The Selectivity Relationship

Brown developed the selectivity relationship before the introduction of aromatic reactivities following the Hammett model. The former, less direct approach to linear free-energy relationships was necessary because of lack of data at the time. 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

A Quantitative Treatment of Reactivity of the Electrophile the Selectivity Relationship... 

A modification of the Hammett approach, suggested by Brown, called the selectivity relationship is based on the principle that reactivity of a species varies inversely with selectivity. Table 11.3 " shows how electrophiles can be arranged in order of selectivity as measured by two indexes (1) their selectivity in attacking toluene rather than benzene, and (2) their selectivity between the meta and para positions in toluene. As the table shows, an electrophile more selective in one respect is also more selective in the other. In many cases, electrophiles known to be more... 

Given this behavior (little selectivity in distinguishing between different substrate molecules), the selectivity relationship would predict that positional selectivity should also be very small. However, it is not. For example, under conditions where nitration of p-xylene and 1,2,4-trimethylbenzene takes place at about equal rates, there was no corresponding lack of selectivity at positions within the latter. Though... 

It is clear that the selectivity relationship has broken down and it becomes necessary to explain why such an extremely rapid reaction should occur with positional selectivity. The explanation offered is that the rate-determining step is formation of an encounter complex (12, p. 506).84 Since the position of attack is not determined in the rate-determining step, the 5/6 ratio is not related to the reaction rate. Essentially the same idea was suggested earlier85 and for the same reason (failure of the selectivity relationship in some cases), but the earlier explanation specifically pictured the complex as a it complex, and we have seen... 

One interesting proposal86 is that the encounter pair is a radical pair N02 ArH formed by an electron transfer (SET), which would explain why the electrophile, once in the encounter complex, can acquire the selectivity that the free N02+ lacked (it is not proposed that a radical pair is present in all aromatic substitutions only in those that do not obey the selectivity relationship). The radical pair subsequently collapses to the arenium ion. There is evidence87 both for and against this proposal.88... 

In 1956, as additional information was obtained for a range of substitution reactions, it was proposed that the linear relationship (8) be designated as the Selectivity Relationship (Brown and Smoot, 1956). The reactivity ratio, pfe/mfe, representing a measure of the discriminatory property of the electrophile, was employed to define the Selectivity Factor, 8t (9). In cases where the partial rate factors are not known, 8f... 

The empirical expression (8) is then rewritten as the Selectivity Relationship (11). 

Ample data (Table 2) are now available to permit a full and adequate test of the Selectivity Relationship (11). Figure 7 illustrates the remarkably precise linear relationship between log pfa and 8t. A previous least-squares analysis of the results for 47 reactions indicated... 

The empirical relationship between the para/meta ratio for substitution in toluene and the value of pfe (the Selectivity Relationship (11)), is closely obeyed by the substitution and displacement reactions of toluene and its derivatives (Section II). 

Algebraic treatment of this equation, as for expressions (16) and (17), leads to the Selectivity Relationship (21). 

Figure 29 presents an analysis of the data for p-phenyl groups in the Extended Selectivity Treatment. The reactivity of the para position increases significantly with an increase in the electron demand of the substitution reaction. This result is confirmed by an analysis of the data through the Selectivity Relationship in which a linear relationship is predicted for a diagram of log pfh against S (Fig. 30). Again, curvature is evident. It must be concluded that the substitution reactions of biphenyl do not adhere to a linear free-energy relationship (Eabom and Taylor, 1961b Stock and Brown, 1962a).

The failure of the Selectivity Relationship and the Extended Selectivity Relationship presented a serious problem. The p-phenyl substituent activates the aromatic ring by the same mechanism as the p-methoxy group. Accordingly, it was suggested (Knowles et al., 1960) that the variation in reactivity was a reflection of the variability of resonance stabilization merely as a function of electron demand. The... 

A more detailed exploration of the reactivity of biphenyl resolves the problem. The ra-phenyl substituent reduces the rate of substitution in the benzene nucleus (Table 7). Qualitatively, this effect is in agreement with the predictions based on the rate of solvolysis of ra-phenylphenyl-dimethylcarbinyl chloride (Brown and Okamoto, 1958) and with the expected electron-withdrawing properties of the phenyl group. The data conform to the Selectivity Relationship with reasonable precision (Fig. 31). In view of the activation of the ortho and para positions, direct evaluation of the partial rate factors for the deactivated meta position is not always possible. Hence, indirect kinetic procedures were employed in several cases, halogenation and acylation, to estimate the values. Graphical analysis of the data shows that mfb is independent of the reagent selectivity. Deviations from the relationship are no greater than for the ordinary side-chain reactions. 

The Selectivity Relationship was shown to be applicable for substitution in the meta and para positions of toluene (Section II). The fine adherence of the -methyl group to a linear free-energy relationship (Fig. 37) is apparently typical of the behavior of the other alkyl substituents, as illustrated for the p-ethyl, p-i-propyl, and p-t-butyl groups (Figs. 38-40). Indeed, the data for electrophilic substitution in toluene are better correlated by a linear relationship than are the data for ordinary side-chain reactions of p-tolyl derivatives (Stock and Brown, 1959a). In the Extended Selectivity Treatment (Fig. 25) the side-chain reactions show a slightly greater scatter from the correlation line than the aromatic substitution reactions. 

The observations for the electrophilic substitution reactions of the monosubstituted benzenes have been examined for adherence to a linear free-energy relationship. As shown, the Selectivity Relationship,... 

Stock and Brown (1959) have carried out a detailed study of reactions involving the electrophilic substitution of hydrogen in toluene, and have shown that the selectivity relationship (eqs. (11), (20-23) of article by Stock and Brown) is applicable. 

The partial rate factors vary within wide limits. Electrophilic species of lower activity, such as molecular bromine, are more selective, i.e., more capable of discriminating either between thiophene and benzene or between positions a and j8 of thiophene. Quantitatively, a linear trend is observed (Fig. 1) between loga/j3 and logotf. This is a correlation formally analogous to the selectivity relationship proposed by Brown and Nelson188 for the reactions of monosubstituted benzenes. 

The data indicate a marked selectivity of the dicyanobutane for aromatics as compared with other hydrocarbons. This selectivity is greatest over paraffins, somewhat less over naphthenes, and appreciably less over unsaturated naphthenes. Selectivity for aromatics increases with increasing molecular weight of paraffins and naphthenes and with decreasing molecular weight of the aromatic. In comparison, sulfolane shows a lower solvency for all the hydrocarbons. Thus, dicyanobutane on an equal volume basis has a higher capacity for aromatic hydrocarbons, while the selectivity relationships are comparable. 

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