How to prepare alcohol from benzene


Synonym: benzene
English: benzene

1 definition

benzene is the simplest aromatic hydrocarbon with the empirical formula C.6H6.

2 structure

The six carbon atoms are arranged in a six-ring system, which is also known as the benzene ring or benzene nucleus. The molecule contains three carbon double bonds and is a mesomeric stabilized aromatic hydrocarbon. The molecular weight of benzene is 78.11 g / mol.

In the six-ring system of benzene, all six carbon atoms are equivalent. In the case of disubstituted products, depending on the position of the substituents, a distinction is made between an ortho- (substituent on carbon atom 1 and 2); meta- (substituents on carbon atoms 1 and 3 or 1 and 5) and para- (substituents on carbon atoms 1 and 4) position of the substituents.

3 properties

3.1 Physical properties

Benzene is one of the liquid, toxic (carcinogenic) substances and is highly volatile. Its viscosity is lower than that of water. With a refractive index of 1.501, it is a highly refractive substance.

The density of benzene at room temperature is 0.88 kg / dm3 and the vapor pressure at this temperature is 110 kPa. The freezing point is 5.5 ° C, the boiling point is 80.1 ° C, at room temperature benzene is in the liquid state.

In non-polar, organic solvents such as ether and alcohol, benzene is almost indefinitely soluble, in water, however, the solubility is very low with a maximum of 1.7 g / L.

3.2 Chemical properties

The combustion of the benzene takes place with a yellow flame and strong soot development. The latter indicates the high carbon content of this molecule. The enthalpy of combustion of liquid benzene is 3257.6 kJ / mol, the calorific value of benzene is 40580 kJ / kg.

The smell of benzene is very characteristic and is noticeable even in small quantities, as the odor threshold is 1.5-900 micrograms per cubic meter of air.

An important chemical characteristic of benzene is the lack of an electrophilic addition reaction with bromine water. While the reaction of cyclohexene with bromine water produces 1,2-dibromocyclohexane by addition, benzene reacts with bromine water in a substitution reaction with the formation of bromobenzene, since the aromatic character is retained in this reaction. Since benzene is a non-activated aromatic, catalysis in the form of a Lewis acid, e.g. FeBr3 necessary.

Benzene forms an explosive mixture at an air concentration of 1.4 to 8%. The properties of pure benzene are specified in DIN standard 51633.

4 occurrences

Benzene occurs naturally primarily in hard coal and petroleum, from which it can also be isolated and obtained by means of chemical processes (thermal dealkylation, cracking from petrol and petroleum (90%)).

In addition, benzene is also found in the incomplete combustion of substances, i.e. in house fires, forest fires and volcanoes. The main part of the benzene emissions are caused by exhaust fumes from motor vehicles, which leads to an average pollution of the population of around 2 micrograms per cubic meter of air. The smoke from cigarettes also contains small amounts of benzene. The value here is around 100 to 1000 μg per cigarette.

About 50% of the benzene in the air is eliminated by reacting with OH radicals in the atmosphere. However, this is not enough for the high production of benzene. Thanks to climate protection, benzene emissions have fallen sharply since their peak in the 1980s, but a further reduction in benzene emissions is desirable.

5 Use

Benzene is used in its pure form as well as chemically modified by oxidation, substitution, addition and other diverse uses.

Benzene itself was often used as a solvent in the chemical industry in the past, but has now been largely replaced by the less carcinogenic substances toluene and xylene. In addition, because of its highly flammable properties, it is added to motor fuels, which explains why 75% of the benzene in the air is caused by emissions from motor vehicles.

In the form of derivatives such as toluene, aniline or phenol, it is used in industry for the production of textiles, perfumes, pharmaceuticals, plastics and dyes. Acetylsalicylic acid and paracetamol also contain a benzene backbone.

6 biochemistry

A substituted benzene ring can be found in various biomolecules. The aromatic six-ring is not built up independently in humans, but is consumed with essential aromatic amino acids (e.g. phenylalanine) through food.

Benzene rings can carry certain functional groups (e.g. in adrenaline, dopamine, vitamin E) and thus become receptor-active or they contribute to the stabilization of complex protein structures through hydrophobic interactions between aromatic rings.

7 toxicology

Benzene is very harmful to the human organism even in small amounts. It can enter the organism through ingestion, inhalation and percutaneously.

The strong carcinogenic effect of benzene results from the formation of carcinogenic metabolites. After the benzene has been absorbed, the benzene is oxidized on the ring and a highly reactive metabolite, epoxide, is formed, which can damage the genetic make-up. The type of damage depends on the duration and dose of benzene exposure.

7.1 Acute exposure

A benzene concentration of 2% in the air you breathe causes death after only 5-10 minutes. The acute lethal oral dose is around 50 mg per kilogram of body weight.

Although even small amounts of ingested benzene are poisonous, symptoms only occur with larger amounts or longer exposure.

The first symptoms of benzene poisoning include dizziness, drowsiness, nausea, and apathy. However, it can also lead to the so-called Benzene addiction come, which manifests itself in euphoria and drunkenness.

In addition to the symptoms mentioned, severe poisoning manifests itself in fever, visual disturbances which can lead to temporary blindness and unconsciousness. With prolonged exposure, benzene is fatal.

7.2 Chronic exposure

If the benzene exposure occurs in very small amounts over a longer period of time, other consequences can initially be observed. Damage to the genetic make-up leads to increasing damage to the internal organs and the bone marrow, which can manifest itself in pancytopenia, anemia with dizziness, palpitations, paleness, tiredness and headaches.

If the bone marrow is damaged, an acute form of leukemia can also develop. The benzene is only slowly metabolized and excreted via the kidneys. Benzene is mainly excreted as phenymercapturic acid. In the long term, however, this image also leads to death.

8 Security Measures

In principle, any unprotected exposure to benzene should be avoided. It should be stored at 15-25 ° C and in the containers provided. Disposal must be carried out professionally by specialist companies, since benzene is not only highly toxic but also environmentally hazardous, so it was classified in water hazard class 3.

When working with benzene, appropriate protective equipment must be worn and the corresponding risk and safety phrases (GHS system: GHS02, GHS07, GHS08; signal word: DANGER; H225-H304-H315-H319-H340-H350-H372; P201-P210-P301 + P310-P305 + P351 + P338-P308 + P313-P331) must be taken into account.

Should any symptoms or discomfort occur during or after exposure to benzene, an appropriate doctor should be consulted for clarification.

9 laboratory medicine

9.1 Material

4 ml EDTA blood or 20 ml spontaneous urine are required for diagnosis. Special tubes must be used for taking blood.

9.2 Reference ranges

10 sources; accessed on February 6, 2021