P-substituted aromatic compounds

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Synthons such as 237 or 238 might appear to be non-productive as the availability of reagents corresponding to these synthons may be questionable. Nevertheless, if one takes into account the options to synthesize p-substituted aromatic compounds and their ease of conversion into ci, s -l,4-disubstituted cyclohexanes via catalytic hydrogenation), the proposed disconnections become immediately feasible. This reasoning leads to routes A and B as realistic pathways for the preparation of 236. 

The foregoing has been illustrated in Table 53, taken from the work of Nixon and Branch. The relative rates of the alcoholyses of two types of p-substituted aromatic compounds, benzoyl chlorides and trityl chlorides, have been calculated from the rates for the p-bromo derivatives, using Hammett s values for observed values. The alcoholysis of the benzoyl chlorides has a positive value of p, and the calculated and observed values agree as closely as can be expected. The alcoholysis of the trityl chloride has a negative value of p, and here the calculated and observed values agree as closely as can be expected except that of... 

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