More than one transmission path for

The transmission of the effect from substituent to reactive site can be considered to go through essentially one path. In the case of meta or ortho substituents there are two formal paths respectively through C2 or C4 or direct (Eqn. 55). 

Because a is defined by a similar model the action of meta or ortho substituents is tantamount to a single interaction pathway. When the transmission path in the ionisation of benzoic acids differs from that in the unknown reaction special treatment is required [42]. In the case of substituted aspirins the carboxyl group is involved in catalysis. Thus two pathways exist for substituent transmission not comparable with the simple Hammett definition of a an alternative approach is to look at the substituents and realise that a problem exists in assigning a values (Eqn. 56). 

A substituent in the 4 position is para to A but meta to B. Two transmission routes exist, one to A and one to B, and the linear free energy correlation will be 

The data lead to and pg values corresponding to interaction of the substituents with carboxyl and ester respectively of - 0.52 and — 0.96. 

The 4-parameter Hammett plot will in any case give a better correlation than a 

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