Nitration

The nitration is in chemistry the introduction of a nitro group ( NO2) in an organic molecule.

Nitration of aromatics

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

Since carbon atoms are separated by nitrogen atoms of oxygen atoms in the nitrated organic compounds, it may happen that the nitrogen atom leaves the bond and the carbon atom to the oxygen atom reacts violently. Therefore, some nitrated organic compounds are usable as explosives. Well-known examples are trinitrotoluene ( abbr.: TNT) and picric acid.

Reaction mechanism

The nitration of aromatics is the classic example of electrophilic aromatic substitution. As an electrophilic particle acts the nitronium ion ( NO2 ). This is caused by dehydration ( dehydration ) from the protonated nitric acid and is quite stable in strongly acidic medium. The dehydration of the nitric acid is accomplished by the addition of concentrated sulfuric acid. The resulting mixture of concentrated sulfuric and nitric acid will be referred to as a " mixed acid ". The weaker nitric acid is protonated by sulfuric acid, whereby water is split off as a breakaway group from the nitric acid and the nitronium ion ( NO2 ) forms. The released water is again protonated by sulfuric acid and produce the oxonium ion.

Ultimately resulting from the multi-stage protonation four species: the nitronium ion, the oxonium ion H3O and two bisulfate anions HSO4 -. Is experimentally confirmed the occurrence of these products through analysis of colligative effects: the freezing point depression of the nitrating acid is about four times larger than one would suppose in theory if the mixture. This note can be interpreted that four new species must be responsible for the freezing point depression. The oxonium ions generate the strongly acidic environment, which stabilizes the nitronium ion.

The resulting nitronium ion is in some well spectroscopically investigated compounds can be isolated (eg, (NO2 ) ( BF4) or (NO2 ) ( S2O7H ) ).

A first indication of the presumed mechanism provided analyzes of the kinetics of the reaction. They found the following rate equation for the reaction:

K here is a reaction-specific rate constant. The basis of this equation it is clear that one hand, Ar -H ( wherein Ar is any aryl group ) and are also involved in the NO2 overall reaction rate.

The aromatic compound (1) occurs in loose interaction with the nitronium ion, which is also referred to as π - complex ( 2a). This results in the so-called σ - complex is formed, the positive charge is delocalized over the ring (2b). The intermediates which occur are referred to as Wheland complex. In this process, the aromaticity is canceled. Finally, a deprotonation of the sigma- complex is carried out, so that the nitrated aromatic is released (3).

Nitration of aliphatic

The technical production of nitromethane is carried out by nitration of methane or by the gasphase nitration of propane, where it is contained at about 25 % in the resulting nitro alkane mixture.