Nitration benzene and

The range of preparatively useful electrophilic substitution reactions is often limited by the acid sensitivity of the substrates. Whereas thiophene can be successfully sulfonated in 95% sulfuric acid at room temperature, such strongly acidic conditions cannot be used for the sulfonation of furan or pyrrole. Attempts to nitrate thiophene, furan or pyrrole under conditions used to nitrate benzene and its derivatives invariably result in failure. In the... 

Unfortunately, the several investigators who have examined the nitration of the halobenzenes did not adopt similar conditions. Ingold and his associates (1931) nitrated benzene and toluene by the addition of nitric acid to acetic anhydride and nitromethane. In acetic anhydride, nitric acid is slowly converted to acetyl nitrate (Bordwell and Garbisch, 1960). The relative rates, kT/kB, observed were 23 and 21 for reaction in acetic anhydride and nitromethane respectively. Bird and Ingold (1938) adopted different conditions. In a study of the halobenzenes, these workers added pre-formed acetyl nitrate to the halobenzene in acetic anhydride, acetonitrile, and nitromethane. The results for fluoro-, chloro-, and bromobenzene are summarized in Table 17 (p. 78). 

In 1950 Ingold, Hughes and Reed [39], when studying the kinetics of nitration with nitric acid in the presence of acetic acid, found the reaction to be of zero order in the case of aromatic compounds which are readily nitrated (benzene and its homologues), and its rate Va could be expressed by the approximate equation ... 

Nitric acid alone fails to nitrate benzene and sulfuric acid also does not readily react with it, yet the mixed acid is an efficient nitrating reagent. Solutions of nitric acid in sulfuric acid show an approximately four-fold molar freezing-point depression and this has been attributed to the generation of four ions, as shown in equation (1) ... 

Soviet method (according to Chekalin, Passet and Ioffe) Bofors-Nobcl-C hcmatur method of nitrating benzene and toluene to mononitro poxlucis Dinitrntoluenes... 

Industrial methods of mono-nitration benzene and toluene Removal of phenolic by-products Periodic nitration Continuous nitraiit>n... 

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... 

Olah el al. found that the ion is capable of transfer nitrating benzene and mesitylene [134]. The transfer nitrating ability of the nitrohe.ramethyl-benzenium ion is interesting as addition of hexamethyl benzene as a com-plexing agent to nitronium salt nitrations of aromatics can affect regio-selectivity. 

Bromine, which is an ortho, para director, can be introduced directly onto the benzene ring by reaction with bromine and FeBr. The amino group of aniline is also an ortho, para director, but it can only be introduced by first nitrating benzene and then reducing the nitro compound. 

This reaction is carried out at about 50 °C. There are two phases in the reactor, an organic phase (benzene and the various nitrated benzenes) and an aqueous phase (originally a mixture of sulfuric and nitric acid, which becomes diluted by the water that is formed as the reactions proceed). Mononitrobenzene is the desired product m-dinitrobenzene is an undesired by-product. 

The nitration, sulphonation and Friedel-Crafts acylation of aromatic compounds (e.g. benzene) are typical examples of electrophilic aromatic substitution. 

One of the characteristic properties of phenol is the ease with which it gives substitution products, this property being particularly well shown by the ready nitration, sulphonation and bromination which the benzene ring in the phenol molecule undergoes. 

Benzene and some of its derivatives react with solutions of mercuric nitrate in concentrated nitric acid to give nitrophenols. These reactions, known as oxynitrations may proceed by mercuration followed by nitroso-demercuration the resulting nitroso compound becomes a diazonium compound and then a phenol, which is nitrated. ... 

Our knowledge of the mechanism of the reaction in this medium comes from an investigation of the nitration of nitrobenzene, /)-chloronitro-benzene and i-nitroanthraquinone. These compounds underwent reaction according to the following rate law ... 

This consideration prompted an investigation of the nitration of benzene and some more reactive compounds in aqueous sulphuric and perchloric acids, to establish to what extent the reactions of these compounds were affected by the speed of diffusion together of the active species. ... 

The phenomenon was established firmly by determining the rates of reaction in 68-3 % sulphuric acid and 61-05 % perchloric acid of a series of compounds which, from their behaviour in other reactions, and from predictions made using the additivity principle ( 9.2), might be expected to be very reactive in nitration. The second-order rate coefficients for nitration of these compounds, their rates relative to that of benzene and, where possible, an estimate of their expected relative rates are listed in table 2.6. 

Under conditions in which benzene and its homologues were nitrated at the zeroth-order rate, the reactions of the halogenobenzenes ([aromatic] = c. o-1 mol 1 ) obeyed no simple kinetic law. The reactions of fluorobenzene and iodobenzene initially followed the same rates as that of benzene but, as the concentration of the aromatic was depleted by the progress of the reaction, the rate deviated to a dependence on the first power of the concentration of aromatic. The same situation was observed with chloro- andjbromo-benzene, but these compounds could not maintain a zeroth-order dependence as easily as the other halogenobenzenes, and the first-order character of the reaction was more marked. 

Compound nitrated Zeroth order Benzene and homologues Toluene (benzene) Mesitylene... 

Solutions of dinitrogen pentoxide have been used in preparative nitrations.Benzene, bromobenzene, and toluene were nitrated rapidly in solutions of the pentoxide in carbon tetrachloride nitrobenzene could not be nitrated under similar conditions, but reacted violently with solid dinitrogen pentoxide. 

Kinetic studies of nitration using dilute solutions of dinitrogen pentoxide in organic solvents, chiefly carbon tetrachloride, have provided evidence for the operation, under certain circumstances of the molecular species as the electrophile. The reactions of benzene and toluene were inconveniently fast, and therefore a series of halogenobenzenes and aromatic esters was examined. 

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). 

Characteristics of the system as nitrating reagents Wibaut, who introduced the competitive method for determining reactivities (his experiments with toluene, benzene and chlorobenzene were performed under heterogeneous conditions and were not successful), pointed out that solutions of nitric acid in acetic anhydride are useful in making comparisons of reactivities because aromatic compounds are soluble in them. ... 

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. 

Acetoxylation and nitration. It has already been mentioned that 0- and m-xylene are acetoxylated as well as nitrated by solutions of acetyl nitrate in acetic anhydride. This occurs with some other homologues of benzene, and with methyl phenethyl ether,ii but not with anisole, mesitylene or naphthalene. Results are given in table 5.4. 

The addition of sulphuric acid increased the rate of nitration of benzene, and under the influence of this additive the rate became proportional to the first powers of the concentrations of aromatic, acetyl nitrate and sulphuric acid. Sulphuric acid markedly catalysed the zeroth-order nitration and acetoxylation of o-xylene without affecting the kinetic form of the reaction. ... 

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... 

Nitration (Section 12 3) The active electro phile in the nitration of benzene and its... 

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