How are you going to convert benzene to acetophenone



Structural formula
General
Surname benzene
other names

Benzene

Molecular formula C.6H6
CAS number 71-43-2
Brief description colorless liquid with a characteristic odor[1]
properties
Molar mass 78.11 g mol−1
Physical state liquid
density 0.8842 g · cm−3 [1]
Melting point 5.5 ° C[1]
boiling point 80.1 ° C[1]
Vapor pressure

100 hPa (20 ° C)[1]

solubility
  • very soluble in gasoline and alcohol
  • very sparingly soluble in water: 1.77 g · l−1 [1]
safety instructions
MAK

not specified because it is carcinogenic

LD50

930 mg kg–1 (Rat, oral) [2]

WGK 3 - highly hazardous to water[1]
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

  benzene (according to the IUPAC nomenclature as Benzene designated; but is neither an alkene nor an alcohol) is a colorless liquid with a characteristic odor. It belongs to the aromatic hydrocarbons. The molecular formula is C.6H6. Benzene is carcinogenic and has therefore largely been replaced as a solvent by the less dangerous methyl-substituted benzenes toluene and xylene. Benzene is therefore almost exclusively used today as a raw material for the production of some industrial chemicals. As an accompanying substance caused by crude oil, it is still allowed to contain up to 1% of petrol in car fuel in Germany, in the USA it is completely forbidden.

history

The name benzene is derived from the Arabic word “luban dschawi”, which means “incense from Java”. This word came to Europe through trade. After the first syllable was dropped, it became “benjui” in Italian and “Benzoé” in Middle Latin.

In the second half of the 17th century, benzene was discovered by Johann Rudolph Glauber, who also discovered Glauber's salt, during the distillation of coal tar. However, the composition was unknown to him and so he called it a "subtle and lovely oleum". In 1825, the English physicist Michael Faraday discovered benzene in coal gas, namely by isolating this oil from liquid residues that separated from the gas phase when whale oils were burned in London's street lamps. He therefore suggested the name "Pheno" (Greek phainein = to shine). A year later this oil was recognized as a hydrocarbon. In 1834, the German chemist Eilhard Mitscherlich obtained benzene from benzoic acid and calcium oxide, and also converted benzene into nitrobenzene, azobenzene and benzenesulfonic acid. He called the substance "gasoline" because of its relationship to benzoic acid. He also created the correct empirical formula C.6H6. In the same year Justus von Liebig renamed “benzene” to benzene. In 1845 the English chemist Charles Mansfield isolated benzene from coal tar while working under the direction of August Wilhelm von Hofmann.

A long scholarly dispute smoldered about the correct structural formula of benzene. Initial proposals such as the prism structure, that of benzvalene, as well as dicyclopropenyl and dewar benzene (by James Dewar) turned out to be wrong. It was not until 1861 that Johann Josef Loschmidt, an Austrian school teacher, formulated some possible structural formulas for benzene, which the German chemist and professor of chemistry August Kekulé then possibly adopted in 1865 as a suggestion for his Kekulé structural formula (see lower right figure). According to legend, this idea came to Kekulé in a dream. In this dream, snakes took turns biting their tails, creating a ring structure. It takes into account the experimental finding that all carbon atoms in benzene are equivalent. However, this formula has not yet been used to explain all the peculiarities of benzene, such as its unusually low reactivity. In particular, the absence of an addition reaction with bromine water, as would actually be expected according to the Kekulé structural formula, was puzzling. The proof of the equivalence of the hydrogen in the benzene molecule could be provided from 1869 to 1874.

In 1872, Kekulé formulated his oscillation hypothesis of a permanent change in place of single and double bonds. It was not until the 20th century that the phenomenon of delocalized electron clouds, which give the benzene molecule a particular stability, could be demonstrated using X-ray structure analysis. In 1988, Kekulé's structural formula could be proven in the grid tunnel model.

In 1849 the industrial production of benzene on the basis of hard coal began. Until a few decades ago, no one was aware of the toxicity of benzene. Benzene was handled carelessly. However, awareness campaigns about the dangers of benzene followed later.

Structural formula and nomenclature

   

Creating the correct structural formula for benzene was a problem for a long time (see history), which is partly due to the large number of theoretically possible structural formulas. A computer would do the empirical formula C6H6 217 Find structural formulas.

Each carbon atom has four valence electrons, two of which connect the atom to the neighboring carbon atoms. An electron binds the associated hydrogen atom. The remaining six π electrons formally form three π bonds, as expressed by the double bonds in the structural formula. In the orbital model that is valid today, the six π electrons form a delocalized charge cloud (delocalized 6 π electron system) above and below the level of the carbon ring. Kekulé expressed this fact of mesomerism through the two structural formulas, each of which symbolizes only one extreme point of the cloud of charge. Due to the mesomerism, the carbon ring is more stable than the hypothetical cyclohexatriene with localized double bonds, i.e. with double bonds at fixed positions. In the simplified notation, the carbon ring is now represented as a hexagon and the electron cloud as an inscribed circle. Benzene is the simplest uncharged aromatic molecule. It is a planar molecule. The carbon atoms are sp² hybridized. Benzene is the basic building block for many aromatics.

Since the ending -ol is used for alcohols in the systematic chemical nomenclature, the historically determined term benzene, which is most commonly used in Germany, is misleading; the name Benzene was determined by the IUPAC as the official nomenclature for this hydrocarbon.

properties

Physical Properties

Benzene is a colorless, clear, highly refractive, highly volatile and easily flammable liquid with a refractive index of 1.5011. The refractive index of benzene agrees quite well with that of window glass, so a glass rod immersed in benzene is almost invisible. The viscosity of benzene is lower than that of water (it is thinner). It freezes at 5.5 degrees Celsius and boils at 80.1 degrees Celsius. At room temperature (20 degrees Celsius) it has a density of 0.88 kilograms per liter and a vapor pressure of 110 hPa. The properties of pure benzene are specified by DIN standard 51633.

Benzene has good to unlimited solubility in non-polar, organic solvents such as ether and alcohol, but only poorly in water (maximum 1.7 grams per liter).

A special feature is that benzene has six bonds of equal length at 139 pm. This value lies between that for a single bond and a double bond and is an indicator of the aromatic character of benzene.

chemical properties

Benzene burns with a yellow flame to form water and carbon dioxide, whereby the strong soot development indicates the high carbon content of the compound. The calorific value of benzene is 40,580 kilojoules per kilogram, the enthalpy of combustion is 3257.6 kilojoules per mol for liquid benzene and 3301 kilojoules per mol for gaseous benzene. Benzene has a characteristic odor. The odor threshold is very low and is 1.5 to 900 milligrams per cubic meter of air.

An important (recognition) feature is the absence of an addition reaction with hydrobromic acid or bromine water. The increasing reactivity towards the addition of hydrogen bromide in the series of the four structurally similar compounds cyclohexane, cyclohexene and cyclohexadiene would suggest an even greater reactivity of benzene (i.e. cyclohexatriene), because in the first three compounds an increase in reactivity caused by a higher number of carbon double bonds can be determined. Benzene does not add hydrobromic acid due to its special, aromatic stability, which results from the delocalization of the electrons.

The thermal coefficient is 0.001187.

spectroscopic properties

in the 1In the H-NMR spectrum, the hydrogen atoms show an unusually strong deshielding (typically signals at 7.28 ppm), which is caused by the ring current induced by the magnetic field of the spectrometer.

in the 13C-NMR spectrum shows benzene a signal at 128.5 ppm.

In the IR spectrum, the phenyl-hydrogen oscillation appears at around 3,035 cm-1. The C-C stretching oscillation appears at 1,500 to 2,000 cm-1. At 650 to 1,000 cm-1 are the C-H deformation vibrations.

With UV spectroscopy, benzene can still be detected in high dilution at two typical absorptions (π-π * transitions) in the range of 200 - approx. 250 nm.

Hazards and Precautions

 

Benzene is highly flammable. Benzene fumes are poisonous when inhaled; the symptoms of acute poisoning only appear at relatively high concentrations. Mild poisoning manifests itself in dizziness, nausea, drowsiness and apathy. Severe poisoning results in fever and visual disturbances up to temporary blindness and loss of consciousness. The so-called benzene addiction, which can occur when breathing in benzene, leads to feelings of drunkenness and euphoria. Benzene can be fatal with prolonged exposure.

The toxic effects as well as the carcinogenic effects are due to the formation of a carcinogenic metabolite. In the body, benzene is oxidized on the ring. The resulting highly reactive epoxide reacts with numerous biological compounds and can also damage the genetic make-up. Long-term ingestion of smaller amounts of benzene mainly leads to damage to the internal organs and the bone marrow. The latter results in a decrease in the number of red blood cells (anemia), which manifests itself in palpitations, eye flickering, tiredness, dizziness, paleness and headache. Benzene is stored in the brain, bone marrow, and adipose tissue. It is only slowly excreted through the kidneys. The degradation takes place via various conversion products such as catechol, phenol, hydroquinone and benzoquinone. Finally, the main excretion product is phenyl mercapturic acid (N-acetyl-S-phenyl-cysteine).

With 2% air volume fraction of benzene in the breath, death occurs after 5 to 10 minutes. The acute lethal dose (oral) in humans is 50 milligrams per kilogram. Benzene forms explosive mixtures between an air volume fraction of 1.4 to 8%.

Benzene should be handled with particular care because of these hazards. Benzene must be stored at 15 ° C to 25 ° C. The TRK value is 1 milliliter per cubic meter of air (or 3.25 mg / m³ air). Any exposure to benzene should be avoided or reduced as much as possible; special instructions should be obtained before using benzene. In the event of an accident or if you feel unwell, a doctor should be consulted immediately. Places where benzene leaks or could leak should be vacated immediately and only entered again in full protective suits. Benzene is very hazardous to water.

Occurrence and benzene emissions

Benzene is found in coal tar and petroleum. When smoking cigarettes, small amounts of benzene vapor (10-100 µg per cigarette) are released; traces of benzene are also produced in volcanic eruptions and forest fires. Half of the benzene present in the atmosphere is broken down after two to five days, as this reacts with hydroxyl radicals (free OH groups). Incomplete combustion of organic material also produces benzene, but it is mainly emitted from gasoline through car exhaust fumes; 75% of emissions are attributable to motor vehicles. The average pollution of the population is around 2 µg / m³ air, but this value can be significantly higher depending on the environment (for example at petrol stations, in poorly ventilated garages, etc.). There was a sharp drop in benzene emissions around 1980 and have remained roughly the same since then. Lower gasoline consumption in cars could significantly reduce benzene emissions.

Manufacturing

Benzene can be obtained by cracking gasoline or petroleum. In most cases, hexane is dehydrated to cyclohexane and then to benzene. Another method of producing benzene is that thermal dealkylation. The methyl group of the toluene used is split off at 780 degrees Celsius and a pressure of 40 bar. The carrier gas in the reactor is 90 percent hydrogen. After cooling, gas-liquid separation and cleaning, pure benzene is obtained. Benzene can also be produced from ethyne molecules in the presence of a catalyst.

From 1940 to around 1960, most of the benzene was produced on the basis of hard coal. It has also been cracked from crude oil since 1950. In 1992, around 90% of benzene in Western Europe was obtained from crude oil (65% from pyrolysis gasoline, 17% from reformate gasoline, 9% by dealkylation) and 10% from coal and coal tar. Around 35 million tons of benzene are produced worldwide every year.

Reactions

Complete oxidation (combustion) of benzene:

Benzene reacts with oxygen to form carbon dioxide and water.

Example of an electrophilic substitution reaction (nitration):

Benzene reacts with nitric acid (in the presence of the sulfuric acid catalyst) to form water and nitrobenzene.

Example of a radical addition reaction (chlorination):

Benzene reacts with chlorine to form lindane.

Friedel-Crafts acylation:

Compared to alkenes, there are no radical substitutions on benzene due to the stabilization through the aromaticity. The exception to this is the so-called Sandmeyer reaction, in which a phenyl radical is formed from diazonium salts in a homolytic cleavage of molecular nitrogen catalyzed by copper. However, this is highly reactive and continues to react immediately.

Benzene derivatives and important substituents

Many important chemicals have a benzene ring as their backbone. These include, for example:

Alkyl substituents:

  • Methylbenzene - toluene (solvent)
  • Dimethylbenzene (mixture of isomers) - xylene (solvent)
  • 1,3,5-trimethylbenzene - mesitylene (solvent)

other functional groups:

  • Hydroxyl group (-OH), phenol (antiseptic)
  • Amino group (-NH2), Aniline (for dye production)
  • Alkoxy group (-OCnH2n + 1), Anisole (in perfume)
  • Alkenes (-CnH2n-1), Styrene (important monomer for the production of plastics and synthetic resins)

Benzene products

further aromatic rings lead to polycyclic aromatic hydrocarbons (PAHs) such as:

Applications

Today, due to the serious dangers, substances with a benzene concentration above 0.1% are banned in Germany. Fuels are an exception: Benzene increases the anti-knock properties of petrol, which is why it played an important role in the development of petrol, but is now only permitted in a concentration of 1% as an accompanying substance. Benzene is banned as a fuel additive in the United States. In larger quantities it may only be used in closed systems and for industrial or research purposes. In the first half of the 20th century it served as fuel for the so-called benzene locomotives as well as pure (rarely) or mixed with gasoline (see Bibo) to drive cars.

Benzene is used in the chemical industry for the synthesis of many compounds, such as aniline, styrene, nylon, synthetic rubber, plastics, detergent substances, insecticides, dyes and many other substances. In addition, many aromatics such as phenol, nitrobenzene, aniline, chlorobenzene, hydroquinone and picric acid are obtained through substitution.

In the past, benzene was used as a good solvent and cleaning agent in many areas. As a solvent for rubber varnishes, waxes, resins and oils, benzene is increasingly being replaced by less toxic substances such as the non-carcinogenic toluene. As a good solvent, it is still sometimes used as a “universal cleaner” in laboratories despite warnings, but in this role it has been largely replaced by acetone. In the normal household, however, it is not used due to its toxicity and carcinogenic effect. B.enzol belongs next Toluene and Xylol to the so-called BTX aromatics, for which strict limit values ​​apply for industrial use.

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  1. abcdefG Entry on benzene in the GESTIS substance database of the BGIA, accessed on October 14, 2007 (JavaScript required)
  2. Benzene at ChemIDplus

literature

  • Edgar Heilbronner, Jean Jacques: Paul Havrez and his benzene formula. in: Chemistry in our time. 32.1998,5, pp.256-264. ISSN 0009-2851
  • Elke Schimming: Human exposure to benzene and other aromatic hydrocarbons with special consideration of the contribution of indoor air . Cuvillier, Göttingen 1998. ISBN 3897121956
  • Reichelt Pfeifer: H2O & Co organic chemistry. Oldenburg, Munich 2003, ISBN 3-486-16032-X
  • C.J. Estler: Pharmacology and toxicology. Schattauer, Stuttgart 1992, 2005. ISBN 3-7945-2295-8
  • Nabil Osman (editor): Small lexicon of German words of Arabic origin. 6th edition, Munich 2002. ISBN 3-406-475841

See also

  • Properties, hazards, use and manufacture
  • Effects, tendencies, production
  • Properties, dangers and legal guidelines
  • International Chemical Safety Card
  • Structure of benzene

Categories: Flammable Substance | Toxic substance | Aromat