Location of Covalent Hydration

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. 

Applying mass spectrometry to the hydrated and anhydrous forms of 4-trifluoromethylpteridine, which interconvert only slowly, it was found that each had its characteristic mass spectrum.9 The fragmentation 

Albert, Heterocyclic Chemistry, 2nd ed., p. 56, Athlone Press, London, and 

Apparently the first published use of PMR to study hydration was the report that the proton in the 4-position of quinazoline moved upfield (t 0.63 — 3.58) after the molecule was converted into the cation by aqueous acid.10 Normally a downfield shift would be expected on ionization, and this reversal of direction afforded a further and convincing proof of the covalent character of hydration, as well as demonstrating the value of this technique in determining the position of hydration. Contemporaneous application of PMR to the hydration of 8-azapurine (6) cations established the scope and value of this technique.11 

In deuterium oxide, the parent substance, 8-azapurine (neutral species), showed two peaks at r 0.32 and 0.80 (1H each) corresponding to the two aromatic-type protons in the 6- and 2-positions, respectively (see Table I). The anhydrous cation was easily demonstrated in tri-fluoroacetic acid, through the downfield displacement of these signals by the usual 0.4-0.6 ppm. However in deuterium oxide-deuterium 

It is a simple matter to determine an ionization constant and also to predict its magnitude. When these values do not agree, and if ringopening has been carefully excluded, the likelihood of covalent hydration must be considered. Equilibria encountered during the determination of the ionization constant of a hydrating heteroaromatic base are shown in the following diagram. Similar equilibria exist for 

On the other hand, if the equilibria for and are attained slowly (see Fig. 1) and the optical density or pH readings are measured rapidly, either the pA or value can be obtained directly, 

Albert and E. P. Serjeant, Ionization Constants of Acids and Bases. Methuen, London, 1962. 

The pA/ value always lies between the pAa and values. Because aromatic or partly aromatic heterocycHc species, e.g. 3 (the concentrations of which are included in the pA expression), are weaker bases than the corresponding carbinolamines, e.g. 4 (the concentrations of which are involved in the expression), it follows that pA a V a- Because the anhydrous species is aromatic (or partly aromatic, if some of the conjugation may be in a —CO. NH— group) the basic value is always higher than that which would be 

It is presumptuous to report that a substance is not hydrated simply because there are no drifts in the readings obtained during potentio-metric measurements or because the experimentally determined p a value is not very different from the predicted value. A small amount of hydration may cause only a small difference in the ionization constant and hence other tests should be applied. A number of heterocyclic compounds which have seemingly normal pvalues may well be partially hydrated. 

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Some of the scatter within the groups is undoubtedly due to differences in bond order and also to whether or not the nitrogens are located in the same ring. Nevertheless, some striking exceptions are apparent in which the pA values are much higher than expected. These include quinazoline, 1,3,5-, 1,3,7-, 1,3,8-, and 1,4,6-triazanaph-thalene, pteridine, and 1,4,5,8-tetraazanaphthalene. In all these cases, covalent hydration of the cation has been shown to occur, so the measured pA values are, in fact, equilibrium values involving both hydrated and anhydrous species. The hydrated species are, without... 

This general view on the EDL ignores the mechanism of surface charging and ion adsorption in the Stern layer. It cannot, therefore, reveal the fine details of real EDLs. For instance, ions may adsorb in the Stem layer with or without keeping their hydration shells and may accordingly be located on different planes parallel to the surface (inner and outer Helmholtz plane— IHP and OHP) co-ions may be covalently bound to the siuface and, thus, virtually increase the surface charge and last but not least, the structure of the EDL may be affected by surface morphology (Hunter 1988, Chap. 2). 

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