Factors in the stabilization of covalent hydrates

8-double bond of 6-hydroxypteridine. Such reagents include ammonia, hydroxylamine, methanol, and the Michael reagents (such as ethyl cyanoacetate, and even acetone) (cf. Section III, A). Also, a second molecule of the hydroxypteridine can be added to give a dimer.  

However, water is a much weaker nucleophilic reagent than most of the above substances and therefore would not remain strongly bound unless some further forces were operative. It is our belief that resonance is the principal cause of this extra stabilization. We have found that these resonances are varied in nature, but they all fit into one or the other of the following principal classes. 

The amidine-type resonance, the well-known base-strengthening resonance of aliphatic amidines, is most prominent in the cations (30) but is not negligible in the neutral species (31). Quinazoline, the 

The gtianidine-type resonance, which is present in aliphatic guanidines, is stronger than that in the corresponding amidines, at least in the case of the cationic forms. 2-Aminopteridine and 

2-aminoquinazoline cations are two examples of compounds in which hydration is stabilized by the presence of a guanidino group (32). 

The amidine-type resonance, the well-known base-strengthening resonance of aliphatic amidines, is most prominent in the cations (30) but is not negligible in the neutral species (31). Quinazoline/ the l,3,x-triazanaphthalenes (wher hydration is confined largely to the cation), and pteridine (where the neutral species is hydrated to the extent of 22%) are examples of compounds in which stabilization of hydration by amidine resonance occurs. 

The urea-type resonance is illustrated, as the neutral species, by 33. Resonance of this type does not operate in anionic species because one of the two possible canonical forms would have to carry the negative charge on the oxygen atom and the other on the nitrogen atom, and these forms would be unequivalent. The urea-type resonance is exemplified by the neutral species of 2-hydroxypteridine, which is strongly hydrated and has an anhydrous anion. 

The 4-aminopyridine-type resonance, one of the most interesting methods by which hydration is facilitated, is illustrated by the small resonance in the neutral species of 4-aminopyridine (34) and the far greater resonance in the cation of the same substance (35). 4-Amino-pyridine owes its strongly basic properties (pA 9.2, as compared to 

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