Nitro aromatics
The term nitroaromatics (also called nitroarenes ) includes all organic-chemical compounds that have at least one nitro group directly on an aromatic molecular structure. The fact that a compound belongs to the group of nitroaromatics is independent of the presence of further functional groups and substituents ; in such cases it would belong to several substance classes.
This class of substances can be roughly divided into mononuclear nitroaromatics, whose basic structure has only one aromatic ring, and polynuclear nitroaromatics, whose aromatic system consists of at least two rings. Another possibility of subdivision is the grouping into mononitroaromatics, dinitroaromatics, trinitroaromatics etc., depending on the number of nitro groups present in the molecule. The latter classification is closely related to the reactivity and stability of the compounds.
The simplest and best-known representative is nitrobenzene , a mononuclear mononitroarene.
synthesis
The most common method of producing nitroaromatics is the direct nitration of aromatics with the aid of nitrating acid . The mechanistic type of this reaction is electrophilic aromatic substitution , S E (Ar). A suitable aromatic starting material is usually treated with a mixture of concentrated nitric acid and concentrated sulfuric acid while cooling . Under carefully controlled reaction conditions, only one nitro group is usually introduced into the molecule.
It is problematic to control the reaction when using activated aromatics, e.g. B. phenols or phenol ethers . If the cooling is insufficient, multiple nitration can occur. In most cases, nitric acid alone is sufficient for the nitration of activated aromatics.
Deactivated aromatics (e.g. aromatic carboxylic acids ), on the other hand, usually have to be heated with nitrating acid or fuming nitric acid in order to achieve nitration.
properties
The properties of the nitroaromatics depend largely on the structure of the overall molecule and can not be described uniformly given the great variety of derivatives . The nitro group gives the molecule an additional dipole moment due to its charges and the resulting polarity . Nevertheless, even multiply nitrated aromatics are usually only poorly soluble in water if they do not contain other polar groups, e.g. B. hydroxyl groups wear.
Effects on the aromatic system
Nitro groups are functional groups with a pronounced electron-withdrawing character and, in addition to an inductive effect (–I effect), also have a very strong mesomeric effect (–M effect) on the aromatic backbone. Result from it
- a decrease in the charge density in the aromatic system,
- a lower reactivity of the nitroarene towards electrophilic reagents (compared to the reactivity of the corresponding aromatic without a nitro group),
- a regioselectivity in favor of the meta position (s), relative to the nitro group, with further electrophilic substitution on the nitroarene.
Explosiveness
A striking property of the multiply nitrated aromatics is their explosiveness , which increases with the number of nitro groups per molecule. While nitrobenzene and the dinitrobenzenes are comparatively harmless compounds, trinitrobenzene , tetranitrobenzene and especially hexanitrobenzene can explode violently when blown or heated. This property stems from the fact that nitro groups are a kind of “internal oxidizing agent ”. As a result, a compound with enough nitro groups reacts relative to its hydrocarbon component (= combustible component of the compound) similar to a mixture of flammable substance and fire-promoting substance, i.e. a "classic" explosive mixture (e.g. black powder ), but its effect is usually more violent because Fuel and oxygen carriers are “mixed” at the molecular level.
use
Explosives
The already mentioned explosiveness of multiply nitrated aromatics makes them interesting as explosives. Examples that should be mentioned here are 1,3,5-trinitrobenzene (TNB), probably the best-known 2,4,6- trinitrotoluene (TNT), picric acid (2,4,6-trinitrophenol), and N- methyl - N , 2,4,6-tetranitroaniline ( tetryl ) and the 2,2 ', 4,4', 6,6'- hexanitrostilbene (HNS). Picric acid is particularly in the form of its heavy metal salts , e.g. B. lead picrate, very sensitive to impact and is used as an initial explosive . The HNS is a temperature-resistant explosive that is made to explode when it is ignited.
Fragrances
Some artificial musk fragrances also belong to the nitro aromatics:
- Musk xylene (systematic: 1- tert -butyl-3,5-dimethyl-2,4,6-trinitro-benzene)
- Musk ketone (systematic: 1- (4- tert -butyl-2,6-dimethyl-3,5-dinitro-phenyl) -ethanone)
These substances are very soluble in fat. Residues of these compounds have been found in fish and in breast milk. In some countries the use of these compounds is severely restricted.
Synthesis intermediates
Nitro aromatics can be converted into aromatic amines in high yields using a variety of reducing agents . Aromatic amines are basic chemicals used in the production of a wide range of synthetic dyes, and were mostly obtained in this way from nitroaromatics by the chemical dye industry that was burgeoning in the second half of the 19th century. By the reduction of nitrobenzene with in situ from iron, zinc or tin and hydrochloric acid formed is hydrogen , to give aniline which u. a. paved the way to industrially produced artificial indigo .
Especially in laboratory synthesis and research, nitroaromatics are still important synthesis intermediates for the production of aromatic amines. By using milder reduction methods, aromatic nitroso compounds and N- hydroxylamines can also be obtained from nitroaromatics. Since nitro groups on aromatics are stable to a large number of chemical conversions and reaction conditions (with the exception of reductions), they are often used in multi-stage syntheses instead of the more reactive amino groups and only converted into amino groups at the appropriate point in the synthesis sequence.
toxicity
The poisonous effect of nitroaromatics is primarily based on the endogenous conversion into amines or their primary metabolites and is similar to that of aromatic amines, i.e. H. it results from the reduction of the nitro group to the amino or N-hydroxylamino group.
Nitro aromatics are available through the GIT. the respiratory tract and skin are very well absorbed and can, for. T. are stored in adipose tissue. Monocyclic nitroaromatics are mainly eliminated renally. If they come into contact with the skin, they can lead to dermatitis and, in the eye, to conjunctivitis and corneal lesions as well as painful gastric colic and diarrhea after ingestion. The chronic toxicity in humans is mainly characterized by the occurrence of liver damage and leads to yellowing of the skin and nails as well as a brownish-red discoloration of the hair. Almost all nitro aromatics are mutagenic. The carcinogenicity has so far only been clearly proven in animal experiments.
Individual evidence
- ↑ a b c Encyclopedia of Nutrition: Nitroaromatics - Lexicon of Nutrition , accessed on August 17, 2018
- ↑ a b c Stefanie Federle, Stefanie Hergesell, Sebastian Schubert: The substance classes of organic chemistry explained in a practical and compact way by students . Springer-Verlag, 2017, ISBN 978-3-662-54968-1 , pp. 172 ( limited preview in Google Book search).
- ↑ a b c Franz-Xaver Reichl: Pocket Atlas of Toxicology Substances, Effects, Environment . Georg Thieme Verlag, 2002, ISBN 978-3-13-108972-4 , p. 126 ( limited preview in Google Book search).
- ↑ Toxicology of Substances: Toxicology Volume 2 - Toxicology of Substances . John Wiley & Sons, 2012, ISBN 3-527-63555-6 , pp. 139 ( limited preview in Google Book search).
- ↑ Musk ambrette, musk mosque, and musk Tibetan may no longer be used in cosmetic products according to Regulation (EC) No. 1223/2009 .
- ↑ M. Uhl, HP Hutter, G. Lorbeer: Polymoschus compounds in human blood II: Human biomonitoring of musk fragrances . Ed .: Federal Ministry for Health and Women . Vienna 2005, ISBN 978-3-900019-71-6 ( PDF - free full text).