Calculation from structural group contributions compound

Here k f k2f and kare microscopic ionization constants defined in section 4.6. From this it was possible to calculate and plot variation in Dnoh with pH for pentazocine as shown in Figure 2. The fourth partition coefficient was obtained by structural group contribution to the measured partition coefficient of a reference compound (morphine). This was done following the Hansch approach. 

The ability to reproduce thermodynamic quantities by means of the structural parameters is usually better than 5% for substances belonging, or similar, to those taken into consideration in the fitting of the data. It is however very difficult to make prediction about the ability of the structural parameters to reproduce satisfactory values for compounds which are not similar to those considered in the calculation of the group contributions. As an example the experimental value of the AG of acetamide (= -40.6 K J mol )(21) resulted very different with respect to that expectable from Hine s bond contributions (= -31.4 K J mol )(3). That is not surprising in view of the characteristics the water has to discriminate the various type of solutes better than others solvents do, see f.i. the peculiarity that the Barclay-Butler s plot presents in water with respect to non-aqueous solvents. 

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