Partial rate factors table

The low selectivity affects the synthetic interest of homolytic arylation from two points of view. The first concerns the position of substitution generally all the free positions are substituted, giving very complex mixtures of isomers. Thus, for example, quinoline gives all the seven possible isomers in appreciable amounts.This is in contrast to all the homolytic substitutions described in the previous sections, which lead to exclusive attack at the 2- and 4-positions. The other aspect concerns the conversions of the heterocyclic compounds, which are always very low, usually lower than 1%. If the conversions are high, the mixture of the reaction products becomes much more complex. Thus with quinoline it can be easily foreseen from the partial rate factors (Table IX) that not only all the possible 21 diphenylquinoline isomers, but also... 

Fluorination of toluene gives a mixture of ortho- and para- uorotoluene, as expected for an electrophilic process (B), but the partial rate factors (Table 4) [139] show a very high ortho para ratio indicating that radical processes (A) must also be involved. Furthermore, fluorination of the methyl group, giving benzyl fluoride, also occurs in increasing yield as the reaction temperature is raised. 

TABLE 4.2 Nitration of aromatic compounds isomer proportions and partial rate factors ... 

Dewar and his co-workers, as mentioned above, investigated the reactivities of a number of polycyclic aromatic compounds because such compounds could provide data especially suitable for comparison with theoretical predictions ( 7.2.3). This work was extended to include some compounds related to biphenyl. The results were obtained by successively compounding pairs of results from competitive nitrations to obtain a scale of reactivities relative to that of benzene. Because the compounds studied were very reactive, the concentrations of nitric acid used were relatively small, being o-i8 mol 1 in the comparison of benzene with naphthalene, 5 x io mol 1 when naphthalene and anthanthrene were compared, and 3 x io mol 1 in the experiments with diphenylamine and carbazole. The observed partial rate factors are collected in table 5.3. Use of the competitive method in these experiments makes them of little value as sources of information about the mechanisms of the substitutions which occurred this shortcoming is important because in the experiments fuming nitric acid was used, rather than nitric acid free of nitrous acid, and with the most reactive compounds this leads to a... 

TABLE 7.1 Partial rate factors for some electrophilic substitutions of toluene... 

As has been noted above, there is no gross change in the mechanism of nitration of PhNH3+ down to 82 % sulphuric acid. The increase in o- andp-substitution at lower acidities has been attributed differential salt effects upon nitration at the individual positions. The two sets of partial rate factors quoted for PhNH3+ in table 9.3 show the effect of the substituent on the Gibbs function of activation at the m- and -positions to be roughly equal for reaction in 98 % sulphuric acid, and about 28 % greater at the -position in 82 % sulphuric acid. ... 

These partial rate factors have been recalculated from the experimental data of Dewar and Urch. Their reported values for diphenylmethane are not seriously discrepant with the values now given, but this is not so for the values for fluorene. As given, and copied in the literature, the values were /j = 2040 / = 60 fi = 944. There are consequent errors in table 8 and figs. 16 (reproduced as fig. 9. i of this volume) and 32 of ref. 22. 

The significance of Dewar s results for a series of polynuclear hydrocarbons, as well as for various compounds containing hetero atoms, has been discussed ( 5.3.2). Though the differences are not often important, we have not in all cases been able to reproduce the values for the partial rate factors reported by these authors, by recalculation from their reported results in table 5.3 the figures in parentheses are some examples of our recalculations. 

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

The first quantitative studies of the nitration of quinoline, isoquinoline, and cinnoline were made by Dewar and Maitlis, who measured isomer proportions and also, by competition, the relative rates of nitration of quinoline and isoquinoline (1 24-5). Subsequently, extensive kinetic studies were reported for all three of these heterocycles and their methyl quaternary derivatives (table 10.3). The usual criteria established that over the range 77-99 % sulphuric acid at 25 °C quinoline reacts as its cation (i), and the same is true for isoquinoline in 71-84% sulphuric acid at 25 °C and 67-73 % sulphuric acid at 80 °C ( 8.2 tables 8.1, 8.3). Cinnoline reacts as the 2-cinnolinium cation (nia) in 76-83% sulphuric acid at 80 °C (see table 8.1). All of these cations are strongly deactivated. Approximate partial rate factors of /j = 9-ox io and /g = i-o X io have been estimated for isoquinolinium. The unproto-nated nitrogen atom of the 2-cinnolinium (ina) and 2-methylcinno-linium (iiiA) cations causes them to react 287 and 200 more slowly than the related 2-isoquinolinium (iia) and 2-methylisoquinolinium (iii)... 

A more detailed study of the nitration of quinolinium (l) in 80-05 % sulphuric acid at 25 °C, using isotopic dilution analysis, has shown that 3-) 5-) 6-, 7- and 8-nitroquinoline are formed (table 10.3). Combining these results with the kinetic ones, and assuming that no 2- and 4-nitration occurs, gives the partial rate factors listed in table 10.4. Isoquinolinium is 14 times more reactive than quinolinium. The strong deactivation of the 3-position is in accord with an estimated partial rate factor of io for hydrogen isotope exchange at the 3-position in the pyridinium ion. It has been estimated that the reactivity of this ion is at least 10 less than that of the quinolinium ion. Based on this estimate, the partial rate factor for 3-nitration of the pyridinium ion would be less than 5 x io . 

TABLE 10.4 Theoretical and experimental partial rate factors for the nitration of the quinolinium ion... 

Partial rate factors for the nitration of 4-hydroxyquinoline and its derivatives are given in table 10.6. Comparison with the values for quinolinium (table 10.4) show that the introduction of a 4-hydroxy or a 4-methoxy group into the latter activates the 6-position by factors of 3-3 X 10 and 1-6 X 10 , respectively, and the 8-position by factors of 29-5 and 23, respectively. What has been said above makes the significance of partial rate factors which may be calculated for 4-hydroxy-cinnoline uncertain. 

Phenyl radicals attack azoles unselectively to form a mixture of phenylated products. Relative rates and partial rate factors are given in Table 7. The phenyl radicals may be prepared from the usual precursors PhN(NO)COMe, Pb(OCOPh)4, (PhC02)2 or PhI(OCOPh)2. Substituted phenyl radicals react similarly. 

Table 7 Relative Rates and Partial Rate Factors for the Homolytic Phenylation of Five-membered...

Table 32 Logarithms of the Partial Rate Factors Calculated According to the Standard Conditions...

Table 10.8. Partial Rate Factors for Hydrogen Exchange in Some Substituted Aromatic Compounds...

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 ... 

Finally, it is customary to compare the partial rate factors obtained under different conditions to indicate the reactivity of the electrophile. Unfortunately, the medium, nitric acid in sulphuric acid, in which the nitronium ion is most clearly established as the electrophile, is such a poor solvent for aromatics that meaningful competitive nitrations are impossible and kinetic studies are hampered by the difficulties noted above. However, since the isomer distribution is a function of the rate factors, inspection of these distributions (Table 15) shows a very... 

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. 

Gallium bromide was used as the catalyst in a determination346 of the partial rate factors, by the competition method, for ethylation of some substituted benzenes in 1,2-dichloroethane at 25 °C. No direct rate measurements were made and the results, summarised in Table 80, show the low selectivity and high steric hindrance in the reaction. 

Nesmeyanov et a/.546 have also measured the effects of substituents in deuteration of ferrocene by deuterated trifluoroacetic acid in dichloromethane at 25 °C. Rate coefficients were measured for ferrocene and its derivative in a range of such acid mixtures, the composition of which was omitted, and in some cases the rate of exchange for ferrocene was calculated on the basis of a linear relationship between log and —H0. Results including the calculated knl values are given in Table 161. It should be noted that, in discussing those results, the authors quoted the incorrect partial rate factors for dedeuteration of toluene arising from the use of the incorrect data for benzene (see p. 199). This should be taken into account... 

The lack of steric hindrance is also shown by the kinetic data for p-xylene, mesitylene, and durene, the observed reactivities being close to those calculated by the additivity principle. The additivity principle has also been tested for the last seven compounds in Table 177, and for the first five of these it holds very well. If one assumes a value for/3Me0 of ca. 4.0 and takes the average of the values listed in the table for the methyl substituent partial rate factors, then the observed calculated reactivity ratios are 1.6, 0.85, 0.75, 1.4 and 1.0. For the last two compounds in the table the ratios are 5.3 and 4.1, the reason for this being unknown. 

From this it is possible to calculate the overall theoretical rate ratio for acetylation of m-xylene relative to benzene, since this is one-sixth the sum of the partial rate factors (in this case 1130), and the isomer distribution if the reaction is kinetically controlled. The overall rate ratio actually is 347 and the calculated and observed isomer distributions are listed in Table 11.2. In this case, and in many others, agreement is fairly good, but many cases are known where the effects are not additive. For example. 

TABLE 14.2 Partial Rate Factors for Attack of Substituted Benzenes by Phenyl Radicals Generated from BZ2O2 (Reaction 14-21)... 

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