Hydrogen exchange mechanisms

Figure 14.1. Hydrogen exchange mechanisms (a) Amide exchange at neutrai pH invoives base cataiyzed proton abstraction and acid cataiyzed transfer of deuterium from solvent. Measurable isotope effects on the amide hydrogen and a lack of a solvent isotope effect indicate that proton abstraction is rate limiting [1]. (b) Schematic diagram of the two-state model for hydrogen exchange in native proteins and kinetic prediction. The rate of...

Hydrogen exchange, in thiazole, especially deuteration, has been quantitatively investigated (379,380), but the mechanism of the reaction carried out at acidic or neutral pH corresponds to a protonation-deprotonation process (380), different from electrophilic substitution and is discussed in section I.3.E. 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts Herman Pines and Joost Manassen Complex Adsorption in Hydrogen Exchange on Group VIII Transition Metal Catalysts... 

Kinetic studies of the mechanism of acid-catalysed hydrogen exchange... 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Since much work has gone into eventually disproving this mechanism for hydrogen exchange it is worth examining the soundness of the experimental data upon which the theory was based. Firstly, the log rate coefficient versus acidity function plots should have had unit slopes and the fact that they did not was attributed to salt effects. Secondly, different rate coefficients were obtained at the... 

Mechanism Kinetic" Order P-Hydrogen Exchange Faster Than Elimination General or Specific Base Catalysis hAd Electron Withdrawal atCp Electron Release at C Leaving- Group Isotope Effect or Element Effect... 

FIGURE 2.1 Classical Horiuti-Polanyi half-hydrogenated state mechanism for hydrogenation, double bond migration, cis-trans isomerization, and deuterium exchange. 

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