Benzene (C6H6) A hydrocarbon, the simplest aromatic hydrocarbon, is a sweet, flammable, carcinogenic, colorless, transparent liquid at room temperature with a strong aromatic odor. Benzene Ring

It is hardly soluble in water, soluble in organic solvents, and itself as an organic solvent. The ring system of benzene is called Benzene Ring. The structure after the benzene ring is removed by one hydrogen atom is called Phenyl group, which is represented by Ph.

Structure of Benzene

Therefore, the chemical formula of benzene can also be written as PhH. Benzene is a basic raw material for the petrochemical industry. Its production level and technical level of production are one of the indicators of the development level of the national petrochemical industry.

On October 27, 2017, the World Health Organization’s International Agency for Research on Cancer published a preliminary list of carcinogens, benzene in a list of Carcinogens.

English name  = Benzene, benzol, benzene

Nickname   =    Rest oil

Chemical formula  =    C6H6

Molecular weight  =    78.11

CAS registration number   =    71-43-2

EINECS login number  =    200-753-7

Melting point   =    5.5°C

Boiling point    =    80.1 ° C

Water soluble   =    0.18g/100ml

Density    =      0.8765g/cm3 (20°C)

Flashpoint    =    -11 ° C

Application  =  Used as a fragrance, dye, plastic, medicine, explosives, rubber, etc.

Risk description  =   Flammable substances, toxic substances

Dangerous goods transport number   =    UN1114/1115

Chemical category   =    Organic matter– Homologue of benzene

A brief history of Research

Benzene was first discovered in 1825 by the British scientist Faraday (Michael Faraday, 1791 – 1867). At the beginning of the 19th century, the United Kingdom, like other European countries, used gas throughout the city.

After the gas is produced from the raw material for producing gas, the remaining oil-like liquid is left unattended for a long time. Faraday was the first scientist interested in this oily liquid.

He separated the oily liquid by distillation to obtain another liquid, which was actually benzene. At the time, Faraday called this liquid “a heavy carbon compound of hydrogen.

In 1834, the German scientist Mihirrich (Ernst Eilhard Mitscherlich, 1794 – 1863) obtained by distilling a mixture of benzoic acid and lime. A liquid identical to the liquid produced by Faraday and named benzene.

Benzene Discovery

After the establishment of the correct molecular concept and valence concept in organic chemistry, the French chemist Rilar (Charles Frederic Gerhardt, 1816 – 1856) et al.

It was also determined that the relative molecular mass of benzene was 78 and the molecular formula was C6H6.


The relative amounts of carbon in the benzene molecule are so high that chemists are surprised. For how to determine the structural formula of benzene, chemists have encountered a problem.

The carbon and hydrogen ratio of benzene is so large, indicating that benzene is a highly unsaturated compound, but it does not have the typical addition of typical unsaturated compounds. The nature of the reaction.

Roche Mitt Benzene Cyclic Chemical Structure

Austrian chemist Roche Mitt (ie John Joseph Roche Mitt, Johann Jasef Loschmidt) in his “chemistry” (published in 1861), a book depicts the 121 benzene and other aromatic compounds the cyclic chemical structure.

German chemist Kekule (ie Friedrich August Kekule von Leibniz Manchester, Colorado, Friedrich August Kekulé von Stradonitz, 1829 Year – 1896) also see throughout this book, in a letter to his students on January 4, 1862, it was confusing to mention that Lohmit’s description of the molecular structure was confusing.

Lohmitt Benzene Ring

However, Lohmitt painted the benzene ring into a circle.

Kekule is an imaginative scholar who has proposed the important doctrine that carbon tetravalent and carbon atoms can be linked into a chain.


For the structure of benzene, he analyzed a large number of experimental facts. He is a very stable “nucleus“, the bond between the six carbon atoms is very strong, and the arrangement is very compact,

It can be connected with other carbon atoms. Aromatic compound. Thus, Kaikole concentrated on studying the “nucleus” of these six carbon atoms.

After proposing a variety of open-chain structures but negating them because of their inconsistency with the experimental results, in 1865 he finally realized that the form of closed chains was the key to solving the molecular structure of benzene.

The story of Kaikule’s understanding of the cyclic structure of benzene molecules has been an anecdote in the history of chemistry.

Kaikule’s Anecdote About Benzene

In 1890, at the conference in Berlin’s City Hall to celebrate the 25th anniversary of Kaikool’s discovery of the benzene ring structure, according to himself, this came from a dream.

That was when he was teaching at Ghent University in Belgium. One night, he was dozing off in the study, and there were rotating carbon atoms in front of him. The long chain of carbon atoms coiled like a snake, and suddenly a snake caught its tail and kept spinning.

He woke up like an electric shock, sorting out the hypothesis of the structure of the benzene ring, and was busy all night. In this regard, Kekule said: “We should dream! Then we can discover the truth, but don’t announce our dreams before the clear sense of reason.”

It should be noted that Kaikool can It is not accidental to be inspired by dreams and successfully propose important structural doctrines.


However, in 1992, John H. Wotiz (1919-2001), a professor of chemistry at the University of Southern Illinois, was in The Mystery of KaikoleThe Challenge to Chemists and Psychologists (The Kekulériddle , a Challenge to Chemists and Psychologists ) questioned the role of Kaikule in the establishment of the benzene ring structure.

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As early as 1854, the French chemist Lauren (Auguste Laurent, 1807-1853) has drawn the molecular structure of benzene into a hexagonal ring structure in the book “Chemical Methods“.

In the file of Kaikool, Watts also found a letter he wrote to the German publisher on July 4, 1854, in which he proposed that he translated Lauren’s book from French to German. Text.

This shows that Kaikole has read and is familiar with Lauren’s book. However, Kekule did not mention Lauren’s study of the structure of the benzene ring in the paper, only the other work of Lauren.

Benzene Material structure

The benzene ring is the simplest aromatic ring. It consists of six carbon atoms and forms a six-membered ring. Each carbon atom is connected to a group. The six groups of benzene are all hydrogen atoms.

However, experiments have shown that benzene does not discolor bromine or acid KMnO4, indicating that there are no carbon-carbon double bonds in benzene.

Studies have shown that the carbon atoms in the main chain of the benzene ring are not arranged by the previously known single bond and a double bond (Kekule), the bond between every two carbon atoms is the same, The double bond is also not a single -key bond (large π bond).

Valence bond

Carbon atoms 4n + 2 (n is a positive integer, i.e., benzene, n = 1), and having a single double bond are alternately arranged structure cycloolefin referred annulene (annulene), a benzene annulene.

The benzene molecule is a planar molecule, 12 atoms are in the same plane, 6 carbons and 6 hydrogens are equal, the CH bond length is 1.08 Α, and the CC bond length is 1.40 Α, which is between the single and double bond lengths.

All bond angles in the molecule are 120°, and the carbon atoms are sp 2 hybridized. There is one p orbit remaining in each carbon atom perpendicular to the plane of the molecule, with one electron in each orbit.


The six orbital overlaps form a large delocalized π bond. The resonance hybridization theory proposed by Linus Pauling believes that benzene possesses a resonance hybrid which is a very stable benzene ring and directly leads to the aromaticity of the benzene ring.

Molecular orbital model

From the molecular orbital theory, it can be considered that the six p orbitals of benzene interact to form six π molecular orbitals, which can be represented by ψ1, ψ2, ψ 3, ψ4, ψ5, and ψ6 respectively. Among them, ψ1, ψ2, and ψ3are the lower energy bonding orbits, and ψ4, ψ5, and ψ6 are the higher energy anti-bond orbits. ψ2 and ψ3, ψ4 and ψ5are two pairs of degenerate orbits.

The electron cloud distribution of benzene in the ground state is the result of the superposition of three bonding orbitals, so the electron cloud is evenly distributed on the upper and lower sides of the benzene ring and on the ring atoms to form a closed electron cloud. It is the source of benzene molecules that generate ring currents in a magnetic field.

Physical properties

Benzene is a colorless, sweet, transparent liquid at room temperature, which has a density lower than that of water and has a strong aromatic odor.

The boiling point of benzene was 80.1 °C and the melting point was 5.5 °C. Benzene has a lower density than water and has a density of 0.88 g/cm3, but its molecular weight is heavier than water.

Benzene is hardly soluble in water. It dissolves up to 1.7g of benzene in 1 liter of water. However, benzene is a good organic solvent. It has a strong ability to dissolve organic molecules and some non- polar inorganic molecules.

In addition to glycerol, glycol and other polyols. The external energy is miscible with most organic solvents. In addition to the slight dissolution of iodine and sulfur, the inorganic substances do not dissolve in benzene.

Benzene and water can form an azeotrope with a boiling point of 69.25 °C and benzene 91.2%. Therefore, benzene distillation is often added to the reaction in which water is formed to carry the water out.

The molar mass was 78.11 g/mol.

Minimum ignition energy: 0.20mJ.

Upper explosion limit (volume fraction): 8.0%.

Lower explosion limit (volume fraction): 1.2%.

The heat of combustion: 3303.08 kJ/mol (25 ° C, gas).

Solubility: Insoluble in water, soluble in most organic solvents such as ethanol, ether, and acetone.

Chemical properties

There are three kinds of chemical reactions involving benzene, one is the substitution reaction between other groups and the hydrogen atom on the benzene ring.

The other is the addition reaction occurring on the benzene ring (Note: the benzene ring has no carbon Double bond, but a unique bond between a single bond and double bond); one is the general combustion (oxidation reaction) (cannot make acid potassium permanganate fade).

Substitution reaction

Hydrogen atoms on the benzene ring may under certain conditions be a halogen, a nitro group, a sulfonic acid group, a hydrocarbon group, and the like substituent to form the corresponding derivatives. Different numbers and structures of isomers can be produced due to differences in substituents and differences in the positions of hydrogen atoms.

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The electron cloud density of the benzene ring is large, so the substitution reaction occurring on the benzene ring is mostly an electrophilic substitution reaction. The electrophilic substitution reaction is a representative reaction of an aromatic ring. When the benzene substituent is electrophilically substituted, the position of the second substituent is related to the type of the original substituent.

Halogenation reaction

The general formula for the halogenation reaction of benzene can be written as:

+ X-PhH 2 – catalyst (FeBr.3 / of Fe) + the HX → PHX

During the reaction, the halogen molecules are split by the action of benzene and a catalyst, X + attacks the benzene ring, and X  is combined with the catalyst.

Taking bromine as an example, liquid bromine is mixed with benzene, bromine is dissolved in benzene to form a reddish-brown liquid, and no reaction occurs. When iron filings are added, bromine reacts with benzene under the catalytic action of iron tribromide formed.

The mixture is slightly boiling, and the reaction is exothermic with reddish brown bromine vapor, and the condensed gas appears white mist (HBr) in the air. Catalytic history:

FeBr3 + Br -——→ FeBr4

PhH + Br + FeBr4 -——→PhBr + FeBr3 + HBr

The reacted mixture was poured into cold water, and a reddish-brown oily liquid (dissolved with bromine) was allowed to settle to the bottom of the water and washed with a dilute alkali solution to obtain colorless liquid bromobenzene.

In the industry, the substitution of chlorine and bromine in halogenated benzene is of the utmost importance.


Benzene and nitric acid in concentrated sulfuric acid may be generated as a catalyst under conditions Nitrobenzene

PhH + HO-NO2 —– H2SO4 (concentrated) △—→ PhNO2 + H2O

The nitration reaction is a strongly exothermic reaction, and it is easy to form a substitute, but the reaction rate is slower. Among them, concentrated sulfuric acid is used as a catalyst, and the reaction is heated to 50-60 degrees Celsius.

If heated to 70-80 ° C, benzene will be sulfonated with sulfuric acid, so the temperature is generally controlled by a water bath heating method. After a nitro group is attached to the benzene ring, the nitro group has an inhibitory effect on the further nitration of benzene, and a nitro group is a passivating group.

Sulfonation reaction

The benzene can be sulfonated to benzenesulfonic acid at a higher (70-80 ° C) temperature with fuming sulfuric acid or concentrated sulfuric acid.

PhH + HO-SO3H——△–→ PhSO3H + H2O

When a sulfonic acid group is introduced into the benzene ring, the reaction ability is lowered, and it is difficult to further sulfonate, and a higher temperature is required to introduce the second and third sulfonic acid groups.

This indicates that both the nitro group and the sulfonic acid group are passivating groups that are groups which hinder the electrophilic substitution again.

Fu-gram reaction

Under the catalysis of AlCl3, benzene can also react with alcohols, olefins and halogenated hydrocarbons, and the hydrogen atom on the benzene ring is substituted with an alkyl group to form an alkylbenzene.

This reaction is referred to as an alkylation reaction, also known as a Friedel-Craft alkylation reaction. For example, alkylation with ethylene to form ethylbenzene

PhH + CH2 = CH2 —-AlCl 3 —→ Ph-CH2CH 3

During the reaction, R groups may undergo rearrangement: for example, 1- chloropropane reacts with benzene to form cumene, since the free radicals tend to be in a stable configuration.

In strong sulfuric acid catalyst, and benzene halide or carboxylic acid anhydride the reaction, the benzene ring hydrogen atoms by an acyl group substituted with phenyl group generated.

The reaction conditions are similar to the alkylation reaction and are referred to as the Friedel-Craft acylation reaction. For example, the reaction of acetyl chloride:

Ph + CH3COCl – AlCl3 – → PhCOCl

Addition reaction

Although the benzene ring is very stable, the addition reaction of the double bond can also occur under certain conditions. Usually, by catalytic hydrogenation, nickel as a catalyst, benzene can form cyclohexane, but the reaction is extremely difficult.

Further, the reaction of producing hexachlorocyclohexane ( hexachlorocyclohexane ) from benzene can be obtained by addition of benzene and chlorine under ultraviolet irradiation. This reaction is an addition reaction of benzene and a radical.

Oxidation reaction

Benzene, like other hydrocarbons, can burn. When oxygen is sufficient, the product is carbon dioxide and water. But when burning in the air, the flame is bright and there is thick black smoke. This is due to the large mass fraction of carbon in benzene.

2C6H6 + 15O2 – Ignition – → 12CO2 + 6H2O

Benzene itself cannot react with the acidic KMnO4 solution, but after the benzene ring is connected with C directly attached to H, the acidic KMnO4 solution can be discolored.

Ozonation Reaction

Benzene can also be oxidized by ozone under certain conditions, and the product is glyoxal. This reaction can be regarded as the ozonation reaction of the cyclic polyene produced after the delocalization of benzene.

Under normal conditions, benzene cannot be oxidized by strong oxidizing agents. However, in the presence of a catalyst such as molybdenum oxide, it reacts with oxygen in the air to selectively oxidize benzene to maleic anhydride.

This is one of the few reactions to the six-membered carbon ring system that destroys benzene. (Maleic anhydride is a five-membered heterocyclic ring.)

This is a strongly exothermic reaction.


When benzene is used as a catalyst at a high temperature using iron, copper or nickel, a condensation reaction can be carried out to form biphenyl. And formaldehyde and hypochlorous acid in the zinc chloride in the presence of chlorine may generate methylbenzene.

The reaction with an alkyl metal compound such as ethyl sodium produces a phenyl metal compound. The phenyl Grignard reagent can be formed by reacting magnesium with tetrahydrofuran, chlorobenzene or bromobenzene.

Benzene does not react with potassium permanganate and fades. When mixed with bromine water, the only extraction occurs. In benzene and its derivatives, only the carbon atom attached to the benzene ring in the substituent on the side chain of the benzene ring is connected to hydrogen.

In this case, potassium permanganate can be discolored (essentially oxidation reaction), and this article is also applicable to aromatic hydrocarbons (if there is an unsaturated bond on the substituent, it must react with potassium permanganate to discolor).

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Note here: 1 only when the carbon atom attached to the benzene ring on the substituent; 2 this carbon atom is connected to the hydrogen atom (bonding).

As for bromine water, benzene and benzene derivatives and saturated aromatic hydrocarbons can only be extracted (provided that there is no unsaturated bond on the substituent, or an addition reaction will still occur).

The treatment of benzene waste gas is also important.


Benzene can be converted to Dewar Benzene under intense light conditions:

The properties of Dewar Benzene are very active (benzene itself is a stable aromatic state with very low energy, and turning into Dewar benzene requires a lot of light energy, so Dewar benzene is very energetic and unstable).

Under the action of the laser, it can be converted into a more active prismatic crystal: the prismatic crystal exhibits a stereoscopic state, which leads to a large mutual exclusion between the π bonds formed by the carbon atom sphybrid orbital, so it is more unstable.