How can we convert nitrobenzene into trinitrobenzene



Structural formula
General
Surname Trinitrotoluene
other names

1-methyl-2,4,6-trinitrobenzene, TNT, Trotyl, AN, Tol, Tolite, Tritol, Trisol, Tutol, trinitrotoluene

Molecular formula C.7H5N3O6
CAS number 118-96-7
Brief description light yellow, needle-shaped crystals
properties
Molar mass 227.13 g mol−1
Physical state firmly
density 1.65 g cm−3
Melting point 80.35 ° C
boiling point Decomposition: 300 ° C
Vapor pressure

0.057 Pa (81 ° C)

solubility

good in ether, acetone, benzene, pyridine, bad in water

safety instructions
MAK

0.1 mg m−3

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

Trinitrotoluene (TNT) is an explosive.

The correct name for TNT is according to the IUPAC nomenclature1-methyl-2,4,6-trinitrobenzene. This article describes the more common names Trinitrotoluene and toluene (instead of toluene or methylbenzene) used.

TNT was first manufactured by Joseph Wilbrand in 1863; large-scale production began in Germany in 1891.

The explosive power of TNT has become the measure of the strength of bombs and other explosive devices (see TNT equivalent).

properties

Trinitrotoluene forms light yellow, needle-shaped crystals and can be distilled in vacuo. It is very sparingly soluble in water, but readily soluble in ether, acetone, benzene and pyridine. With its low melting point of 80.8 ° C, TNT can be melted in water vapor and poured into molds. TNT is poisonous and can cause allergic reactions if it comes into contact with the skin. It gives the skin a bright yellow-orange color.

  • Water solubility: 130 mg / L at 20 ° C
  • Vapor pressure at 20 ° C: 1.5 −6 mbar
  • Detonation speed: 6700–7000 m / s, 6900 m / s (density: 1.6 g / cm³)
  • Lead block bulge: 300 ml / 10 g
  • Impact sensitivity: 15 Nm (1.5 kpm)
  • Friction sensitivity: no reaction up to 353 N (36 kp)
  • Specific calorific value: 4.25 MJ / kg

Manufacturing

past

TNT was first synthesized in impure form in 1863 by Joseph Wilbrand. With the development of pure synthesis in 1880 by P. Hepp and the discovery of TNT as a suitable explosive by Häussermann in 1889, large-scale production began in Germany from 1901. Due to the military's need for TNT to fill grenades (first in Germany from 1902), numerous factories quickly sprang up. The starting material for the production of trinitrotoluene, toluene (toluene), could only be produced in limited quantities at that time, as it was dependent on the extraction of coal tar, a mixture of thousands of individual substances that is obtained during coke production. From today's perspective, however, this method is no longer economical because the proportion of toluene in coal tar is relatively low.

During the Second World War, TNT was again increasingly produced. So-called “sleep factories” were built before the war began, mostly with two installations, so that explosives could continue to be produced in the event of destruction or damage. The quantities produced had increased dramatically. The amount of TNT produced in the German Reich was 18,000 tons per month, while a total of around 0.8 million tons were produced during the war. This increase was possible because the necessary educt could now also be obtained from crude oil. In a two-stage process, the “German process”, the toluene was initially simply nitrated. The resulting mononitrotoluene (MNT) was cleaned of undesired by-products and nitrated again, whereby the desired crude TNT was produced via dinitrotoluene (DNT). After washing and drying several times, it could be granulated and then processed. Security measures were neglected in order to ensure supplies at the front. Since TNT was considered non-toxic for a long time, the waste was only neutralized and allowed to flow into natural waters, where it was partly deposited in the form of sludge and, as old armaments, harmed the environment. Regarding the unknown toxicity, it is known that between 1911 and 1915, 279 munitions workers died from ingesting small amounts through the skin and respiratory tract.

presence

Nowadays, technical extraction takes place through the thermal cracking of crude oil and platinum reforming with subsequent dehydrocyclization, or platforming for short, in which aromatics are obtained from alkanes and cyclo-alkanes. While hydrogen is split off by catalysts made of platinum or aluminum oxide (dehydrogenation), the alkanes change their structure to rings (cyclization) and then form the delocalized electron system. In this way, heptane can be converted into toluene.

It is also possible to introduce an alkyl group into an already existing aromatic. In 1877 Charles Friedel and James Mason Crafts developed this process, which is why it was named after them. With aluminum halides as a catalyst, benzene reacts with alkyl halides.

In the technical implementation, it is aluminum chloride that activates the alkyl halide. This is able to polarize the latter more and more until the alkyl group is cleaved by the halogen and has an electron deficiency, so that it becomes an electrophilic alkyl radical.

This interacts more and more with the π electron cloud (π complex) of the benzene molecule. The electrophilic attack occurs. A carbenium ion is formed consisting of the benzene and the alkyl radical, which are connected to one another via a σ bond (σ complex). Now only the proton (hydrogen) has to be split off (re-aromatization), which means that this type of raw material extraction is based on an electrophilic substitution reaction on an aromatic SE.Ar based. The more detailed examination of this reaction, however, takes place at a different point.

The Friedel-Crafts alkylation has limiting factors which considerably reduce the yield and thus the general economic importance. In electrophilic substitution on aromatics, alkylbenzenes are more reactive than benzene itself; therefore the recently formed alkylbenzene tends to react to form two or more alkylated products. The extraction of toluene for TNT will decrease:

In fact, immense amounts are used, as the raw materials are inexpensive, and the toluene formed is continuously isolated. Because the chemical processes all take place in the form of equilibrium reactions and are thus shifted to monomethylbenzene.

Inductive and mesomeric effect

The reaction speed of the electrophilic substitution on aromatics is decisively determined by the activation energy and two effects, the inductive and the mesomer, which both have an effect on the electron density in the aromatics.

The inductive effect is a polarization effect on the "key atom" via single bonds due to the electronegativity of the atoms in the molecule. + I effects, as they occur with methyl groups, increase the electron density on the aromatic, –I effects, with carboxyl and hydroxyl groups, reduce them. For thisE.Ar is important: the higher the electron density, the more reactive the aromatic.

However, the mesomeric effect is usually stronger than the inductive effect. It comes about when free electron pairs or π electrons from double bonds of a substituent interact with the electron system of the carbon ring and also either increase or decrease the electron density. In the case of the positive mesomeric effect, a free electron pair can slide into the nucleus, which clearly increases the reactivity. Examples of these are the action of the substituents: hydroxylate group, amino group and halogen. The –M effect reduces the electron density in the ring system and has a deactivating effect on the reactivity of an aromatic. The nitro group is one of these substituents.

Second substitution: nitration of the toluene

For the production of TNT, the toluene only needs to be nitrated with nitrating acid. Nitric acid is a special acid consisting of 1/3 sulfuric acid and 2/3 nitric acid. Due to its strength, the former is able to polarize the nitric acid further and further and finally to protonate. The remaining NO2+Ion, the nitryl cation or nitronium ion, is electron deficient and is therefore extremely electrophilic, making the nitrating acid stronger than normal nitric acid.

In contrast to the formation of toluene, the appearance of the molecule changes depending on the carbon on which it is attacked. Because a total of eight different trinitrotoluene are conceivable, of which only the symmetrical, 2,4,6-trinitrotoluene, is the desired one. The explanation can be found in the stability of the interducts of the resulting trinitrotoluenes. The more mesomeric stabilized a product is, the more likely it is to form. We can check this by capturing and comparing the limit formulas for possible structures.

Five different points of attack are conceivable, two two times being identical due to the symmetry of the molecule. These points, where the electrophile attacks after the carbon atoms have been declined based on the functional group, are called the ortho (2nd carbon atom), meta- (3rd carbon atom), and para position (4th carbon atom) -Atom).

Due to the inductive effect of the methyl group in toluene, there is a 96% probability that 2-, 4- and 6- mononitrotoluene are formed, all of which can be used for 2,4,6-trinitrotoluene. Accordingly, methyl groups cause an electrophilic attack at the ortho and para positions. It is “directed” there, because in each of these two positions a limiting formula of the carbenium ion has an energetically particularly favorable structure compared to the others and is therefore generally more mesomerically stable.

Third and fourth substitution

The mononitrotoluene formed now has a further functional group that helps determine the further reaction behavior. The nitro group lowers the electron density in the nucleus, since the nitrogen atom receives a positive partial charge from the strongly electronegative oxygen and therefore cannot provide electrons to increase the electron density. NO2 + therefore has a negative inductive and mesomeric effect. If another nitronium ion attacks our MNT electrophilically, the methyl and nitro groups together determine the point of attack. Again, it is important to consider the possible limit formulas in order to understand the processes. First we follow the further reaction behavior of the most likely product, 2- or 6-mononitrotoluene.

Interestingly, two carbenium ions favored by the methyl group are again conceivable with attack in the ortho and para positions (B + D). But at the same time the nitro group also promotes these possibilities by avoiding the other two positions for the nitronium ion, since the meta positions (A + C) are less stable. The reason for this is the proximity of the two positive partial charges above the carbon ring and the nitrogen atom, which both repel each other. References indicate that in the case of the electrophilic second substitution of an aromatic with the first substituent NO2 93% products are found in the meta, 6% in the ortho and 1% in the para position. Our carbenium ion actually has a meta position if the nitro group as a functional group is decisive for the carbon numbering. Further statements can therefore be made about the yield. Data for the second substitution are:

First substitute ortho position para position meta position
CH3 58 38 4
NO2 6,4 0,3 93,3

The gray colored fields indicate the same position in this case. The meta position for the nitro group can therefore favorably occur either together with the ortho position or with the para position for the methyl group. It is therefore reasonable to assume that the yields of the DNT are almost identical to those of the MNT.

The further reaction of the 4-mononitrotoluene can now be easily understood. Due to symmetry, only two carbenium ions are relevant, with their mesomerism in turn an already known stable and a new less favorable one occurs. The nitro groups reduce the occupation of the ortho and para positions. The final path from DNT to TNT is now evident. Because with the highest probability the sixth carbon will be attacked and we will get a symmetrical structure.

Nobody should dare to manufacture trinitrotoluene without appropriate laboratory experience and appropriate technical equipment. The energy released during the formation of the nitronium ions in the nitrating acid can already cause the products to explode. Adequate cooling is of the utmost importance. Toluene is harmful to health and the work requires an air vent, the nitrating acid is dangerous because of possible severe burns, and nitrous gases are also released during its production. Furthermore, the end product TNT is poisonous and its great explosive power can easily be underestimated by laypeople. Furthermore, the private handling of explosives is subject to authorization and the unauthorized acquisition or handling is punishable (§§ 27, 40 SprengG).

Laboratory scale

In principle, toluene is dripped into a vessel with the nitrating acid; the vessel with the nitrating acid must always be cooled. In a first step the toluene is nitrated to mono-, in a second to di- and finally to trinitrotoluene; To ensure that all of the toluene is converted to TNT, the nitrating acid must be sufficiently strong. The TNT crystallizes to a solid and can be removed from the nitrating acid using a filter.

In practice, harsh conditions have to prevail for the conversion of nitrotoluene to TNT to take place. The second substitution only takes place at temperatures around 100 ° C; TNT production requires pressure and heat-resistant reaction vessels.

use

TNT is still the most important military explosive today and has a detonation speed of 6900 m / s. It is used both in the military and in the commercial sector in mixtures as a safety explosive that can only be detonated by initial ignition (for example by means of a detonator). Cast TNT even requires a booster charge for reliable ignition. TNT alone cannot explode by fire or heat; it just burns down. Due to the high manufacturing costs (around 20 times the cost of commercial explosives), however, its main area of ​​application is primarily the military sector, in which it is used as a combat load of, for example, grenades, bombs and mines.

The energy content in SI units is:

1 kg TNT = 4.6 megajoules (4.6 x 106 Joules)

The TNT equivalent unit, which is often used in connection with atomic explosions, is based on the calorie and is defined by

1 KT (kiloton TNT) = 1012 cal = 4,184 x 1012 J.

The units megaton and gigaton are defined analogously. The capitalization is intended to prevent confusion with the unit of mass kilotons.

Why is TNT exploding?

TNT is one of the best known chemically homogeneous explosives. Like all homogeneous explosives, it owes its explosiveness to its chemical instability. Chemically homogeneous explosives do not consist of a mixture of fuel and oxygen carrier (such as black powder), but only of a single substance. The fuel and the oxidizer required for the explosion are bound in the substance's molecule. Due to the proximity of the reaction partners on the molecular level and the fine distribution on the smallest possible (atomic) scale, an optimal conversion of the reaction partners can be achieved.

If energy is supplied to the TNT molecule through a suitable process (pressure and heat), the nitrogen atom, which is stored in a limited stable manner, between carbon and oxygen is removed and the conversion of the substance begins. If a sufficient amount of the substance has been ignited, the released energy maintains the reaction and the entire amount of substance reacts. The reaction takes place in a very fast, narrow reaction zone through which the substance runs like a wave. With powerful explosives, the speed of this reaction zone reaches several thousand meters per second, i.e. it exceeds the internal speed of sound. The energy released and the formation of gases as reaction products lead to an extremely steep rise in pressure and temperature, which is the reason for the efficiency of explosive explosives.

The picture shows the basic principle. In reality, when TNT detonates, other reaction products are created. Because the oxygen content in the molecule is too low, toxic substances such as carbon monoxide and hydrogen cyanide (hydrocyanic acid) are produced.

Practically all military explosives are based on homogeneous explosives, which, however, are almost always provided with various additives to increase security, increase performance or expand the range of applications.

Chemically related explosives

literature

  • Richard Escales: Nitrous explosives. Survival Press, 1915 Reprint 2003, ISBN 3-8330-0114-3
  • Wolfgang Asselborn, among others: Chemistry today. Upper secondary education. School band Schroedel Verlag, Hanover 1999, ISBN 3-5071-0618-3
  • Hans-Jürgen Quadbeck-Seeger, among others: Chemistry records. People, markets, molecules. 2nd edition, Wiley-VCH, 1999, ISBN 3-5272-9870-3
  • Dr. H.-D. Barke, K. Dehnert, M. Jäckel, Dr. J. Jaenicke, G. Krug, H. Oehr, E. Petrak, W. Rauh, U. Rehbein, Dr. K. Risch: Chemistry today. Lower secondary education. Schroedel Verlag, Hanover 1988
  • M. E. Walsh, T. F. Jenkins, P. S. Schnitker, J. W. Elwell, M. H. Stutz: Evaluation of SW846 Method 8330 for characterization of sites contaminated with residues of high explosives, CRREL Report 93-5, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH

swell

  1. abc ESIS-European chemical Substances Information System

Categories: Explosive substances | Toxic substance | Environmentally hazardous substance | Nitro compound | Aromat | explosive