Stabilization of covalent hydrates

Increasing numbers of nitrogen atoms increase not only the kinetic susceptibility toward attack but also the thermodynamic stability of the adducts. Reversible covalent hydration of C = N bonds has been observed in a number of heterocyclic compounds (76AHC(20)117). Pyrimidines with electron-withdrawing groups and most quinazolines show this phenomenon of covalent hydration . Thus, in aqueous solution the cation of 5-nitropyrimidine exists as (164) and quinazoline cation largely as (165). These cations possess amidinium cation resonance. The neutral pteridine molecule is covalently hydrated in aqueous solution. Solvent isotope effects on the equilibria of mono- (166) and dihydration (167) of neutral pteridine as followed by NMR are near unity (83JOC2280). The cation of 1,4,5,8-tetraazanaphthalene exists as a bis-covalent hydrate (168). 

The added electron deficiency at positions 2 and 5 in the nitro-1,6-naphthyridines together with the resonance stabilization of the hydrated cations (150<—>151 152<—>153) is thus required for covalent hydration. The 3-nitro-1,5-naphthyridine satisfies the first require-... 

In aqueous alkaline conditions with chloroacetic acid the pyrido[4,3- f]pyrimidinethione (80) undergoes facile ring opening, attributed to the resonance stabilization of a delocalized covalent hydrate dianion intermediate (81) (82). Pyrido[2,3- f]pyrimidine-4-thiones (and... 

In all the examples studied, the difference in the free energy between the anhydrous and hydrated species is 4 kcal/mole or less. ° Both electron deficiency and resonance stabilization are necessary for covalent hydration to be measurable. The necessity for electron deficiency is clearly shown in the following examples. The cation of 1,4,5-triazanaphthalene is anhydrous, but the cation of 1,4,5,8-tetraazanaphthalene is predominantly hydrated. 1,6-Naphthyridine cation is anhydrous, whereas the cations of the 3- and 8-nitro derivatives are predominantly hydrated. Also, the percentages of the hydrated form in the neutral species of 2-hydroxy-1,3-diaza-, 1,3,8-... 

In general, electron-releasing groups (e.g. —NH2, —OH) diminish or prevent covalent hydration by decreasing the electron deficiency in the nucleus. This diminution becomes ineffective if a new kind of stabilizing resonance is facilitated by the substituent, e.g. the urea-type resonance and the 4-aminopyridine-type resonance in 2- and 6-hydroxypteridine, respectively. The reluctance of the anions of these substances to form hydrates is attributed to the stable benzenoid system, e.g. 42, in the anhydrous anion compared with the predominantly lactam form of the neutral species, e.g. 43. 

There are some special cases where tetrahedral intermediates are unusually stable there are three phenomena which lead to this stability enhancement. The first is an unusually reactive carbonyl (or imine) compound which is very prone to addition. An example of such a compound is trichoroacetaldehyde or chloral, for which the covalent hydrate can be isolated. A simple way to recognize such compounds is to think of the carbonyl group as a (very) stabilized carbocation, bearing an substituent. 

The maximum stability for vanadium as the V3 4 ion in acidic solution can be understood in terms of the maximization of the enthalpy of hydration for the +3 ion, above which hydrolysis alters the form and stability of the higher states. The + 4 and + 5 states are considerably electronegative compared to the lower oxidation states, and are able to engage in covalent bonding to ligand oxide ions to form V=0 bonds. 

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

In all the examples studied, the difference in the free energy between the anhydrous and hydrated species is 4 kcal/mole or less. Both electron deficiency and resonance stabilization are necessary for covalent hydration to be measurable. The necessity for electron deficiency is clearly shown in the following examples. The cation of... 

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