Nitrating acid

Nitric acid is a mixture of concentrated nitric acid and concentrated sulfuric acid with different mixing ratios. The molar ratio is usually set between 1: 2 and 2: 1. The nitrating acid is mainly used for the preparation of nitro compounds, which in turn mainly serve as starting materials for the production of amines and hydroxylamines .

Reaction mechanism

For aromatics, the nitration process can be explained as part of an electrophilic aromatic substitution , otherwise the explanations are based on the mechanism of nucleophilic substitution .

In both cases, a highly reactive nitronium ion is used as the electrophile . This is created by protonation and subsequent dehydration of the nitric acid. This protonation is only possible with a strong acid such as sulfuric acid:

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

${\displaystyle \mathrm {H_{2}NO_{3}^{+}\ \rightleftharpoons \ H_{2}O+NO_{2}^{+}} }$

NO ₂ ^{+, which} is isoelectronic to carbon dioxide, has a linear structure. The positive charge on the nitrogen atom is therefore particularly easily accessible to a nucleophilic attack. The nitration of benzene is an example of electrophilic aromatic substitution :

Electrophilic aromatic substitution of benzene (1). A loose interaction with the linear nitronium ion ( π complex , 2a) forms the σ complex (2b), which has the mesomerism shown . After splitting off a proton, nitrobenzene (3) is finally formed .

The substitution on an alcohol is analogous:

One or more nitro groups can be introduced through the mechanisms shown; explosive substances are usually multiply nitrated substances. Multiple nitriding is often difficult due to electronic effects, which is why higher reaction temperatures are necessary for this. However, the explosiveness of the reaction mixture also increases with an increasing degree of nitration and an increasing temperature. Reactions with nitrating acid are therefore carried out and supervised by trained specialists under carefully controlled conditions. Since the production of nitrating acid, like any dilution of concentrated mineral acids, is a strongly exothermic process, its production must only be carried out by trained specialists.

Products

By nitration on a large scale u. a. manufactured the following products:

• Aniline (from nitrobenzene with H ₂ on Pd / C )
• Cellulose nitrate (from cellulose )
• Glycerol trinitrate (from glycerol )
• Nitrobenzene (from benzene )
• Picric acid (from phenol )
• TNT (from toluene )

Individual evidence

1. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 948.
2. ↑ R. Brückner: reaction mechanisms: organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Spectrum, ISBN 3-8274-1579-9 , pp. 222 .

Web links

• Internship in organic chemistry for students of the teaching profession (accessed on May 14, 2020)
• Manufacture of gun cotton, nitrocellulose (accessed May 14, 2020)
• Intensification of heat and mass transport during the nitration of benzene in the capillary microreactor (accessed on May 14, 2020)
• Nitrocellulose (gun cotton) (accessed May 14, 2020)
• Alfred Nobel and the discovery of dynamite with special consideration of ethical issues in natural science (accessed on May 14, 2020)