nitration

This article covers a chemical process. For a heat treatment or coating process in materials technology, see nitriding .

In chemistry, nitration is the introduction of a nitro group (-NO ₂ ) into an organic molecule . For historical reasons, nitrates are sometimes referred to as nitro compounds, and glycerol trinitrate as nitroglycerine. However, nitro compounds have a CNO bond, in contrast to nitrates, which have a CON bond.

history

Eilhard Mitscherlich.

Eilhard Mitscherlich described the first conversion of benzene with fuming nitric acid to nitrobenzene in 1834. The aniline obtained from it by hydrogenation was an important raw material for the pharmaceutical and paint industry. Industrial production of nitrobenzene started as early as 1847, initially in England. In 1863 Julius Wilbrand reported on the nitration of toluene to trinitrotoluene using nitrating acid , a mixture of concentrated nitric and sulfuric acid .

In 1880, Friedrich Konrad Beilstein examined the effect of nitric acid on an aliphatic gasoline fraction and, in addition to oxidation products, also demonstrated the formation of nitro compounds on a small scale. The work on the nitration of aliphatic hydrocarbons was continued in the 1890s by Vladimir Wassiljewitsch Markovnikov and Mikhail Ivanovich Konovalov , but was forgotten until the 1930s.

In the early 1940s , Christoph Grundmann , the discoverer of the Grundmann aldehyde synthesis , succeeded in nitrating higher molecular weight hydrocarbons by nitrating hydrocarbons that were liquid at 160 to 180 ° C using superheated nitric acid vapor. Grundmann also the idea of Drucknitrierung of came cyclohexane by nitrogen dioxide .

Nitration of aromatics

The term nitration usually stands synonymously for the nitration of aromatic compounds, which can be characterized as electrophilic aromatic substitution .

Since in the nitrated organic compounds carbon atoms are separated from oxygen atoms by nitrogen atoms, it can happen that the nitrogen atom leaves the bond and the carbon atom reacts violently with the oxygen atom. Therefore some nitrated organic compounds can be used as explosives . Well-known examples are trinitrotoluene (TNT) and picric acid .

Reaction mechanism

The nitration of an aromatic is the classic example of electrophilic aromatic substitution. The nitronium ion (NO ₂ ⁺ ) acts as an electrophilic particle . This arises from dehydration (elimination of water) from the protonated nitric acid and is quite stable in a strongly acidic medium. The dehydration of nitric acid is done by adding concentrated sulfuric acid. The resulting mixture of concentrated sulfuric and nitric acid is called "nitrating acid". The weaker nitric acid is protonated by the sulfuric acid, whereby water as a leaving group is split off from the nitric acid and the nitronium ion (NO ₂ ⁺ ) is formed. The released water is in turn protonated by the sulfuric acid and the oxonium ion is formed.

Ultimately, four species arise from the multistage protonation reaction: the nitronium ion, the oxonium ion H ₃ O ⁺ and two hydrogen sulfate anions HSO ₄ ^- . The oxonium ions generate the strongly acidic environment that stabilizes the nitronium ion.

${\displaystyle \mathrm {HNO_{3}+2\ H_{2}SO_{4}\longrightarrow NO_{2}^{+}+2\ HSO_{4}^{-}+H_{3}O^{+}} }$

The resulting nitronium ion can be isolated in some compounds that have been well examined spectroscopically (e.g. (NO ₂ ) (BF ₄ ) or (NO ₂ ) (S ₂ O ₇ H)).

Analyzes of the kinetics of the reaction provided a first indication of the presumed mechanism . The following rate equation was found for the reaction:

${\displaystyle {\text{Geschwindigkeit}}=k\ \mathrm {[Ar{-}H][NO_{2}^{+}]\!\,} }$

Here k is a reaction-specific rate constant. This equation makes it clear that, on the one hand, Ar – H (where Ar is any aryl radical) and NO ₂ ^{+ are} involved in the overall reaction rate.

Reaction mechanism for the nitration of benzene

The aromatic ( 1 ) interacts loosely with the nitronium ion, which is referred to as the π complex ( 2a ). This forms the so-called σ-complex , the positive charge of which is delocalized over the ring ( 2b ). The intermediate stages that occur are also known as the Wheland complex . In this process, the aromaticity is canceled. Finally, the sigma complex is deprotonated so that the nitrated aromatic is released ( 3 ).

Nitration of aliphatics

The technical production of nitromethane takes place through the nitration of methane or through a gas phase nitration of propane , where it is contained in the resulting nitroalkane mixture with about 25%. The nitration takes place in the gas phase either with gaseous nitric acid or nitrogen dioxide at temperatures of around 200 to 400 ° C.

Special reagents for nitration

Nitrations can also be carried out with special reagents. Example is the acetyl nitrate , which u. a. CH-acidic compounds such as acetoacetate ester nitrated.

Web links

Commons : Nitration  - collection of images, videos and audio files

literature

• Peter Sykes: How do organic reactions work? 2nd Edition. Wiley-VCH, 2001, ISBN 3-527-30305-7 , p. 36 ff.
• JG Hoggett, RB Moodie, JR Penton, K. Schofield: Nitration and aromatic reactivity. Cambridge University Press, 1971, ISBN 0-521-08029-0 .
• GA Olah, R. Malhotra, SC Narang: Nitration: Methods and Mechanisms. Wiley-VCH, 1989, ISBN 0-471-18695-3 .

Individual evidence

1. ↑ Eilhard Mitscherlich: About the composition of nitrobenzide and sulfobenzide. In: Annals of Pharmacy. 12, 1834, p. 305, doi: 10.1002 / jlac.18340120281 .
2. ↑ Julius Wilbrand: Note on trinitrotoluene. In: Annals of Chemistry and Pharmacy. 128, 1863, pp. 178-179, doi: 10.1002 / jlac.18631280206 .
3. ↑ Friedrich Konrad Beilstein, A. Kurbatov: Btr. Dtsch. Chem. 13, 1880, p. 2029.
4. ↑ Mikhail Iwanowitsch Konowalow: Btr. Dtsch. Chem. 26, 1893, p. 878.
5. ↑ Wladimir Wassiljewitsch Markownikow: Btr. Dtsch. Chem. 32, 1899, p. 1441.
6. ↑ Christoph Grundmann: About the nitration of higher molecular paraffin hydrocarbons. In: The chemistry. 56, 1943, pp. 159-163, doi: 10.1002 / anie.19430562303 .
7. ↑ Christoph Grundmann, H. Haldenwanger: About the nitration of cyclohexane. In: Angewandte Chemie. 62, 1950, pp. 556-558, doi: 10.1002 / anie.19500622303 .
8. ^ S. Hauptmann, J. Grafe, H. Remane: Textbook of Organic Chemistry . German publishing house for basic industry, Leipzig 1980, p. 480 .
9. ^ Brockhaus ABC chemistry . tape 2 . FA Brockhaus Verlag, Leipzig 1971, p. 951 .
10. ^ HB Hass, Julian Dorsky, EB Hodge: Nitration of Propane by Nitrogen Dioxide. In: Industrial & Engineering Chemistry. 33, 1941, pp. 1138-1143, doi: 10.1021 / ie50381a011 .
11. ↑ S. Sifniades: nitration of acetoacetate ester by acetyl nitrate. High yield synthesis of nitroacetoacetate and nitroacetate esters. In: J. Org. Chem. 40, 1975, pp. 3562-3566, doi: 10.1021 / jo00912a020 .