Nitration, aromatic table

T. Urbanski and Rabek-Gawronska [11] found that cyclonite dissolves in molten, highly-nitrated aromatic hydrocarbons, substituted urea derivatives, and camphor to form eutectics of the composition given in Table 18. It is almost insoluble in molten diphenylamine. 

Using Menke s conditions, Smith et al.[29,30] have described a method for the nitration of benzene, alkylbenzenes and halogenobenzenes using zeolites with different topologies (HBeta, HY, HZSM-5 and HMordenite) as catalysts and a stoichiometric amount of nitric acid and acetic anhydride. The reactions were carried out without solvent at temperatures between -50 °C and 20 °C. For the nitration of toluene, tridirectional zeolites HBeta and HY were the most active catalysts achieving >99 % conversion in 5 min reaction time. However, HY exhibited selectivity to the p-nitrotoluene very similar to the homogeneous phase, while with HBeta, selectivities to p-nitrotoluene higher than 70% could be achieved. HBeta zeolite exhibited excellent para-selectivity for the nitration of the different monosubstituted aromatics (Table 5.1). The catalyst can be recycled and the only by-product, acetic acid, can be separated by vacuum distillation. 

It forms eutectics with other high nitrated aromatic compounds and certain nitric esters (Table 40). (See also Burkhardt [65].)... 

Trinitrotoluene, like other highly nitrated aromatic hydrocarbons, easily forms addition products with polycyclic hydrocarbons and amines. The addition products formed by a- trinitrotoluene have been extensively studied by Hepp [35] and Kremann [104a,104b]. Some of the products are listed below (Table 69). 

This Table shows that TNB is the most powerful explosive among the nitrated aromatic hydrocarbons. One amino group, as in TNA, reduces its strength and so hydroxyl group as in TNPh(PA). Two hydroxyl groups... 

Ordinary eutectics are formed by nitromannitol with higher nitrated aromatic compounds (Table 27). 

BZ was al so partially oxidized by Ag(II) in a small H-cell with stationary platinum electrodes. Compounds identified in anolyte extracts included phenol, hydroquinone, benzoquinone, benzaldehyde, benzoic acid, methyl benzoate, benzonitrile, benzonitrile aldehyde, and 4-nitro butylnitrile. The yellow color of the anolyte was probably due to benzoquinone, which had a relatively high concentration. A compound which was tentatively identified as benzoquinone epoxide ( 11403) was present at the highest concentration and is believed to be a product of the oxidation of benzoquinone. Numerous nitrated aromatics were also detected and include nitrobenzene, dinitrobenzene isomers, nitrophenol isomers, and dinitnophenol isomers. Intermediates are summarized in Table 3 and classified as I. BZ substrate II. nitrated BZs HI. phenols, quinones, and epoxides IV. nitrated phenols V. BZ substituted with aliphatic and aromatic... 

Some acylated aromatic amines may be characterized by nitration (see Table 19). 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

For the last two compounds, first-order rates were observed towards the end of the reactions, enabling the reactivities of these compounds relative to that of water to be estimated (table 2.1). The nitration of 2-mesitylethanesulphonate anion was independent of the concentration of the aromatic over 80 % of its course, and because the final part of the reaction did not obey a truly first-order law its reactivity could not be estimated. 

A simple kinetic order for the nitration of aromatic compounds was first established by Martinsen for nitration in sulphuric acid (Martin-sen also first observed the occurrence of a maximum in the rate of nitration, occurrii for nitration in sulphuric acid of 89-90 % concentration). The rate of nitration of nitrobenzene was found to obey a second-order rate law, first order in the concentration of the aromatic and of nitric acid. The same law certainly holds (and in many cases was explicitly demonstrated) for the compounds listed in table 2.3. 

Many aromatic compounds are sufficiently basic to be appreciably protonated in concentrated sulphuric acid. If nitration occurs substantially through the free base, then the reactivity of the conjugate acid will be negligible. Therefore, increasii the acidity of the medium will, by depleting the concentration of the free base, reduce the rateof reaction. This probably accounts for the particularly marked fall in rate which occurs in the nitration of anthraquinone, benzoic acid, benzenesulphonic acid, and some nitroanilines (see table 2.4). 

For nitrations in sulphuric and perchloric acids an increase in the reactivity of the aromatic compound being nitrated beyond the level of about 38 times the reactivity of benzene cannot be detected. At this level, and with compounds which might be expected to surpass it, a roughly constant value of the second-order rate constant is found (table 2.6) because aromatic molecules and nitronium ions are reacting upon encounter. The encounter rate is measurable, and recognisable, because the concentration of the effective electrophile is so small. 

A similar circumstance is detectable for nitrations in organic solvents, and has been established for sulpholan, nitromethane, 7-5 % aqueous sulpholan, and 15 % aqueous nitromethane. Nitrations in the two organic solvents are, in some instances, zeroth order in the concentration of the aromatic compound (table 3.2). In these circumstances comparisons with benzene can only be made by the competitive method. In the aqueous organic solvents the reactions are first order in the concentration of the aromatic ( 3.2.3) and comparisons could be made either competitively or by directly measuring the second-order rate constants. Data are given in table 3.6, and compared there with data for nitration in perchloric and sulphuric acids (see table 2.6). Nitration at the encounter rate has been demonstrated in carbon tetrachloride, but less fully explored. ... 

TABLE 4.1 Nitration of aromatic compounds relative rates at 25 °C... 

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

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

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 5.3 The nitration of polycyclic aromatic compounds in solutions of acetyl nitrate in acetic anhydride... 

Expts. 16, //. Pure nitric acid was used. In expt. 16 the reaction was of the first order in the concentration of the aromatic, and of half-life 1-1-5 minutes (similar to that of toluene under the same conditions). In expt. 17 the sodium nitrate slowed the reaction (half-life c. 60 min). About 2 % of an acetoxylated product was formed (table 5-4). 

Melander first sought for a kinetic isotope effect in aromatic nitration he nitrated tritiobenzene, and several other compounds, in mixed acid and found the tritium to be replaced at the same rate as protium (table 6.1). Whilst the result shows only that the hydrogen is not appreciably loosened in the transition state of the rate-determining step, it is most easily understood in terms of the S 2 mechanism with... 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

Table 31 Kinetic Nitration Studies on Pyrazoles and Aromatic Pyrazolones (Reagent HNO3/H2SO4)...

Table 2 summarizes some of the transformations of substituents which have been carried out on azetidines without effect on the ring <79CRV33l). Other transformations of interest are the base catalyzed epimerization, H exchange and alkylation of the activated H-3 in azetidines (26) (69JHC153) and the nitration, reduction, diazotization and hence further modification of the aromatic ring in 3-phenyl-fV-acetylazetidine (27) (61LA 647)83). 

Comparison of Table 5.4 and 5.7 allows the prediction that aromatic oils will be plasticisers for natural rubber, that dibutyl phthalate will plasticise poly(methyl methacrylate), that tritolyl phosphate will plasticise nitrile rubbers, that dibenzyl ether will plasticise poly(vinylidene chloride) and that dimethyl phthalate will plasticise cellulose diacetate. These predictions are found to be correct. What is not predictable is that camphor should be an effective plasticiser for cellulose nitrate. It would seem that this crystalline material, which has to be dispersed into the polymer with the aid of liquids such as ethyl alcohol, is only compatible with the polymer because of some specific interaction between the carbonyl group present in the camphor with some group in the cellulose nitrate. 

Because nitration has been studied for a wide variety of aromatic compounds, it is a useful reaction with which to illustrate the directing effect of substituent groups. Table 10.3 presents some of the data. A variety of reaction conditions are represented, so direct comparison is not always valid, but the trends are nevertheless clear. It is important to remember that other electrophiles, while following the same qualitative trends, show large quantitative differences in position selectivity. 

Table 10.7. Relative Reactivity and Fosition Selectivity for Nitration of Some Aromatic...

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