Parameters, relationships among substituent

Hammett, after illustrating the existence of linear relationships among the data for a variety of side-chain reactions, defined the (7-constants to characterize the behavior of substituent groups. In the application of the Hammett equation the cr-parameters are assumed to be constant. The assessment of the validity of this same assumption for substituents in aromatic substitution reactions is the major problem which must be considered prior to the adoption of a simple two-parameter linear free-energy relationship for these reactions. Preliminary evaluations of linear relationships were undertaken through somewhat modified procedures as discussed in Section IV. Now, however, with many quantitative data available it is no longer necessary to rely on the less direct Selectivity Relationship. Rather, the more straightforward conventional Hammett approach is applicable. This procedure requires the adoption of the a1 -constants derived from the study of substituted phenyldimethylcarbinyl chlorides and the assumption of constancy of the values. This assumption is shown to be fully justified in subsequent tests of the relationship. 

Among Type A relationships, the Hammett and Taft equations are most frequently employed for noncatalytic reactions. When utilizing them for catalytic reactions we must consider the reliability of the substituent parameters and their suitability for the given structural type of reactants. The Hammett equation... 

In 1959, four independent and simultaneous reports defined the problem, van Bekkum et al. (1959), not distinguishing among the ordinary and electrophilic side-chain reactions, showed the o--constants to be broadly variable. They tabulated the a-parameters for twelve groups as derived from each reaction of an extended series. Just as illustrated in Fig. 50 for the p-methoxy, p-methyl, and p-chloro substituents, they detected extensive variations in the apparent values of the constants. The authors concluded that the postulation of two or three sets of -constants could not be regarded as sound. By eliminating all those reactions involving important resonance contributions Wepster and his associates (van Bekkum et al., 1959) obtained a series of reactions similar to those presented in Table 23, and hence derived a series of normal, an, parameters for meta and para substituents. These normal values, presumably representing the reactions for which the Hammett relationship is most precise, exhibit standard deviations of 10-30%. 

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