Nitration benzenes, yields

First-order nitrations. The kinetics of nitrations in solutions of acetyl nitrate in acetic anhydride were first investigated by Wibaut. He obtained evidence for a second-order rate law, but this was subsequently disproved. A more detailed study was made using benzene, toluene, chloro- and bromo-benzene. The rate of nitration of benzene was found to be of the first order in the concentration of aromatic and third order in the concentration of acetyl nitrate the latter conclusion disagrees with later work (see below). Nitration in solutions containing similar concentrations of acetyl nitrate in acetic acid was too slow to measure, but was accelerated slightly by the addition of more acetic anhydride. Similar solutions in carbon tetrachloride nitrated benzene too quickly, and the concentration of acetyl nitrate had to be reduced from 0-7 to o-i mol 1 to permit the observation of a rate similar to that which the more concentrated solution yields in acetic anhydride. 

Nitration of benzene yields nitrobenzene, which is reduced to aniline, an important intermediate for dyes and pharmaceuticals. Benzene is chlorinated to produce chlorobenzene, which finds use in the preparation of pesticides, solvents, and dyes. 

WOLFFENSTEIN-BOTERS REACTION. Simultaneous oxidation and nitration of aromatic compounds to nitrophcnols with nitric acid or the higher oxides of nitrogen in the presence of a mercury salt as catalyst. Hydroxynitration of benzene yields picric acid. 

Phenol can be nitrated upon nitrate irradiation, yielding 2- and 4-nitrophenol [54,58,79,99,100]. The generation of OH + NO2 upon nitrate photolysis (reactions 1 and 2) would suggest the possibility that phenol nitration might follow an OH-mediated pathway as in the gas phase [80,81]. Furthermore, hydroxyl-mediated nitration in aqueous solution has been described in the case of benzene [107]. However, the addition of hydroxyl scavengers to the system (formate [79], 2-propanol [58]) favours the formation of ni-trophenols, while an OH-mediated nitration would be inhibited by hydroxyl consumption. The positive effect of the scavengers can be accounted for in the hypothesis that phenol nitration takes place upon reaction with nitrogen... 

Beuzene-1.2-diamine hydrochloride (4.0 g, 22 mmol) was dissolved in distilled Il O (100 mL) in a 250-mL round-bottom flask equipped with a magnetic stirrer. The pH was adjusted to 6.0 with 3 M NaOH. 30% H2O2 (5.0 mL) was then added. The flask was chilled in an ice bath to approximately 10 and horseradish peroxidase (50 mg, 0.001 mol), dissolved in distilled HjO (l.OmL), was added. After 15 min the icc bath was removed and the reaction was allowed to continue for 16 h. The precipitate which had formed was isolated by nitration and rccrystallized from dioxane/benzene yield 2.08 g (42%). 

One of the higher homologues of benzene yields a very interesting nitro product. When i-methyl 3-tertiary bulyl benzene is nitrated to a tri-nitro product the three nitro groups enter the 2-4-6 positions. 

Nitric acid salts in the presence of other acids have been previously described (Vol. I, p, 46). It was recently reported by Crivello [125] that metal nitrates in trifluoroacetic anhydride (TFAA) can nitrate aromatic compounds at room temperature in very good yields. Thus ammonium nitrate with TFAA nitrated benzene to nitrobenzene with a yield of 95%. 

Several papers appeared after the pioneer work of Mine and co-workers (Vol. I, p. 126). Papers by Chernova and co-workers [S3] and Sugimoto and co-workers [54] have shown that even dilute nitric acid (0.1-1.7 N) can nitrate aromatic compounds when subjected to gamma radiation from Co °. Benzene yielded nitrobenzene and p-nitrophenol as the result of free radical reactions. Nitric acid is decomposed by radical mechanism according to equation [55] ... 

Ox3mitration. An interesting reaction occurs between benzene and approximately 50 per cent nitric acid containing 0.2 molar mercuric nitrate which yields up to 85 per cent dinitrophenol and picric acid. This process is known as oxynitration. It has been shown by Westheimer and coworkers that this reaction occurs in the following steps ... 

Equimolar amounts of NO2 BFT and 21-crown-7 or 18-crown-6 ethers yielded homogeneous solutions in nitromethane and dichloromethane. Using these systems in nitrating benzene and toluene, both substrate and positional selectivities were altered and were dependent upon the nature of the crown ether and (crown ether-NO BFT) ratio. The linear nitronium ion obviously forms a complex with the crown ether. However, a guest-host complex with the linear nitronium ion inside the cavity would hardly allow the aromatic access. Therefore, it is more probable that the crown ether complexes the nitronium ion on the outside. The ortho/para ratio in nitration of toluene can be varied from 1.5 to 0.3 on changing the (21-crown-6)-(N02 BF4) ratio from 1 to 6. The isomer distribution of the... 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

In nitration with nitronium salts in sulpholan, nitrobenzene was substituted in the following proportions 8% ortho, 90% meta and 2% paraf under the same conditions benzylidyne trifluoride yielded 8%, 88% and 4% of 0-, m- and p-nitro compound respectively Both of these aromatic compounds were stated to be io -10 times less reactive than benzene. "... 

Nitration using this reagent was first investigated, by Francis. He showed that benzene and some of its homologues bromobenzene, benzonitrile, benzoyl chloride, benzaldehyde and some related compounds, and phenol were mono-nitrated in solutions of benzoyl nitrate in carbon tetrachloride anilines would not react cleanly and a series of naphthols yielded dinitro compounds. Further work on the orientation of substitution associated this reagent with higher proportions of o-substitution than that brought about by nitric acid this point is discussed below ( 5.3.4). 

TABLE 5.4 Yields moles %) in the nitration and acetoxylation of some derivatives of benzene at 25 °C... 

The overall reactivity of the 4- and 5-positions compared to benzene has been determined by competitive methods, and the results agreed with kinetic constants established by nitration of the same thiazoles in sulfuric acid at very low concentrations (242). In fact, nitration of alkylthiazoles in a mixture of nitric and sulfuric acid at 100°C for 4 hr gives nitro compounds in preparative yield, though some alkylthiazoles are oxidized. Results of competitive nitrations are summarized in Table III-43 (241, 243). For 2-alkylthiazoles, reactivities were too low to be measured accurately. 

Nitration of (trifluoromethyl)benzene on the other hand yields almost exclusively m nitro(trifluoromethyl)benzene (91%) The ortho and para substituted isomers are minor components of the reaction mixture... 

The behavior of aromatic aldehydes is typical Nitration of benzaldehyde takes place sev eral thousand times more slowly than that of benzene and yields m mtrobenzaldehyde as the major product... 

Nitrations can be performed in homogeneous media, using tetramethylene sulfone or nitromethane (nitroethane) as solvent. A large variety of aromatic compounds have been nitrated with nitronium salts in excellent yields in nonaqueous media. Sensitive compounds, otherwise easily hydroly2ed or oxidized by nitric acid, can be nitrated without secondary effects. Nitration of aromatic compounds is considered an irreversible reaction. However, the reversibihty of the reaction has been demonstrated in some cases, eg, 9-nitroanthracene, as well as pentamethylnitrobenzene transnitrate benzene, toluene, and mesitylene in the presence of superacids (158) (see Nitration). 

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