Tritium isotope exchange reactions

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. 

Isotopic Exchange Reactions. Exchange reactions between the isotopes of hydrogen are well known and well substantiated. The equihbrium constants for exchange between the various hydrogen molecular species have been documented (18). Kinetics of the radiation-induced exchange reactions of hydrogen, deuterium, and tritium have been critically and authoritatively reviewed (31). The reaction T2 + H2 — 2HT equiUbrates at room temperature even without a catalyst (30). 

By contrast with the radiation-induced procedures, isotope exchange reactions catalyzed by Group VIII transition metals are applicable to both deuterium and tritium labeling of heterocyclic compounds. Because of recent mechanistic developments in this field, it is possible to predict with some degree of certainty the reactivity of a molecule for deuteration and also for moderate levels of tritiation. If compensation for additional radiation-induced interactions is made, then the theory also satisfactorily explains high specific activity tritiations. 

With the development of liquid scintillation counters which permit tracer levels of tritium to be measured accurately, isotope exchange reactions are now usually carried out with tritium rather than deuterium. Frequently, however, it is useful to compare rates of exchange for all three isotopes and deuterium exchange may be followed using, for example, mass spectrometry [22]. An example of the application of isotope exchange to proton transfer from carbon is shown in eqns. (8) and (9) for the hydroxide ion catalysed exchange of phenylacetylene in aqueous solution [23], viz. 

Since 1952, most of the tritium measured in the atmosphere originates from thermonuclear explosions. Like hydrogen, deuterium and tritium also exhibit molecular isomerism. Because of the important differences between the relative atomic masses of the three isotopes, their physical properties (e.g., density, enthalpy of vaporization) differ greatly. This allows an easier isotopic separation than for any other element. Several separation processes are used for the enrichment and separation of hydrogen isotopes. Most of these processes use isotopic exchange reactions (e.g., H D-H O or NH3-HD) and to a lesser extent fractional distillation and water electrolysis (e.g., Norway, Canada). 

The well-known iridium catalyst [Ir(COD)(Cy3P)-(Py)]PF, where COD = 1,5-cyclooctadiene and Py = pyridine, demonstrates excellent regioselectivity in isotopic exchange reactions of acetanilides and other substituted aromatic substrates. NMR spectroscopy is invaluable in identifying the site(s) of tritium incorporation - there are many instances where the broad signals in the corresponding NMR spectra are much less informative. 

Preparation of Tritium-Labeled Compounds by Isotope Exchange Reactions... 

Phase transfer catalysis has been exploited with beneficial effect in isotope exchange reactions. Chemically sensitive compounds that would not survive under the conventional base catalysis conditions can be labeled successfully. For example 23 was tritiated in benzene solution using HHO/NaOH/n-Bu4NBr, with a recovery of 80%, and the base-sensitive steroids 2A and were tritiated without double bond isomerization (location of tritium not reported) (Figure 3.11) ... 

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