Partial rate factors for bromination

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. 

Scheme 8.8. Partial rate factors for bromination benzimidazole, 5-bromobenimidazole, and indazole.

Rates of bromination of several methylpyridines by hypobromous acid in aqueous perchloric acid at 25°C (Table 9.9) have been measured. Rate-acidity profiles and comparison with model compounds showed that the conjugate acids were the reacting species (74JOC3481). From this work the partial rate factor for bromination of the 3-position of the pyridinium ion was estimated as 2-6 x 10 which gives a o+3 value of —2.0. 

Rf.lative Rates and Partial Rate Factors for Bromination of Pyridinium Ions"... 

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... 

The relative rates of bromination from the above and additional studies yielded the following relative rates with partial rate factors for the indicated position in parentheses. These rates were mostly obtained by extrapolation of observed rates to those expected in 50% aqueous acetic acid and therefore probably contain small errors benzene, 1.0 (1.0) biphenyl, 1.54 x 103(4-,4.34 x 103) naphthalene, 1.24 x 105(1 -, 1.84 x 10s 2-, 1.86 x 103) phenanthrene, 7.43 x I03(9—, 2.23 x 10s) f uoranthrene, 2.30 x 106(3 —,6.90x 106) chrysene,... 

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,... 

Table 9 Relative Rates and Partial Rate Factors for the Bromination of some Pyridinium Perchlorates with Hypobromous Acid in Perchloric Acid at 25 °C...

Because the rate of substitution varies with position, in a benzene derivative it is more informative and frequently more useful to talk about partial rate factors than about relative rates. A partial rate factor is defined as the rate at one particular position in the benzene derivative relative to the rate of substitution at one position in benzene. Let us, for example, calculate the para and meta partial rate factors (pf and mf, respectively) for bromination of toluene with bromine in aqueous acetic acid. Toluene brominates 605 times faster than benzene under these conditions. The product is 66.8 percent p-, 0.3 percent m-, and 32.9 percent o-bromotoluene. Attack at the para position of toluene occurs 0.668 x 605 times as fast as attack at all six positions of benzene but (0.668 x 605 x 6 = 2420) times as fast as at one position of benzene. Therefore pfCH for bromination of toluene under these conditions is 2420. There are only three times as many total carbons in benzene as meta carbons in toluene. Therefore mfca3 = 0.003 x 605 x 3 = 5.5. The definitions of the partial rate factors for monosubstituted benzenes (—R) are given in Equations 7.78-7.80. 

Table 9.12 compares partial rate factors for substitution by phenyl radical with those for electrophilic bromination. Selectivity is clearly much lower for the radical substitution furthermore, for attacking phenyl radical, nearly all positions in the substituted benzenes are more reactive than in benzene itself, a finding that reflects the tendency for most substituents to stabilize a radical, and thus to lower transition state energy for formation of the cyclohexadienyl intermediate, when compared with hydrogen. The strong polar effects, which cause the familiar pattern of activation and deactivation in the electrophilic substitutions, are absent. One factor that presumably contributes to the low selectivity in radical attack is an early transition state in the addition step, which is exothermic by roughly 20 kcal mole-1.178... 

The additivity treatment also allows one to evaluate the influence of substituents which are otherwise obtainable only with difficulty. The study of the non-catalytic bromination of the halo-substituted poly-methylbenzenes by Illuminati and Marino (1956) allowed the evaluation of the partial rate factors for the highly deactivating m- and p-halogens. These data for the slow, highly selective bromination are inaccessible by other techniques. Analysis of the relative rates is made by application of the additivity equations (5) and (6) as described in Section I. An important aspect of the chemistry of the substituted polymethyl-benzenes, in contrast to the monosubstituted benzenes, is the large difference in p for bromination. The partial rate factors derived for each reaction are correlated with good precision by the tr4 -constants (Figs. 11 and 19). Yet the susceptibility of the reactions to the influence of substituents is altered by more than 25%. As already noted, this aspect of the problem is not well defined and is worthy of additional attention. 

Figures 3. The relationships between the partial rate factors for hypobromous acid-catalyzed bromination and a...

Partial rate factors for the SgAr bromination of toluene. (Data from reference 166.)... 

Partial rate factors calculated for indazole bromination indicate that the benzo derivative is less reactive than pyrazole a positional reactivity order of 5 > 3 > 7 (in the ratio 10.7 6.9 I) was obtained [78JCS(P2)865]. 

Novel techniques for the study of fast reactions were employed to study the bromination of AT.AT-dimethylaniline and its derivatives by Bell and Ramsden (1958). The second-order rate constant for the bromination of N,AT-dimethylaniline in acid solution was approximately 109 1. mole-1 sec-1. An estimate of the rate of bromination relative to the rate of substitution of benzene indicates that the methylated aniline is 1019 times more reactive (Robertson et al., 1953). The large influence of the p-dimethylamino substituent has discouraged extended quantitative study. Nevertheless, Eabom and his associates (Eabom, 1956 Eaborn and Pande, 1961a Eabom and Waters, 1960) assessed the influence of the group in several displacement reactions. Eaborn points out, however, that the resulting partial rate factors are only approximate. 

Another illustration of the application of the additivity procedure is the scheme employed to determine mp for non-catalytic bromination (Brown and Stock, 1957a). Direct analysis of the products of bromination of toluene indicates only 0.3% m-bromotoluene providing mfe is 5.5 (Table 2). The difficulties in the analysis of the products for the small quantity of meta isomer introduce considerable uncertainty in the partial rate factor. As a test of the observation, the rate of bromination of p-xylene was examined. The additivity treatment (40) yields expression (41),... 

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. 

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