Covalent Hydration in Nitrogen Heterocycles

This chapter discusses the covalent character of the hydration that has been firmly established by mass spectroscopy, X-ray crystallography, and proton magnetic resonance (PMR). Of the new methods that have emerged for studying covalent hydration, PMR has been most used. With its aid added to the two most fruitful of the earlier methods—ionization constants and ultraviolet (UV) spectra—many new and often unsuspected facts have emerged. The various physical methods have been particularly revealing when a nucleus with substituents that display a range of inductive, mesomeric, and steric properties has been used. As a result, more is now known about the mechanism of hydration; and many new patterns of hydration have emerged. Furthermore, covalent hydrates have been found in many new ring systems, even in single rings. Some cases have been found where two molecules of water are added to a single nucleus. Other molecules have been shown to add one molecule of water to a certain position, under kinetic control, whereas after the lapse of time this water molecule is found to occupy a different position under thermodynamic control.