Separation of inductive, steric and resonance effects

Hammett a values are adequate for reactions and equilibria involving separation of substituent and reaction centre by a phenylene grouping. Examination of aliphatic species necessitates a more sophisticated set of o values as transmission effects are now markedly different from the model. Resonance and inductive effects may be separated if we can use as models reactions with no resonance interactions so that a° type values may be defined. Entirely new approaches have led to the definition of an inductive scale of a parameters (O]). Although some of these parameters are now only of historic value we shall include a brief discussion of early attempts to define the a, scale. 

Roberts and Moreland [49] recommend the dissociation constants of a series of bicyclo[222]octane-l-carboxyhc acids to define a a scale of parameters (Eqn. 62). 

The net polar effect is directed along the axis of the molecule in line with the nitrogen lone pair. Table 10 collects some up values. The Roberts and Moreland approach, while giving excellent values absolutely independently of resonance or steric effects requires the synthesis of some very challenging molecules and the same might be said of Grob s method. Taft and Lewis [51] defined a a, value which is very accessible substituted acetic acids are readily available as are their ionisation 

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