The Dewar-Grisdale Equation

1 The Dewar-Grisdale Equation An approach to field and resonance transmission of the polar effect essentially the same as that of Taft s group was suggested by Dewar and Grisdale. The localised and delocalised polar effects are represented by F and M which are taken to be additive in any given a value. The value a,y is related to the sum of 

The distance between the atoms i and j is the formal charge induced at atom j by CH2- placed at i is and is the atom-atom polarisability of atoms i-j. Since 7u,y and can be calculated for benzene, application of equations (19) and (20) in combination with known values of Hammett s CTy gives a series of F, F, M and M parameters. 

The approach is based on a theoretical model for the transmission of the polar effect but its further extension is beyond the scope of this book. Future models of the polar transmission effect will undoubtedly be firmly based in MO theory.24-25,26 

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The second approach is more rigorous here the p-values are taken as equivalent to that in benzenoid compounds and cr-values are calculated for the thiophene substituents. The calculation may take into account the differing geometries of the two ring systems (use of the Dewar-Grisdale equation). 

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