Benzene

The benzene is a aromatic Hydrocarbure monocyclic, of formula C 6 H 6, also noted pH - H. This Composé colorless organics besides (it has same the Index of refraction that glass) is a Liquide Cancérigène. It is a Solvant very much used in chemical industry, and an important precursor for the chemical synthesis of Médicament S, plastic, synthetic rubber or of dye S. the benzene is a constituent naturalness of the Pétrole gross, but it is generally synthesized from others made up organics present in oil.

History

The benzene was discovered in 1825 by the scientific British Michael Faraday which isolated it from the Pétrole and bicarburet baptized it off hydrogen . In 1833, the German Chimiste Eilhard Mitscherlich produced it by Distillation Benzoic acid (component of the gum Benjoin) and Chaux. Mitscherlich baptized this compound benzin . In 1845, the British chemist Charles Mansfield, working under the direction of August Wilhelm von Hoffmann, isolated it from the coal tar. Four years later, it launched the first production of benzene on the scale Industrie it starting from coal tar. In 1868, Marcellin Berthelot synthesizes it by trimerisation of the Acétylène.

Chemical structure

History

During a few years, the Chemical formula of benzene, C 6 H 6 , caused a great number of interrogations concerning the structure of the compound. Several structures were successively proposed without managing to explain the chemical properties of the compound, among which for example that proposed by James Dewar, presented opposite (on the left).

The first correct structural form was proposed in 1861 by Johann Josef Loschmidt, which provides a base to the first correct interpretation of the structure of benzene proposed by the German chemist Friedrich August Kekulé von Stradonitz in 1865 (on the right). The plan eity of this compound is one of the particularly interesting facts of chemistry. Kekulé highlighted the fact that several representations (known as nowadays of Lewis) of this molecule are equivalent. More precisely, the doubles connections can be placed anywhere on the cycle, in an alternate way.

However, of the researchers discovered by using the diffraction of x-rays that all the connections carbon-carbon of the benzene molecule have same the Length, which is not compatible above with the representation. Indeed, a simple connection is longer than a double linking. Moreover, the length of connection in the benzene molecule is at the same time larger than the length of a connection doubles carbon-carbon, and weaker than that of a simple connection. All occurs as if there was a bond and half between each carbon atom.

It will be necessary to await the theory of the orbital hybrids (worked out by Linus Pauling, Nobel Prize of chemistry and Nobel Prize of peace, in its publication the nature of the chemical bond ) to explain this fact with a great elegance, and this definitively. Indeed the chemical bonds can be described with a reasonable approximation as being formed by atomic orbital coverings of (of method CLOA, Linear Combinaison of Orbital Atomic, atomic N orbital mix to form molecular N orbital). The molecular orbital obtained by this linear combination can induce effects of delocalization of the electrons.

Delocalization-mésomérie

One explains the flatness of benzene by the fact that in this conformation, the orbital 2p (X or there) pure (i.e. not hybridées) of the carbon atoms optimize their side covering. There is not thus really a double connection between two carbons given, but what is called a resonant system pi (or delocalized) that one can describe like a vast electronic cloud distributed equitably between all the carbon atoms.

Electrons pi are thus distributed on all the carbon atoms, and the molecule can be represented like the superposition of the two following forms, called forms mésomères:

Actually, none of the two forms represented above exists. The delocalization must be represented in a different way that by the simple use of simple connections and double linkings. In a organic molecule, the simple connections are connections σ, made of electrons of which the probability of presence is very important between the atoms (axial covering). The double linkings are made at the same time of a connection σ and of a connection π, those being built starting from the electrons 2p (X or there) of the carbon, as illustrated on the following diagram (in the part of left, the connections σ are represented in red, orbital the 2p in white and gray):

Orbital the 2p (X or there) being apart from the plan consisted the atoms, they can interact freely, which leads to the delocalization of the electrons: each electron is not attached specifically to an atom or a connection, but is delocalized on all the ring, reinforcing each connection in an equivalent way:

To represent this delocalized character of the connections, the benzene is generally represented by a circle contained in a hexagon:

Aromaticity

This delocalization of the electrons is called aromaticity. It is responsible for many properties of benzene, and in particular for its great stability.

Compared to a “virtual” system, for which the connections π would be localized (each connection located between two carbon atoms), the real system in which the connections π are delocalized on the totality of the cycle is stabilized of more than 150  kJ/mol. The chemical reactions in which the benzene takes part are thus preferentially those for which this stabilization is preserved.

Physicochemical characterization and properties

The benzene is a colorless Liquide, whose Index of refraction is 1,50 (close to that of the Verre). Its Viscosité is weaker than that of the Eau. It is very soluble in the organic Solvant S polar, but its solubility in water is rather low. It has a characteristic odor, with a threshold of detection of 1,5 with 900  Mg per m of air.

In Infra-red Spectroscopy of absorption , the benzene presents an absorption band in the vicinity of 1500-1600  cm due to the Vibration S of the connections Carbon - carbon, and several peaks of absorption between 650 and 1000  cm due to the vibrations of the connections carbon Hydrogen. The position and the Amplitude of these last peaks give information on possible substitutions of hydrogen atoms.

In nuclear Magnetic resonance (NMR) of the Proton, it presents a peak of chemical shift δ to 7-8 ppm.

Production

The benzene is produced when compounds rich in Carbone undergo a incomplete Combustion. For example, it is produced naturally in the Volcan S or the forest fires. It is also present in the Fumée of Cigarette.

Until the Second world war, the benzene was in major part a secondary product of the production of coke in the Industrie of the Acier. However during the Years 1950, the benzene increasing demand, in particular in the industry of the plastic , involved the need for producing benzene starting from Pétrole. At present, the major part of benzene is produced by petrochemical industry , with a minor share resulting from the coal.

The industrial production of benzene is resulting so as to little close equal from three chemical processes: the catalytic Reforming, the hydrodesalkylation of the Toluene and the vapocraquage. In 1996, the world production benzene was of 33 million ton S including 7 million with the the United States, 6,5 million in the European Union, 4,2 million with the Japan, 1,4 million in South Korea and 1 million in China.

Catalytic reforming

During catalytic reforming, a Mixture of Hydrocarbon S of boiling points ranging between 60 and 200 °C is mixed with Dihydrogène, then passed on Catalyze the USSR (Chlorure of platinum or Chlorure of rhenium) with a Température ranging between 500 and 525 °C and a Pression ranging between 8 and 50 atm. Under these conditions, the Hydrocarbure S Aliphatique S form cycles and lose aromatic atoms of Hydrogène to become . The aromatic compounds produced during the reaction are then separated from the reactional mixture by Extraction by using Solvant S like the Sulfolane or the Diéthylène glycol. The benzene is then separated from the others aromatic compounds by Distillation.

Hydrodésalkylation of toluene

The hydrodesalkylation of the Toluène makes it possible to convert toluene into benzene. In this chemical process, toluene is mixed with Dihydrogène, then passed on a Catalyze ur (Oxyde of Chrome, Molybdène or Platine) at a temperature ranging between 500 and 600 °C and a Pression ranging between 40 and 60 atm. It is also possible to do without catalyst by using higher temperatures. Under these conditions, toluene undergoes a desalkylation (loss of the grouping alkyl, here a grouping Méthyle):

C6H5CH3 + H2 → C6H6 + CH4

The Rendement of this reaction is higher than 95 %. Sometimes, of the aromatic compounds heavier as the Xylène are used in the place of toluene with similar outputs.

Vapocraquage

The vapocraquage is a process used to produce ethylene and others Alcène S starting from hydrocarbons Aliphatique S. Following the starting compound used in the process, the vapocraquage can also produce a secondary product Liquide rich in benzene. This liquid can be mixed with other hydrocarbons as an additive for the gasoline, or distilled to separate it in various compounds of which the benzene.

Uses

Before the Years 1920, the benzene was frequently used like Solvant Industrie L, particularly to degrease the metals. When its Toxicité became obvious, it was replaced by other solvents for the applications requiring a direct exposure of the user.

The benzene is used in major part as intermediary in the synthesis of others chemical compounds. The derivatives of benzene produced in the greatest quantities are the Styrène, used to manufacture Polymère S and plastic, the phenol, used to manufacture resin S and Adhésif S, and the Cyclohexane, used to manufacture the Nylon. Less large quantities of benzene are used in the manufacture of Pneu S, of Lubrifiant S, dye S, Détergent S, Médicament S, Explosif S or of Pesticide S. In the Années 1980, the principal compounds produced starting from benzene were the ethylbenzene (intermediate for the manufacture of the Styrène) with 48% of the consumption of benzene used for the synthesis, the Cumène 18%, the Cyclohexane 15% and the Nitrobenzène 7%.

As an additive with the gasoline, the benzene makes it possible to increase the Number octane, thus acting like anti-knock. So until in the Années 1950 the gasoline frequently contained a few percent of benzene, when it was replaced by the Tétraéthylplomb in the most used anti-knock agents. However, the benzene made its return in many Pays following the regulations concerning the content of Plomb of the gasoline. With the E. - U., the concerns concerning its harmful effects on the Health and the possibility of contamination of the ground water led to the installation of a strict Réglementation concerning the content benzene of the fuels with a limit close to 1%. In Europe, this same limit of 1% was fixed.

Chemical reactions using benzene

The chemical reactivity of benzene is strongly dependant on its aromatic nature . For example, the majority of the Alcène S can be hydrogenated (the double connection is transformed into simple connection by addition of hydrogen) under a pressure of 1 atm with room temperature in a reaction catalyzed by the Nickel. In the case of the benzene, this same reaction must be realized to 180 °C under a pressure of 2000 atm. Indeed, the addition of hydrogen makes lose the aromatic character, and thus the delocalization of the electrons which contributes to strongly stabilize the compound. The benzene will thus tend rather to react by substitution of hydrogen, thus preserving its aromatic character.

aromatic Substitution électrophile

Aromatic substitution électrophile is a generic reaction during which one of the Atome S of Hydrogène is substituted by another functional Groupe. During this reaction, the benzene plays the part of Nucléophile, and reacts with a reagent électrophile such as for example a Carbocation. The simplified mechanism of the reaction is the following:

The load positive range by the intermediary reactional, named intermediate of Wheland , is actually delocalized on the cycle by Mésomérie, which tends to stabilize the benzene carbocation. This reaction generally requires a Catalyze ur of the type Acide of Lewis.

Acylation de Friedel and Crafts

The acylation of Friedel and Crafts is an aromatic particular case of substitution électrophile. This reaction is the Acylation of a aromatic Composé, like benzene, by a chloride acyle. This reaction is catalyzed by a powerful Acide of Lewis (like AlCl3 here):

Alkylation of Friedel and Crafts

The alkylation of Friedel and Crafts is similar to the acylation, with this close which it constitutes the Alkylation of an aromatic compound, like benzene, by a Halogénure of alkyl. It is also catalyzed by a powerful Acide of Lewis:

Substituted benzene

A great number of very important chemical compounds in industry are obtained by replacing one or more Atome S of Hydrogène of benzene by others functional groups.

Substitution by groupings Alkyl S

Substitutions by other groupings

  • the phenol C6H5-OH

  • the Aniline C6H5-NH2
  • the Chlorobenzene C6H5-Cl
  • the Nitrobenzine C6H5-NO2
  • the Picric acid C6H2 (- OH) (- NO2) 3
  • the Trinitrotoluene C6H2 (- CH3) (- NO2) 3
  • the Benzoic acid C6H5-COOH
  • the Salicylic acid C6H4 (- OH) (- COOH)
  • the acetylsalicylic acid C6H4 (- O-C (=O) - CH3) (- COOH)
  • the Paracetamol C6H4 (- NH-C (=O)- CH3) - 1 (- OH) - 4
  • the Phenacetin C6H4 (- NH-C (=O) - CH3) (- O-CH2-CH3)

Welded aromatic rings

Effects on the Health

The intoxication by benzene alone bears the name of benzenism; that by benzene or its derivatives (toluene, xylene…) bear the name of benzolism.

The inhalation of a very high benzene rate can cause the Mort, while high rates can cause somnolences, Vertige S, an acceleration of the Cardiac rhythm, evils of head, tremors, the confusion or the loss of consciousness. An exposure from five to ten minutes toa benzene rate in the air of approximately 2% is enough to result in death. The lethal amount by ingestion is of 50 mg/kg. The ingestion of Food or Boisson S containing of benzene high rates can cause Vomissement S, an irritation of the Estomac, giddinesses, somnolences, Convulsion S, an acceleration of the cardiac rhythm, even death.

The principal effect of a chronic exposure to benzene is a damage of the osseous Moelle, which can cause a decrease of the rate of red globules in the Sang and a Anémie. It can also cause bleedings and a weakening of the Immune system. The effect of benzene on the Fertility of the Man or the good development of the Fœtus is not known. Lastly, the benzene is recognized as being a substance Cancérigène.

The professional exposure limit value in the European Union is fixed by the regulation at 1 ppm is 3,5 mg.m-3 over 8 a.m. The content benzene of the Eau X intended for human consumption should not exceed 1 µg/l (except for mineral water).

One of the exposures, of the general public but also of the professionals, with benzene is on the level of the service stations, since there is always a little benzene in the fuel: at the time of the filling of the tank, the gun can let escape a small portion of benzene (volatile), which is then inhaled by the customer (or the professional). The guns can be provided with a protection.

Carcinogenic properties of benzene

The carcinogenic properties of benzene come from what it behaves as a agent intercalating (i.e. it slips between the nucleotidic bases of the nucleic acids, of which DNA, causing replication and/or misreadings). There exist other intercalating agents (like Bromure of éthidium, or Study Bureau, used in experimental biology to mark the DNA in particular during the electrophoresis S). All the plane compounds are not however carcinogenic. The Benzoic acid , for example, very near to the benzene, and whose combined base is absolutely plane, is not carcinogenic (it is used like preserving in various types of soda). In the same way the Phenylalanine, an Amino-acid which comprises a grouping phenyl (a benzene cycle), is not carcinogenic.

Sentences of risk and council of prudence

The benzene is a dangerous chemical compound, which must be handled and used with many precautions. It must be stored between 15 and 25°C.

According to the National research institute and of Safety (France)

  • R: 11 (Easily flammable)
  • R: 48/23/24/25 (Poison: risk serious effects for health in the event of exposure prolonged by inhalation, contact with the skin and ingestion)
  • R: 45 (Can cause cancer)
  • S: 45 (In the event of Accident or of Malayan to consult a Doctor immediately and to show him the Packing or labels it)
  • S: 53 (To avoid the exposure and to get special instructions before the use)

According to the international card of safety

  • R: 45 (Can cause cancer)
  • R: 46 (Can cause hereditary genetic deteriorations)
  • R: 11 (Easily flammable)
  • R: 36/38 (Irritating for the eyes and the skin)
  • R: 48/23/24/25 (Poison: risk serious effects for health in the event of exposure prolonged by inhalation, contact with the skin and ingestion)
  • R: 65 (Harmful: an attack of the lungs in the event of ingestion)
  • S can cause: 53 (To avoid the exposure and to get special instructions before the use)
  • S: 45 (In the event of Accident or of Malayan to consult a Doctor immediately and to show him the Packing or labels it)

Anecdotes

Catastrophe of the petrochemical factory with Jilin

See also: Catastrophe of the petrochemical factory of Jilin

Following the explosion of a petrochemical factory in the town of Jilin in Popular republic of China on November 13rd, 2005, a quantity of benzene estimated at a hundred ton flowed in the river Songhua, an important affluent of the river Amour. This accident involved many water cuts in the cities located downstream, in particular Harbin (5 million inhabitants).

See too

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