Electrophilic substitution, aromatic partial rate factors

The applicability of the two-parameter equation and the constants devised by Brown to electrophilic aromatic substitutions was tested by plotting values of the partial rate factors for a reaction against the appropriate substituent constants. It was maintained that such comparisons yielded satisfactory linear correlations for the results of many electrophilic substitutions, the slopes of the correlations giving the values of the reaction constants. If the existence of linear free energy relationships in electrophilic aromatic substitutions were not in dispute, the above procedure would suffice, and the precision of the correlation would measure the usefulness of the p+cr+ equation. However, a point at issue was whether the effect of a substituent could be represented by a constant, or whether its nature depended on the specific reaction. To investigate the effect of a particular substituent in different reactions, the values for the various reactions of the logarithms of the partial rate factors for the substituent were plotted against the p+ values of the reactions. This procedure should show more readily whether the effect of a substituent depends on the reaction, in which case deviations from a hnear relationship would occur. It was concluded that any variation in substituent effects was random, and not a function of electron demand by the electrophile. ... 

Partial rate factor (Section 12 10) In electrophilic aromatic substitution a number that compares the rate of attack at a particular nng carbon with the rate of attack at a single po sition of benzene... 

The table below gives first-order rate constants for reaction of substituted benzenes with w-nitrobenzenesulfonyl peroxide. From these data, calculate the overall relative reactivity and partial rate factors. Does this reaction fit the pattern of an electrophilic aromatic substitution If so, does the active electrophile exhibit low, moderate, or high substrate and position selectivity ... 

These relative rate data per position are experimentally detennined and are known as partial rate factors. They offer a convenient way to express substituent effects in electrophilic aromatic substitution reactions. 

There is, however, no very satisfactory explanation of why such m-attack as does take place on QH5Y should also be faster than attack on QHg or of why attack on the o-position in C6H5Y is commonly faster than attack on the p-position. The relatively small spread of the partial rate factors for a particular QH5Y means that homolytic aromatic substitution normally leads to a more complex mixture of products than does electrophilic attack on the same species. 

Dewar, M. J. S., T. Mole, and E. W. T. Warford, Electrophilic Substitution. Part VI. The Nitration of Aromatic Hydrocarbons Partial Rate Factors and Their Interpretation, J. Chem. Soc., Part 111, 3581-3586 (1956). 

The partial rate factors af and /3f for the a- and /3-positions of thiophene have been calculated for a wide range of electrophilic reactions these have been tabulated (71 AHC(13)235, 72IJS(C)(7)6l). Some side-chain reactions in which resonance-stabilized car-benium ions are formed in the transition states have also been included in this study. A correspondence between solvolytic reactivity and reactivity in electrophilic aromatic substitution is expected because of the similar electron-deficiency developed in the aromatic system in the two types of reactions. The plot of log a or log /3f against the p-values of the respective reaction determined for benzene derivatives, under the same reaction conditions, has shown a linear relationship. Only two major deviations are observed mercuration and protodemercuration. This is understandable since the mechanism of these two reactions might differ in the thiophene series from the benzene case. 

Not all radical aromatic substitutions are as immune to polar effects as is attack by phenyl. Some radicals reveal marked electrophilic or nucleophilic character. Oxygen-centered radicals, for example, are electrophilic, as would be expected if there is substantial polar contribution to the transition state. Table 9.13 lists partial rate factors for substitution by benzoyl radicals note that the orientation and activation trends found in typical electrophilic substitutions have begun to appear, but are still modest compared with the dramatic effects shown in Table 9.12 for a true heterolytic substitution.179... 

The effect of the o-c-Pr group in electrophilic aromatic substitution is anomalous Thus in nitration high o, p-product ratios were obtained (2.0-4.7), whereas in halogenation the ratios are much lower. It was found in aromatic detritiation, a reaction suggested to be free of steric effects that the ratio of partial rate factors for /p//o for cyclopropyl was 9, and this was interpreted that the high fraction of o-nitration was a peculiarity of this particular reaction and not a general effect of the cyclopropyl group. ... 

Equimolar mixtures of toluene and benzene were passed over beds of DHY at low temperatures (25-60°) in experiments where the two aromatics of different reactivity competed for the electrophilic deuterium (75). The distribution of deuterium between toluene and benzene (apparent ACeHsCHs/fcCeHe) and among the ring positions of the toluene-di samples was determined. A plot of log pt vs selectivity factor (St) for these data from the competitive experiments at 25-60° (Fig. 20, black circles) falls on the line obtained from a study of 47 electrophilic substitution reactions by H. C. Brown and associates (83). The partial rate factors pt and mt give the rate of substitution of the para position and one of the meta positions in toluene, relative to the rate of substitution of one of the six equivalent ring positions of benzene. Points a, b, c, d, and e fall quite close to the line, which represents a linear free energy relationship in both positional and substrate selectivity. 

TABLE 11. Partial rate factors for electrophilic aromatic substitution of some aromatic silicon compounds... 

The partial rate factors and the isomer distribution in the amination by di-methylamino radical cation of toluene, isopropylbenzene, -butylbenzene, biphenyl and naphtalene are reported in Table 7. These partial rate factors are far the highest ever observed in homolytic substitutions so that the general character of the homolytic amination allows a more relevant analogy to be drawn with the electrophilic substitutions than with the homol5rtic arylation, the only homol5rtic substitution for which numerous and accurate quantitative data exist in homo-cyclic aromatic series. 

The effect of substituents on electrophilic substitution can be placed on a quantitative basis by use of partial rate factors. The reactivity of each position in a substituted aromatic compound can be compared with benzene by measuring the overall... 

Rate and product studies of electrophilic aromatic substitution in halobenzenes reveal a fairly consistent pattern of reactivity. The partial rate factors for chlorination show that, with one exception, all the ring positions of fluoro-, chloro-, and bromoben-zene are deactivated. The exception is the para position of fluorobenzene, which is slightly more reactive than a single position of benzene. 

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