Anion, definition

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

Most of the work done in the pteridine series has been concerned with the equilibria between the neutral species and the anions. This work was more fruitful than that involving the cations because all three of the values, p /, p a , and pK/ (for definitions, see Section II, A), could be determined, and, from these, ratios of the hydrated to the anhydrous forms were calculated. Furthermore, the kinetics in the... 

According to the Hiickel criteria for aromaticity, a molecule must be cyclic, conjugated (that is, be nearly planar and have ap orbital on each carbon) and have 4n + 2 tt electrons. Nothing in this definition says that the number of p orbitals and the number of nr elections in those orbitals must be the same. In fact, they can he different. The 4n + 2 rule is broadly applicable to many kinds of molecules and ions, not just to neutral hydrocarbons. For example, both the cydopentadienyl anion and the cycloheptatrienyl cation are aromatic. 

It seems reasonable to assume that both polarity and polarization potential of the anions must be taken into account in the definition of anion activity. It is quite indicative to arrange the anions by increasing order of the product of the polarity and polarization potential, a-Z/r,... 

Careful comparison of Pt-P bond lengths for the series trany-Pt(Pcy3)2X2 (X = H, Cl, Br, I) with those for p-any-Pt(PR3)2X2 (PR3 = PMe3 or PEt3) shows a more definite increase in Pt-P with anion size for the cyclohexyl-phosphine complexes (Table 3.16) believed to be owing to intermolecular X- H and X- C non-bonded interactions arising from overcrowding [151]. 

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