Partial rate factors nitration

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... 

Relative electrophilic localization energies vs. logarithms of partial rate factors for nitration (a) Hiickel, (6) PPP with fixed /3. (From Dewar Thompson. ) (iv) Plot of log K vs. AB c. (From Dewar. )... 

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. ... 

The isomer proportions for the nitration of the chlorotoluenes, to be expected from the additivity principle, have been calculated from the partial rate factors for the nitration of toluene and chlorobenzene and compared with experimental results for nitration with nitric acid at o °C. The calculated values are indicated in brackets beside the experimental values on the following structural formulae. In general, it can be... 

Here, and with the chlorotoluenes, the precise values for the calculated figures depend on the values adopted for the partial rate factors in the mono-substituted compounds. These and the relative rates do depend slightly on conditions. As has been pointed out several times previously, comparisons with benzene for nitration in sulphuric acid have to be made with care. 

Using the partial rate factors for nitration of chlorobenzene, Ridd and de la Mare calculated the relative rates of nitration of the dichlorobenzenes, with respect to /)-dichlorobenzene, with the results shown below. Also given are results based on more recent nitrations in mixed... 

Despite the considerable amount of work which has been reported, our knowledge of the nitration of biphenyl is not in a satisfactory state. The 0 p-T3.tw varies considerably with the conditions of nitration, and the cause of the variation is not fully understood. Nitrations with solutions prepared from nitric acid and acetic anhydride have generally given o -ratios greater than unity, the most consistent value being 2-2, obtained at o °C. The corresponding partial rate factors are reported later. 

The nitration of nitro- and dinitro-biphenyls has been examined by several workers. i - As would be expected, nitration of the nitro-biphenyls occurs in the phenyl ring. Like a phenyl group, a nitrophenyl group is 0 -directing, but like certain substituents of the type CH CHA ( 9.1.6) it is, except in the case of w-nitrophenyl, deactivating. Partial rate factors for the nitration at o °C of biphenyl and the nitro-biphenyls with solutions prepared from nitric acid and acetic anhydride are given below. The high o p-v2X o found for nitration of biphenyl... 

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. 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

The partial rate factors for nitration of tert-butylbenzene are... 

All the ring positions of (trifluoromethyl)benzene are deactivated compared with benzene The meta position is simply deactivated less than the ortho and para positions The partial rate factors for nitration of (trifluoromethyl)benzene are... 

A here y is the number of equivalent positions. A partial rate factor calculation for nitration of toluene is given in Example 10.1. 

Example 10.1. The nitration of toluene is 23 times faster than nitration of benzene in nitric acid-acetic anhydride. The product ratio is 63% ortho, 34% para, and 3% meta. Calculate the partial rate factors. 

Toluene is 17 times more reactive than benzene and isopropylbenzene is 14 times more reactive than benzene when nitration is carried out in the organic solvent sulfolane. The o m p ratio for toluene is 62 3 35, and for isopropylbenzene it is 43 5 52. Calculate the partial rate factors for each position in toluene and isopropylbenzene. Discuss the significance of the partial rate factors. Compare the reactivity at the various positions of each molecule, and explain any differences you consider to be significant. 

The outstanding problem is to decide how much, if any, association exists between N02 and X" in the generally rate-determining step of the reaction. Kinetic studies tend to indicate the presence of different electrophiles under different conditions whereas the derived partial rate factors are closely similar and therefore indicate one electrophile common to most, if not all, nitrating agents. The more electron-attracting is X , the more easily is N02 displaced from it and hence a reactivity sequence should be... 

From this work the deactivation of pyridine to benzene was estimated as about 107. The partial rate factors for nitration of the 3 position of pyridine and the corresponding pyridinium ion were 101,7-2,5 and 1020 respectively. 2-Phenylpyridine was evaluated as 4.9 x 10"5 times less reactive than benzene. 

The partial rate factor for nitration of pyridine-N-oxide in the 4 position was estimated as 4x 10"6 which is, therefore, close to that found for the 3 position of pyridine, and 2-phenylpyridine-N-oxide was evaluated as 2xl0-4 times less reactive than benzene from rate measurements in 74.7-78.6 wt. % acid at 25 °C. 

RATE COEFFICIENTS AND PARTIAL RATE FACTORS FOR NITRATION OFPhXMe3+ IN... 

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