Electrophilic aromatic substitution nitration with nitronium ions

Aniline is an important derivative of benzene that can be made in two steps by nitration to nitrobenzene and either catalytic hydrogenation or acidic metal reduction to aniline. Both steps occur in excellent yield. Almost all nitrobenzene manufactured (97%) is directly converted into aniline. The nitration of benzene with mixed acids is an example of an electrophilic aromatic substitution involving the nitronium ion as the attacking species. The hydrogenation of nitrobenzene has replaced the iron-... 

Nitration of toluene is the only important reaction that involves the aromatic ring rather than the aliphatic methyl group. The nitration reaction occurs with an electrophilic substitution hy the nitronium ion. The reaction conditions are milder than those for henzene due to the activation of the ring hy the methyl substituent. A mixture of nitrotoluenes results. The two important monosubstituted nitrotoluenes are o- and p-nitrotoluenes ... 

Figure 12.3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene. The first step is rate-determining in it benzene reacts with nitronium ion to give the cyclohexadienyl cation intermediate. In the second step, the aromaticity of the ring is restored by loss of a proton from the cyclohexadienyl cation. 

These steps illustrate how to generate the electrophile E for nitration and sulfonation, the process that begins any mechanism for electrophilic aromatic substitution. To complete either of these mechanisms, you must replace the electrophile by either or S03H in the general mechanism (Mechanism 18.1). Thus, the two-step sequence that replaces H by E is the same r ardless of E. This is shown in Sample Problem 18.1 u.sing the reaction of benzene with the nitronium ion. 

The question of the identity of the nitrating species can be approached by comparing selectivity with that of nitrations known to involve the nitronium ion. Examination of Part B of Table 9.8 shows that the position selectivity exhibited by acetyl nitrate toward toluene and ethylbenzene is not dramatically different from that observed with nitronium ion. The data for 2-propylbenzene suggest a higher o p ratio for nitronium ion nitrations, however. Several substituted aromatic compounds— for example, anisole and acetanilide — give much higher 0 p ratios when nitrated by acetyl nitrate than when nitronium ion conditions are used, suggesting the involvement of a different electrophile. ... 

Nitration introduces a nitro group (— NO ) onto an aromatic ring. Electrophilic aromatic substitution requires nitric acid (HNOj), with sulfuric acid as a catalyst. Nitronium ion, (NO ), is the electrophile. It forms in two steps by the reaction of nitric acid with sulfuric 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). 

Nitration is widely applicable, can be carried out under a variety of conditions, can usually be stopped cleanly after mononitration, is usually effected by the nitronium ion, can take place on a neutral molecule or a cation, and in many cases can be considered as the standard aromatic electrophilic substitution. However, this last point must be treated with caution. Depending on the reaction conditions and reagents, the mechanism of the reaction does vary, and accompanying reactions such as oxidation (due to the oxidative action of nitric acid), acetoxylation (by acetyl nitrate), and migration of nitro groups following ipso attack (80MI1) can occur. Ipso nitration processes have been extensively studied by Fischer and co-workers. 

As commonly accepted, the nitration of aromatic compounds is a typical reaction of electrophilic substitution, with the N02+ nitronium ion serving as a directly attacking moiety. On nitration by only nitric acid, the nitronium cation is formed via autoprotolysis according to Scheme 1 ... 

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