Basicity of unsaturated compounds

Base catalysis, general, of ester hydrolysis and related reactions, 5, 237 Basicity of unsaturated compounds, 4, 195... 

The preceding section summarized basicity results obtained from the stability constants of 7r-complexes and EDA-complexes. These results can only reflect a qualitative gradation of the basicity. If one moves from these complexes to u-complexes, then exact values for the basicity of unsaturated compounds can be obtained by measuring the formation equilibria of the proton addition complexes in strongly acid solutions. The experimental methods and the calculations have been described in Sections III, A-C. 

The methods for the determination of the basicity of aromatic compounds discussed hitherto have as their starting point the formation of a proton addition complex in an acid solution. In addition to this interaction, numerous intermolecular interactions are known which are also directly connected with the basicity of unsaturated compounds but which do not lead to the formation of a true covalent bond. This interaction was already mentioned in connection with the vapour pressiu-e measurements of the system of aromatic substance-HCl, and leads to a TT-complex (Dewar, 1946). 

Furthermore, in many cases the effect of the third component, namely the solvent, is decisive. For example, the measured Henry s law constant for the system aromatic substance-HCl, only reflects the difference between the chemical potential of HCl in solution, and in the vapour phase, p% (Kortiim and Vogel, 1955). The values obtained therefore do not permit a quantitative interpretation and only give qualitatively the relative order of the basicity of unsaturated compounds. This is also true for partition measurements between an acid and an organic phase, if in such a case the necessary thermodynamic assumptions have not been tested or established by separate investigations. 

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