Reactions hydrogen/proton transfer

As pointed out in Chapter 4, the first step in the reaction is proton transfer to the alcohol from the hydrogen halide to yield an alkyloxonium ion. This is an acid-base reaction. 

The first experimental data for a reaction involving proton transfer from a hydrogen-bonded acid to a series of bases which were chosen to give ApK-values each side of ApK=0 are given in Fig. 15 (Hibbert and Awwal, 1976, 1978 Hibbert, 1981). The results were obtained for proton transfer from 4-(3-nitrophenylazo)salicylate ion to a series of tertiary aliphatic amines in aqueous solution, as in (64) with R = 3-nitrophenylazo. Kinetic measurements were made using the temperature-jump technique with spectrophoto-metric detection to follow reactions with half-lives down to 5 x 10"6s. The reciprocal relaxation time (t ), which is the time constant of the exponential... 

The last of these eight statements has to do with the idea that, if a very small amount of an organic solvent such as methanol is introduced into an aqueous solution, the rate of reaction (involving proton transfer) may speed up because of the increased hydrogen-bonded water structure. 

Hydride-transfer reactions involving other cofactors 64 Hydrogen-atom transfer reactions 67 Proton-transfer reactions 69 Experimental signatures of tunneling 70... 

Lewis, E.S. (1975). In Tunneling in Hydrogen Transfer Reactions in Proton-Transfer Reactions, Caldin, E. and Gold, V. (eds), p. 317-338. Chapman and Hall, London... 

OTHER EXAMPLES OF KINETIC ISOTOPE EFFECTS. The power of kinetic isotope effects in enzymol-ogy is well illustrated in the work of Rose ° and Knowles deahng with hydrogen effects in proton transfer to and from carbon. Abstraction of a proton from a tetrahedral carbon is a fundamental step in many enzyme-catalyzed reactions. Intramolecular proton transfer as well as partial loss (wash-out) migrating protons have provided important clues in mechanistic investigations. Enol and enediolate formation constitute several... 

When a base catalyzed reaction with proton transfer in the first step is carried out in D20 solution, the substrate exchanges its acidic hydrogen with the deuterium of the solvent before the reaction takes place if the mechanism is fast pre-equilibrium proton transfer with subsequent slow step. If, on the other hand, hydrogen exchange does not occur prior to the reaction, it may be concluded that proton transfer is the rate-determining step. 

In recent years, more examples of the unconventional X-H- M or X-H- H-M hydrogen bonds were found and computational studies were carried out more systematically and using better methods, including in most cases the BSSE correction. The application of the AIM theory to detect BCPs and therefore the presence and nature of bonds also increased significantly. The role of the unconventional hydrogen bonds in several reactions, namely proton transfer, has been emphasized. 

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 ... 

The reaction of proton transfer from an ion AH" to the adsorbed state of H at the electrode surface is one of the steps (often the rate determining one) of the electrochemical hydrogen evolution reaction. This reaction is rather important, both with respect to its practical application and from the... 

Harmonic potential energy curves may be linearized only within a narrow energy interval therefore, it is to be expected that the relation between their slopes, and hence a, must vary smoothly over a sufficiently great variation in AZ7 . However, the linearization interval is broader if the curves U X) have an inflection typical of, for example, an anharmonic oscillator, but near the minimum of the curves, where they can always be approximated by a parabola, the linearity is inevitably disturbed. In fact, when a certain series of homogeneous reactions of proton transfer were studied over a sufficiently broad range of Af/° (about 80 kJ moF ), a smooth variation in a was obser-ved " however, nobody has so far reliably established a smooth variation in the true value of a with the potential for any electrode reaction moreover, in the case of the hydrogen evolution reaction, a good constancy in a is observed over an interval of overpotentials of about 1.5 This... 

Large hydrogen isotope effects were found experimentally very soon after the discovery of deuterium in 1932, and there is now an extensive literature on the subject, recently supplemented by work with tritium. Much of this relates to the kinetics of reactions involving proton transfer, but there is also a large amount of information on acid-base equilibria. A recent review estimated that about 300 papers on isotope effects are now published each year, and a considerable proportion of these relate to the isotopes of hydrogen. The present chapter will be devoted to the effect of isotope substitution on acid-base equilibrium constants, while Chapter 12 will deal with kinetic isotope effects in proton transfer reactions. 

Henchman, M. J., Smith, D. and Adams, N. G. Proton-Transfer Reactions, Hydrogen-Exchange Reactions, and Proton-Transfer / Hydrogen-Exchange Reactions, presented at the 28th ASMS Conference, New York City, 1980, paper WAMOC 13 Ausloos, P. and Lias, S. G. 1981, J. Am. Chem. Soc. 

Similarly to the electron transfer reactions, a proton transfer is preceded by the formation of an intermediate associate which in this case is a hydrogen bond complex (H-complex) V ... 

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