Craig plots are two-dimensional plots of one parameter against another (Fig. 3.16). The plot is divided into four sections corresponding to the positive and negative values of the parameters. They are used in conjunction with an already established Hansch equation for a series of related aromatic compounds, to select aromatic substituents that are likely to
Figure 3.16 An example of a Craig plot of para Hammett constants s against para p values. Reproduced with permission from M.E. Wolf (ed.), Burgers Medicinal Chemistry, 4th edn, Past 1, p. 343, 1980 John Wiley & Sons, Ltd.
produce the most active analogues. For example, suppose that a Hansch analysis carried out on a series of aromatic compounds yields the Hansch equation:
To obtain a high value for the activity (1/C) and as a result a low value for C, it is necessary to pick substituents with a positive p value and a negative s value. In other words, if high activity analogues are required, the substituents should be chosen from the lower right-hand quadrant of the plot. However, it is emphasised that the use of a Craig plot does not guarantee that the resultant analogues will be more active than the lead because the parameters used may not be relevant to the mechanism by which the analogue acts.
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