Exchange without Added Acid or Base

Among simple molecules, pyridine undergoes exchange of the C2-H and C6-H on heating in H20 to 225 °C, and a variety of mononuclear and polynuclear nitrogen heteroaromatics and their methyl derivatives do likewise, both adjacent to nitrogen and in the methyl groups , yet the labels in all these compounds are quite stable at ambient 

There are examples of the uncatalyzed exchange with HHO at the a-positions of steroidal ketones . The ketone functions were immediately subjected to synthetic transformations in order to prevent tritium loss by back-exchange. The C2 positions of imidazole-containing pharmaceuticals have been similarly exchanged in this case, the half-time for loss of the tritium label was determined to be 2.4 days at 37 °C and pH 7, adequate for the intended study. This figure is similar to that measured for [imidazole-2- H]histidine, namely 2.8 days at 37 °C and pH 8.2. 

It is with more complex compounds, which are more difficult to label by other means, that exchange in neutral tritiated media can have real value. 

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