Hydrogen isotope effects and

Saunders, W. H., Jr., Ashe, T. A. Mechanisms of elimination reactions. XII. Hydrogen isotope effects and the nature of the transition state in eliminations from alicyclic quaternary ammonium salts. J. Am. Chem. Soc. 1969, 91,4473-4478. 

Both mechanisms involve the breakage of C-H bonds in the transition state, so either would give significant hydrogen isotope effects, and no distinction between these mechanisms can be readied. However, in mechanism (a), no breakage of the C-N bond takes place in the transition state, whereas in the concerted reaction (b) the C-N bond is partially broken. Thus the nitrogen isotope effect favours route (b). 

Table 18.3 Chlorine and hydrogen isotope effects, and element effects associated with alcoholic sodium alkoxide-promoted dehydrochlorination ractions.1 1...

The Primary Hydrogen Isotope Effect and the Nature of the Transition State... 

The oxidation of naphthalene by RUO4 takes place in two distinct steps. The first shows an inverse hydrogen isotope effect and is accelerated by the presence of electron-releasing substituents consistent with the formation of a transient Ru i... 

The Westheimer effect and its implications form the main subject of this chapter, and no attempt is made to consider for example the more traditional application of primary isotope effects to the study of reaction mechanisms. However, a further point that is emphasized is that interpretations of isotope effects may be appreciated without resort to calculations, and before discussing kinetic effects some time is spent in considering, from a qualitative standpoint, the origins of hydrogen isotope effects and the isotopic properties of stable molecules and equilibria. In this preliminary review previous accounts of hydrogen isotope effects are extensively used [2-9] and among these... 

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 ... 

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). 

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. 

When one of the ortho hydrogens is replaced by deuterium, the rate drops from 1.53 X 10 " s to 1.38 X lO s. What is the kinetic isotope effect The product from such a reaction contains 60% of the original deuterium. Give a mechanism for this reaction that is consistent with both the kinetic isotope effect and the deuterium retention data. 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

The idea that hydrogen bonding, as a special ortho effect of the substrate, may be involved in the transition state of the reactions with amines was first proposed by Chapman et al. Attempting to test this hypothesis, Hawthorne investigated the hydrogen-isotope effect in the reaction of o- and p-chloronitrobenzene with... 

The first unequivocal evidence for the AE + DE mechanism came in three papers by Zollinger (1955 a-c) dealing with general base catalysis and primary kinetic hydrogen isotope effects in azo coupling reactions of various types. Three classes of reactions were identified i) reactions with no isotope effects (ArH/A D - 1.0) and no general base catalysis, ii) others with large isotope effects (k /k — 6.5) and (practically) linear base catalysis, and iii) intermediate cases with isotope effects of around 3.0 and less-than-linear base catalysis. 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

Aromatic substitution, a quantitative treatment of directive effects in, 1, 35 Aromatic substitution reactions, hydrogen isotope effects in, 2, 163 Aromatic systems, planar and non-planar, 1, 203 Aryl halides and related compounds, photochemistry of, 20, 191 Arynes, mechanisms of formation and reactions at high temperatures, 6, I A-Se2 reactions, developments in the study of, 6,63... 

Substitution reactions, aromatic, hydrogen isotope effects in, 2, 163 Substitution reactions, bimolecular, in protic and dipolar aprotic solvents, 5,173 Sulphur, organic oxyacids of, and their anhydrides, mechanisms and reactivity in reactions of, 17, 65... 

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