Hydrogen-deuterium exchange rate equations

The rate of hydrogen exchange depends on the protolytic properties of both the solvent and the substrate. In fact there is a correspondence between the magnitude of the rate constants for deuterium exchange with ND3 and the conventional ionization constants of hydrocarbons which were used by Conant and Wheland (1932) and by McEwen (1936) to obtain the first quantitative estimates of the acidity of hydrocarbons. To do this, they determined the equilibrium of metallation of hydrocarbons by organo-alkali metal compounds. This reaction was described by Shorygin (1910) and is represented by the equation... 

Isotope concentration While deuterium model reactions are often conducted with pure deuterium oxide as the isotope source, tritium oxide is rarely used at anything close to nuclidic purity (note that tritiated water at 50 Ci/mL, the highest specific activity normally available commercially, has a tritium/hydrogen ratio of only about 1.6/98.4). Therefore, the concentration of tritium in HHO is usually much lower than that of deuterium in HjO, and this difference will be important if the source concentration is a factor in the rate equation. Analogously, model exchange reactions with deuterium gas are often done at one atmosphere of pressure, whereas in most cases tritium gas is used at lower pressures. This can result in substantially slower tritium exchange rates. 

Let us consider a homogeneous system composed of liquid ammonia, dissolved hydrogen and dissolved catalyst. Because of an important isotopic effect, R, the rate of exchange of the atoms between hydrogen and ammonia (expressed in atg. cm." mn." ), cannot be calculated. However, since only traces of deuterium are present, it is possible to calculate Rpi, where pi, takes into account the isotopic eflFect and is constant at constant temperature. Then, the following equation is valid ... 

In the presence of bases, in complexes containing electron-withdrawing constituents, an exchange of hydrogen for deuterium may take place [equation (8.61)]. The rate of this reaction decreases as the electronegativity of the substituent becomes smaller X = CN > CHO > COPh > COOMe. 

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