Isotope effects reactions

Another kind of isotope effect is the solvent isotope effect. Reaction rates often change when the solvent is changed from H2O to D2O or from ROH to ROD. These changes may be due to any of three factors or a combination of all of them. 

Isbell, Horace S., and Pigman, Ward, Mutarotation of Sugars in Solution Part II, Catalytic Processes, Isotope Effects, Reaction Mechanisms, and Biochemical Aspects, 24, 13-65 Isbell, Horace S. See also, Pigman, Ward. 

Catalytic Processes, Isotope Effects, Reaction Mechanisms, and Biochemical Aspects... 

Part II. Catalytic processes, isotope effects, reaction mechanisms, and biochemical aspects, 24,13-65 Mycobacterium tuberculosis, polysaccharides of, 3,311-336... 

Solvent kinetic isotope effects (SKIEs) in H2O/D2O mixtures on the reaction of /)NPP catalyzed by calcineurin gave a small normal value of 1.35. Proton inventory and fractionation data are consistent with a mechanism involving a single proton transfer from a metal-bound water, although due to the small KSIE value and the inherent experimental error of the proton inventory technique, the participation of a second proton could not he excluded. Further information has been furnished by heavy-atom isotope effects. Reaction of NPP catalyzed by APP shows that phosphoryl transfer is fully rate limiting. However, for calcineurin the... 

Stradiotto and Tobisch collaborated to investigate the proposed mechanism for lr(l)-catalyzed cyclohydroamination of unactivated aUcenes with primary and secondary amines. A combination of kinetic investigations, including kinetic isotope effects, reaction monitoring, substrate scope investigations, and computational... 

One way in which the step of the reaction in which the proton is lost might be slowed down, and perhaps made kinetically important (with i), would be to carry out nitration at high acidities. Nitration of pentadeuteronitrobenzene in 97-4% sulphuric acid failed to reveal such an effect. In fact, nitrations under a variety of conditions fail to show a kinetic isotope effect. 

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... 

Melander, L. (a) (i960). Isotope Effects on Reaction Rates. New York Ronald Press. (6) (1950). Ark. Kemi 2, 211. 

The occurrence of a hydrogen isotope effect in an electrophilic substitution will certainly render nugatory any attempt to relate the reactivity of the electrophile with the effects of substituents. Such a situation occurs in mercuration in which the large isotope effect = 6) has been attributed to the weakness of the carbon-mercury bond relative to the carbon-hydrogen bond. The following scheme has been formulated for the reaction, and the occurrence of the isotope effect indicates that the magnitudes of A j and are comparable ... 

By protodetritiation of the thiazolium salt (152) and of 2 tritiothiamine (153) Kemp and O Brien (432) measured a kinetic isotope effect, of 2.7 for (152). They evaluated the rate of protonation of the corresponding yiides and found that the enzyme-mediated reaction of thiamine with pyruvate is at least 10 times faster than the maximum rate possible with 152. The scale of this rate ratio establishes the presence within the enzyme of a higher concentration of thiamine ylide than can be realized in water. Thus a major role of the enzyme might be to change the relative thermodynamic stabilities of thiamine and its ylide (432). 

An isotopic effect (H or D) has been demonstrated when starting from 2-methyl-4-phenylthiazole or from 2-methyl-4-phenyl-5-D-thiazole (224) in the dimerisation reaction. 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

C. J. Collins and N. S. Bowman, eds.. Isotope Effects in Chemical Reactions, American ChemicalSociety Monograph 167, Van Nostrand Reinhold Co., New York, 1971. 

Most of the chemical properties of tritium are common to those of the other hydrogen isotopes. However, notable deviations in chemical behavior result from isotope effects and from enhanced reaction kinetics induced by the ( -emission in tritium systems. Isotope exchange between tritium and other hydrogen isotopes is an interesting manifestation of the special chemical properties of tritium. 

Methylene-l-pyrazoline Secondary deuterium isotope effects on the reaction rate 81CJC2556... 

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