The Jaffe Relationship

Transmission of the polar effect from substituent (X) to reaction centre (RJ in benzenoid systems involves more than one route. The meta substituent in 3 transmits its effect to the ipso position via the 2-position or via the 4, 5 and 6 positions. The overall effect at is due to the combined transmission routes and this is constant provided the only route to Rx is through the ipso position. 

In some aromatic systems, particularly in fused benzenoid structures, the two transmission routes can be separated experimentally. The function Z in structure 4 lies on the transmission track to and the substituent X is para to Z but meta to R. Which value is to be used, o n or Op Structures 5 and 6 illustrate this problem for reaction at R in substituted benzothiazoles and 7 and 8 for the hydrolysis of salicylate esters which includes transmission via the carboxylate anion. 

A more explicit separation of transmission routes is illustrated in the dissociation of N-phenyl benzene sulphonamides (9) where the effect of the substituent in the aniline residue Y has a manifestly different path from that of X in the benzene sulfonyl residue. 

In this case the ipso position is that on the aromatic ring adjacent to the reaction centre, R,. 

The similarity coefficient for each substituent effect can be solved sep- 

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