibrium. 

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

In analyzing the behavior of these types of tetrahedral intermediates, it should be kept in mind that proton-transfer reactions are usually fast relative to other steps. This circumstance permits the possibility that a minor species in equilibrium with the major species may be the major intermediate. Detailed studies of kinetics, solvent isotope effects, and the nature of catalysis are the best tools for investigating the various possibilities. 

This variation from the ester hydrolysis mechanism also reflects the poorer leaving ability of amide ions as compared to alkoxide ions. The evidence for the involvement of the dianion comes from kinetic studies and from solvent isotope effects, which suggest that a rate-limiting proton transfer is involved. The reaction is also higher than first-order in hydroxide ion under these circumstances, which is consistent with the dianion mechanism. 

Bromination has been shown not to exhibit a primary kinetic isotope effect in the case of benzene, bromobenzene, toluene, or methoxybenzene. There are several examples of substrates which do show significant isotope effects, including substituted anisoles, JV,iV-dimethylanilines, and 1,3,5-trialkylbenzenes. The observation of isotope effects in highly substituted systems seems to be the result of steric factors that can operate in two ways. There may be resistance to the bromine taking up a position coplanar with adjacent substituents in the aromatization step. This would favor return of the ff-complex to reactants. In addition, the steric bulk of several substituents may hinder solvent or other base from assisting in the proton removal. Either factor would allow deprotonation to become rate-controlling. 

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