Partial rate factors for chlorination

The partial rate factors for chlorination of biphenyl are as shown... 

Of the groups shown, which is the most likely candidate for substituent X based on the partial rate factors for chlorination ... 

In sharp contrast, the relative rates, isomer distributional, and partial rate factors for chlorination depend Importantly on the character of the reagent and catalysts. The order-of-magnitude differences in Indicate that there are Important... 

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. 

Chlorination in acetic acid is characterized by a large p value ( — 9 to —10), and the partial rate factor for toluene is 820. Both values indicate a late transition state, which would resemble the rr-complex intermediate. 

Bromination follows the same general pattern as chlorination [78JCS(P2)865] (Scheme 22). A comprehensive kinetic study has demonstrated that benzo derivatives are much less reactive than imidazole itself. Partial rate factors for the bromination of 31 (R = H) were 5-bromination, 6.37 x I07 7-bromination, 2.88 x 106. For the 7-bromination of 6-bromobenzimidazole the factor was also 2.88 x 106, confirming that... 

Comparison of the partial rate factors for nitration of toluene with those for chlorination and bromination (above) show that these differ, both absolutely and relatively, with the attacking electrophile in other words relative directive effects in C6H5Y do depend on E as well as on Y. We notice above that the absolute values of the partial rate factors, i.e. fcY//cH, increase in the order,... 

Chlorination and bromination of pyridine and some alkylpyridines in the (3-position can be effected in the liquid phase at 100°C using excess AlCl3as catalyst. -Bromination of pyridine and 2- and 4-picoline is conveniently effected in oleum at 80-120°C. Bromination kinetics using HOBr in aqueous HC104 indicate that the partial rate factor for bromination of the pyridinium cation is 10 13, comparable to that for nitration. 

Acylation of the amino substituent decreases the rate of bromination by 1010 (Robertson et al., 1953). Acetanilide is, accordingly, somewhat more readily studied. The partial rate factors for bromination, chlorination, and mercuration are presented in Table 4. 

The partial rate factors for the substitution reactions of biphenyl, with the exception of a few observations, are on a firm experimental basis. The chlorination of biphenyl was examined on several occasions (de la Mare et al., 1958a Beaven et al., 1961 Mason, 1959 Dewar and Mole, 1957). There are significant differences in the reported values for the rate relative to benzene. A recent careful examination of the products (Beaven et al., 1961) indicated the formation of 2- and 4-chloro-biphenyl in 76.5% yield with 17.5% of the residual chlorine consumed via addition processes. The partial rate factors presented in the table are corrected on this basis. Two early studies of the nitration of biphenyl with acetyl nitrate in acetic anhydride yield rate data in poor agreement (Dewar et al., 1956 Simamura and Mizuno, 1957). A recent re-examination of the problem (Billings and Norman, 1961) yielded partial rate factors (ofh = 36.4 = 32.6) confirming the results... 

The partial rate factors for naphthalene (Table 9) require comment in two cases. The chlorination data are approximate in that the product distribution is unestablished. Moreover, the reports for the rate of chlorination of naphthalene relative to benzene are in poor agreement (Dewar and Mole, 1957 Mason, 1959). The rate constants for the deuteration of naphthalene are not precise. These rate data are not based on an isomer distribution but rather extrapolated from observations for polydeuteration (Dallinga et al., 1957). 

The application of the Extended Selectivity Treatment to the partial rate factors for the wi-fluoro, ra-chloro, and ra-bromo substituents is shown in Figs. 43-45. Too few data are available for ra-iodo. Inspection of these diagrams reveals that substitution meta to the halogens conforms to a linear relationship. No serious deviations are detected. The log mf values for non-catalytic chlorination are displaced from the correlation line, but this discrepancy is almost certainly the consequence of the failure of the additivity principle on which the results are based (Stock and Baker, 1962). 

Fig. 9.6. Correlation between the interaction energy X. and the log of partial rate factors for mercuration, nitration, and chlorination. Reproduced from J. Phys. Chem. A, 107, 2875 (2003), by permission of the American Chemical Society.

The partial rate factors for nitration are compared with the related data for acetylation and chlorination in Table VIII. 

Relative rates and partial rate factors have been determined for the chlorination of some aromatics by chlorine acetate in 76 % aqueous acetic acid at 25 °C209 these are given in Table 65. The spread of rates is, therefore, smaller than is found with molecular chlorine and this is entirely consistent with the lower reactivity of the latter reagent. 

There are extensive data for the acid-catalyzed protiodesilylation of XCgELrSiMes in methanol-aqueous perchloric acid or acetic acid-aqueous sulphuric acid at 50°C225. Correlation analysis of the partial rate factors (relative rate constants) by means of the Yukawa-Tsuno equation (Section n.B) finds p = —5.3 and r+ = 0.65. These values are consistent with a relatively low demand for stabilization of the transition state by electron delocalization, i.e. the transition state is early along the reaction coordinate, p-NO2 is highly deactivating with / = 14 x 10 but 0-NO2 is even more deactivating, with / = 6.8 x 10-5. This contrasts with the deactivation order discussed above for nitration and chlorination (Table 6), and may be explained in terms of the early transition state, well removed from the Wheland intermediate. 

Chlorination of 9-acetylcarbazole in acetic acid produced values of 4,300 (C-1), 8,600 (C-2), 122,000 (C-3), and 8,600 (C-4) for the partial rate factors. Chlorination of carbazole itself gave values of > 10 and > 10 for the 1- and 3-positions chlorination of carbazole was found to be six times slower than that of diphenylamine. ... 

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