The kinetics of hydrogen isotope exchange reactions. part xiii. isotope effects in the acid-catalysed hydrogen exchange of 1,3,5-trimethoxybenzene

The ratio of the rates of the simultaneous loss of tritium and deuterium from 1,3,5-trimethoxybenzene labelled with both isotopes has been studied in aqueous perchloric acid and acetate buffer solutions. This ratio has been shown to be independent of the concentration of the catalysing acid. In D ₂O-DC1O₄ solution the loss of tritium is 1.68 times faster than in H₂O-HC1O₄ solutions of the same acid concentration. In deuteriated acetate buffer solution the acetic acid-catalysed reaction (general acid catalysis) is 1-46 times slower than in the light medium. On the basis of a reasonable representation of the reaction path, isotope effects for the component steps of the overall reaction have been deduced. These effects should be interconnected if a common reaction mechanism holds for both forms of catalysis, and this prediction is borne out by the data.