Electrophilic aromatic substitution reactions nitrobenzene

In another example of an electrophilic aromatic substitution reaction, benzene reacts with a mixture of concentrated nitric and sulfuric acids to create nitrobenzene. 

Friedel-Crafts type reactions of strongly deactivated arenes have been the subject of several recent studies indicating involvement of superelectrophilic intermediates. Numerous electrophilic aromatic substitution reactions only work with activated or electron-rich arenes, such as phenols, alkylated arenes, or aryl ethers.5 Since these reactions involve weak electrophiles, aromatic compounds such as benzene, chlorobenzene, or nitrobenzene, either do not react, or give only low yields of products. For example, electrophilic alkylthioalkylation generally works well only with phenolic substrates.6 This can be understood by considering the resonance stabilization of the involved thioalkylcarbenium ion and the delocalization of the electrophilic center (eq 4). With the use of excess Fewis acid, however, the electrophilic reactivity of the alkylthiocarbenium ion can be... 

Nitration (Section 18.4) An electrophilic aromatic substitution reaction in which benzene reacts with N02 to give nitrobenzene, C6H5NO2. 

All of these effects are observed when comparing the rates of various electrophilic aromatic substitution reactions. Activating substituents increase the rate of reaction relative to benzene. The rate of reaction for the nitration of anisole, for example, was 9.7 x 10 times faster than nitration of benzene. The reaction of anisole with nitric and sulfuric acids, gave 44% of o-nitroanisole, 56% of p-nitroanisole and < 1% of m-nitro-anisole.2 9 contrasts with reactions involving deactivating substituents, where selectivity for the meta -product is usually very good. Nitration of nitrobenzene, for example, gave 1,3-dinitrobenzene in 94% yield, with only 6% of the ortho product and < 1% of the para product. ... 

Nitro groups are meta-directing. Both nitro groups of m-dinitrobenzene direct an incoming substituent to the same position in an electrophilic aromatic substitution reaction. Nitration of m-nitrobenzene yields 1,3,5-trinitrobenzene. 

The answer is A. This reaction is an electrophilic aromatic substitution reaction. The product formed is nitrobenzene. In the reaction, nitronium ion acts as the electrophile which attacks the benzene to form the cyclohexadienyl intermediate, followed by the elimination of the proton to form nitrobenzene. The reaction mechanism follows ... 

Friedel-Crafts reactions are the slowest of the electrophilic aromatic substitution reactions. Therefore, if a benzene ring has been moderately or strongly deactivated— that is, if it has a meta-directing substituent—it will be too unreactive to undergo either Friedel-Crafts acylation or Friedel-Crafts alkylation. In fact, nitrobenzene is so unreactive that it is often used as a solvent for Friedel-Crafts reactions. 

Pyrimidine forms 4-bromopyrimidine when the hydrochloride is heated with bromine at 160°C (or at 130°C in nitrobenzene) (73JHC153), the process being preceded by a vigorous reaction at lower temperature (57CB1837 58AG571). It is likely that N-bromo compounds and perbro-mides are implicated in these reactions, which occur (3 to the ring nitrogens, and they are not conventional electrophilic aromatic substitutions. 

Energy profiles with a deactivating group. Nitrobenzene is deactivated toward electrophilic aromatic substitution at any position, but deactivation is strongest at the ortho and para positions. Reaction occurs at the meta position, but it is slower than the reaction with benzene. 

Electrophilic aromatic substitution. 8. A kinetic study of the Frledel-Crafts benzylatlon reaction in nitromethane, nitrobenzene, and sulfolane. Substituent effects in Friedel-Crafts benzylatlon. J. Am. Chem. Soc. 1984, 106, 7038-7046. 

Electrophilic aromatic substitution is a situation in which it is useful to discuss TS structure in terms of a reaction intermediate. The ortho, para, and meta directing effects of aromatic substituents were among the first structure-reactivity relationships to be developed in organic chemistry. Certain functional groups activate aromatic rings toward substitution and direct the entering electrophile to the ortho and para positions, whereas others are deactivating and lead to substitution in the meta position. The bromination of methoxybenzene (anisole), benzene, and nitrobenzene can serve as examples for discussion. 

Bromination of nitrobenzene is remarkably good, considering the unreactivity of nitrobenzene in electrophilic aromatic substitution. One recipe uses iron powder and bromine at 140 °C and gives 74% of the meta product. We shaU need these reactions in the next section. 

There are several lines of evidence for formation of cr complexes as intermediates in electrophilic aromatic substitution. One particularly informative approach involves measurement of isotope effects on the rate of substitution. If removal of the proton at the site of substitution were concerted with introduction of the electrophile, a primary isotope effect would be observed in reactions in which electrophilic attack on the ring is rate-determining. This is not the case for nitration nor for several other types of aromatic substitution reactions. Nitration of aromatic substrates partially labeled by tritium shows no selectivity between protium- and tritium-substituted sites. Similarly, the rate of nitration of nitrobenzene is identical to that... 

Because it is aromatic, benzene does not react directly with reagents such as HBr or HCl, or even with diatomic bromine or chlorine. Benzene reacts with cationic species to ve a resonance-stabilized carbocation intermediate, which loses a hydrogen to give a substitution product. This reaction is called electrophilic aromatic substitution. The most common method for generating reactive cations in the presence of benzene is to treat certain reagents with strong Lewis acids. Lewis acids or mixtures of strong acids can be used to convert benzene to chlorobenzene, bromobenzene, nitrobenzene, or benzenesulfonic acid. 

Aromatic rings can be nitrated by reaction with a mixture of concentrated nitric and sulfuric acids. The electrophile is the nitronium ion, N02+, which is generated from HNO3 by protonation and loss of water. The nitronium ion reacts with benzene to yield a carbocation intermediate, and loss of H+ from this intermediate gives the neutral substitution product, nitrobenzene (Figure 16.4). 

Fluorinated nitrobenzenes are highly activated electrophiles, and have been used in PTC-catalyzed nucleophilic aromatic substitution by Jorgensen and colleagues [44,45]. The arylation ratio at C- or O- is heavily dependent on the catalyst structure, counter anion and reaction temperature, and the reaction of ketoesters... 

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