Inorganic chemistry,nitration is a general class ofchemical processes for the introduction of anitro group (−NO2) into anorganic compound. The term also is applied incorrectly to the different process of formingnitrate esters (−ONO2) betweenalcohols andnitric acid (as occurs in thesynthesis ofnitroglycerin). The difference between the resultingmolecular structures of nitro compounds andnitrates (NO−3) is that thenitrogen atom in nitro compounds is directlybonded to a non-oxygen atom (typicallycarbon or another nitrogen atom), whereas in nitrate esters (also called organic nitrates), the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom (nitrito group).
There are many major industrial applications of nitration in the strict sense; the most important by volume are for the production of nitroaromatic compounds such asnitrobenzene. The technology is long-standing and mature.[1][2][3]
Nitration reactions are notably used for the production of explosives, for example the conversion ofguanidine tonitroguanidine and the conversion oftoluene totrinitrotoluene (TNT). Nitrations are, however, of wide importance as virtually all aromatic amines (anilines) are produced from nitro precursors. Millions of tons of nitroaromatics are produced annually.[2]
Typical nitrations of aromatic compounds rely on a reagent called "mixed acid", a mixture of concentratednitric acid andsulfuric acids.[4][2] This mixture produces thenitronium ion (NO2+), which is the active species inaromatic nitration. This active ingredient, which can be isolated in the case ofnitronium tetrafluoroborate,[5] also effects nitration without the need for the mixed acid. In mixed-acid syntheses sulfuric acid is not consumed and hence acts as acatalyst as well as an absorbent for water. In the case of nitration ofbenzene, the reaction is conducted at a warm temperature, not exceeding 50 °C.[6] The process is one example ofelectrophilic aromatic substitution, which involves the attack by the electron-richbenzene ring:
Alternative mechanisms have also been proposed, including one involvingsingle electron transfer (SET).[7][8]
Selectivity can be a challenge. Often alternative products act as contaminants or are simply wasted. Considerable attention thus is paid to optimization of the reaction conditions. For example, the mixed acid can be derived from phosphoric orperchloric acids in place of sulfuric acid.[2]
Regioselectivity is strongly affected by substituents on aromatic rings (seeelectrophilic aromatic substitution). For example, nitration of nitrobenzene gives all three isomers ofdinitrobenzenes in a ratio of 93:6:1 (respectively meta, ortho, para).[9] Electron-withdrawing groups such as othernitro aredeactivating. Nitration is accelerated by the presence ofactivating groups such asamino,hydroxy andmethyl groups alsoamides andethers resulting in para and ortho isomers. In addition to regioselectivity, the degree of nitration is of interest.Fluorenone, for example, can be selectively trinitrated[10] or tetranitrated.[11]
The direct nitration ofaniline withnitric acid andsulfuric acid, according to one source,[12] results in a 50/50 mixture ofpara- andmeta-nitroaniline isomers. In this reaction the fast-reacting and activating aniline (ArNH2) exists in equilibrium with the more abundant but less reactive (deactivated) anilinium ion (ArNH3+), which may explain this reaction product distribution. According to another source,[13] a more controlled nitration of aniline starts with the formation ofacetanilide by reaction withacetic anhydride followed by the actual nitration. Because the amide is a regular activating group the products formed are the para and ortho isomers. Heating the reaction mixture is sufficient to hydrolyze the amide back to the nitrated aniline.
Mixture of nitric and acetic acids or nitric acid and acetic anhydride is commercially important in the production ofRDX, as amines are destructed by sulfuric acid.Acetyl nitrate had also been used as a nitration agent.[14][15]
In theWolffenstein–Böters reaction,benzene reacts with nitric acid andmercury(II) nitrate to givepicric acid.
In the second half of the 20th century, new reagents were developed for laboratory usage, mainly N-nitro heterocyclic compounds.[16]
With aryl chlorides,triflates and nonaflates,ipso nitration may also take place.[17] The phraseipso nitration was first used by Perrin and Skinner in 1971, in an investigation into chloroanisole nitration.[18] In one protocol, 4-chloro-n-butylbenzene is reacted withsodium nitrite int-butanol in the presence of 0.5 mol%Pd2(dba)3, a biarylphosphine ligand and aphase-transfer catalyst to provide 4-nitro-n-butylbenzene.[19]
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