Acid-Catalyzed Hydrogen Exchange as a Quantitative Measure of Reactivity

Acid-catalyzed hydrogen exchange has important advantages as a quantitative measure of heteroaromatic reativity. 

The mechanism of heteroaromatic hydrogen exchange can therefore be determined for a wide variety of substrates in the manner described in the previous sections in particular, the nature of the species undergoing reaction may be elucidated. The next stage is to compare rates, and this can be done provided all the kinetic data can be extrapolated to the same standard conditions of acidity and temperature. These relative rates will, however, as in the case of electrophilic substitution of all heterocycles in solution (especially ir-deficient heterocycles in protic solvents), be subject to the modifying effect of hydrogen bonding, correction for which has so far been applied in only a relatively few cases. 

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