Substitution, electrophilic relative rate ratios

The relative rates for electrophilic substitution para to the halogens are examined graphically in Figs. 46-48. A further analysis of the results is accomplished through the Selectivity Treatment by assessment of the constancy of the logarithmic ratio, ]ogpfjS (32). 

For displacement reactions the partial rate factors af and /3f are calculated directly from the ratios of the rate constants of the heterocyclic and the benzene derivatives. For the electrophilic substitutions in which the proton is replaced, they may be easily calculated from the overall relative rates and the isomer distributions ... 

The existence of polarizability thus renders impossible any unique scale of electrophilic reactivity. The two most common theoretical measures, namely ir-electron densities and localization energies, correspond to transition states approximating the ground state and the Wheland intermediate, respectively, whereas the transition state (the precise structure of which is unknown), lies somewhere in between, it Densities, which relate to a situation where inductive effects are dominant, will tend to predict a relatively low 2- 3-rate ratio since all of the heteroatoms are inductive acceptors (-/). By contrast, since it electrons are delocalized from the heteroatoms more to the 2- than to the 3-position, localization energies will predict a high 2- 3-rate ratio. The importance of these factors becomes particularly evident in consideration of the substitution of benzo derivatives of these molecules (Chapter 8). 

These substitutions are facilitated by electron release from the heteroatom pyrroles are more reactive than furans, which are in turn more reactive than thiophenes. Quantitative comparisons of the relative reactivities of the three heterocycles vary from electrophile to electrophile, but for trifluoroacetylation, for example, the pyrrole furan thiophene ratio is 5 x 10 1.5 x 10 I " in formylation, furan is 12 times more reactive than thiophene, and for acetylation, the value is 9.3. In hydrogen exchange (deuteriodeproton-ation), the partial rate factors for the a and p positions of A-methylpyrrole are 3.9 x 10 ° and 2.0 x 10 ° respectively for this same process, the values for furan are 1.6 x 10 and 3.2 x l(f and for thiophene, 3.9 X 10 and 1.0 x 10 respectively, and in a study of thiophene, a P ratios ranging from 100 1 to 1000 1 were found for different electrophiles. Relative substrate reactivity parallels positional selectivity i.e. the a P ratio decreases in the order furan > thiophene > pyrrole. ° Nice illustrations of these relative reactivities are found in acylations of compounds containing two different systems linked together. ... 

On the whole the effect of substituents on the relative stability of isomeric arenium ions (for details see Sect. IV, 1) is described in the same terms as those used to explain the influence of substituents on the orientation and relative rates of electrophilic aromatic substitution. However, the isomeric composition of electrophilic substitution products is often controlled by kinetic factors while the equilibrium composition of isomeric arenium ions formed in aromatic compound protonation is determined by thermodynamic equilibrium. Therefore, no quantitative agreement may be observed between the relative hydrogen substitution rates at different positions of this compound and the ratio of equilibrium concentrations of the respective arenium ions formed in protonating the same compound even under identical conditions (cf. Sect. IV, 7). 

Evidently S, is a measure of intramolecular selectivity because it involves a ratio, the contribution of the benzene substitution rate disappears, and the selectivity factor expresses the selectivity of the reagent X in Eq. (7-83) for the para position relative to the meta position. Each individual partial rate factor, on the other hand, is expressive of an inteimolecular selectivity thus p is a measure of the selectivity of the reagent for the para position in CgHsY relative to benzene. It was observed that Eq. (7-85), where Cmc is a constant, is satisfied for a large number of electrophilic substitutions of toluene. 

In order to confirm the radical character of 51 and to extend its utility, oxidations of ary-lacetic acids to the corresponding ketones, aldehydes or alcohols have been conducted. Competitive decarboxylation reactions of phenylacetic acid and p-substituted phenylacetic acids were carried out. The ratio of the rate constants for the decarboxylation of various substituted phenylacetic acids relative to that of phenylacetic acid was found to decrease on decreasing the electron density at the benzylic carbon. Consequently, compound 51 shows an electrophilic oxidation ability towards arylacetic acids, giving a Hammett p value of —0.408. 

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