Protic-Acid-Catalyzed Nitration

The scope of nitrating agents, however, by now is much wider (Table I) 13). The discussion in this chapter will primarily emphasize reagents and methods developed by the Olah group in its study of nitration chemistry. 

Recognizing the limited nitrating ability of the nitric acid-anhydrous hydrogen fluoride system, Olah and Kuhn in 1956 introduced nitric 

CHjSOjH CF SOjH RpSOjH FSO3H solid acids 

N02 BF,. NOJPF and other salts N-Nitropyridinium salts N-Niiropyraxole 9-Nitrounthraeenc C lCHri No  

Nitronium tetrafiuoroborate can be isolated as a stable salt and used as such as the nitrating agent or the system can be used for in situ nitration of aromatics. 

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Nitration with nitric acid in the presence of strong protic acids such as H2SO4, FSO3H, and CF3SO3H or Lewis acids such as boron trifluotide requires subsequent separation of spent acid (due to water formed in the reaction) and neutralization of acid left in the product. One is generally left with a large amount of dilute acid for disposal, which is neutralized in the case of sulfuric-acid-catalyzed nitrations to a mixture of ammonium nitrate and ammonium sulfate. By using a solid acid catalyst most of these environmental problems can be eliminated. The solid acid catalyst is simply separated and recycled for subsequent use. 

Both protic- or Lewis-acid-catalyzed nitration of aromatics can be carried out with a/ky/ nitrates (i.e., alkyl esters of nitric acid). Acid catalysts are assumed to form nitronium ion from alkyl nitrates or strongly polarized complexes. 

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