Substitution, electrophilic tritium exchange

One report describes a study of HNaY zeolite for the catalysis of tritium exchange into simple aromatic compounds. In analogy with the use of the water-sensitive EtAlCl2 (Section 3.1.2), the anhydrous active centers of the thermally activated zeolite were exposed to small quantities of tritiated water, and subsequent heating to 175 °C with substrate induced the exchange-labeling of the latter. Success was limited to aromatic compounds without bulky substituents or electron-withdrawing substituents, and the pattern of label incorporation was that of electrophilic substitution. 

Much less studied is the use of zeolites as catalytic entities at elevated temperatures for sol vent-free exchange tritiation with tritium gas. Of the many types of zeolites, only metal-containing ones have been found " to catalyze exchange from tritium gas. Published information in this area consists of pilot studies and the method has not been developed for routine practical use. The mechanism of labeling is evidently similar to that of HSCIE, in that tritium activated by the metal spills over via the zeolite to interact with substrate as an electrophilic entity. The most thorough study yet published includes results for labeling of alkyl-substituted aromatics, halobenzenes, saturated hydrocarbons and several simple heterocycles. 

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