Benzene, is a fundamental aromatic organic compound. It is used to produce various compounds such as polystyrene, phenol, aniline, and detergents. It is primarily derived from coal and oil. There are several methods for the preparation of Benzene such as the decarboxylation of benzoic acid, the reaction of diazonium compounds with hypophosphorus acid, etc. In this article, we look into what Benzene is, preparation with mechanism , properties, Resonance and aromaticity of Benzene, etc.

What is Benzene?

Benzene is a fundamental organic compound with the chemical formula C₆H₆. It is the simplest aromatic hydrocarbon and a natural constituent of crude oil. It is a colorless and highly flammable liquid with a sweet smell. It is the simplest organic, aromatic hydrocarbon and parent compound of important aromatic compounds.

It has a 1:1 carbon-to-hydrogen ratio in the empirical formula and low chemical reactivity. It is also a natural part of crude oil, gasoline, and cigarette smoke and is widely used in producing various chemicals, plastics, resins, and synthetic fibers. However, it is highly toxic and a known carcinogen, with exposure capable of causing leukemia.

Structure of Benzene

Benzene is a planar molecule composed of six carbon atoms joined in a hexagonal ring with one hydrogen atom attached to each carbon atom. The carbon atoms in the benzene ring are sp2-hybridized, and the overlap of the sp2 orbitals around the ring produces a framework of six sigma bonds, while the unhybridized p-orbitals, which are perpendicular to this plane overlap in a side-to-side manner to form three delocalized π orbitals spanning the ring.

In this structure of Benzene, the double bonds are mainly separated by a single bond. This arrangement is recognized to have conjugated double bonds. Due to the delocalized nature of the bonding, Benzene is often depicted with a circle inside a hexagonal arrangement of carbon atoms. This circle represents the delocalized electrons shared by all six carbon atoms in the ring, making Benzene exceptionally stable.

Preparation of Benzene

Benzene can be prepared through several methods. Some standard laboratory techniques for the preparation of Benzene are discussed below:

Preparation of Benzene From Aromatic Acids

Benzene can be prepared from aromatic acids through a decarboxylation reaction. In this process, the sodium salt of benzoic acid (sodium benzoate) is heated with soda lime to produce benzene and sodium carbonate. This method is commonly used in laboratories for the preparation of benzene.

(C₆H₅COO-Na⁺) + NaOH → C₆H₆ + Na₂CO₃

Preparation of Benzene From Aryl Halides

Benzene can be prepared through various methods, including halogenation of benzene and the Sandmeyer reaction. The halogenation of benzene to synthesize aryl halides is a well-known method. These aryl halides can be transformed to aryl radicals via radical anions upon reaction with a single electron donor.

Chlorobenzene + 2[H] ––(Ni-Alloy/NaOH)––› Benzene + HCl

Preparation of Benzene From Alkanes via Dehydrogenation

Benzene can be synthesized from alkanes through a process called dehydrogenation, which involves the removal of hydrogen atoms from the alkane to form a more unsaturated hydrocarbon. The alkane of choice is typically paraffin (alkane with straight chains), such as n-hexane or cyclohexane. The dehydrogenation process is generally carried out at high temperatures (around 500-800°C) and in the presence of a catalyst, such as platinum, palladium, or nickel, on an alumina support. The reaction can be represented as follows:

C_6​H₁₂ ​ → C₆​H_6​ + 3H₂

​This method is not widely used for the industrial production of benzene due to the high cost of the catalysts and the need for high temperatures, which can lead to side reactions and the formation of undesired products.

Preparation of Benzene From Alkynes via Hydrogenation

Benzene can also be synthesized from alkynes through hydrogenation, which involves the addition of hydrogen atoms to the triple bond of the alkyne to form a double bond. The most common alkyne used for this purpose is acetylene (C₂H₂). The hydrogenation process is typically carried out at high pressures (around 100-300 atm) and in the presence of a catalyst, such as nickel, palladium, or platinum, on an alumina support. The reaction can be represented as follows:

C₂​H_2​ + H₂​ → C₆​H_6​

Preparation of Benzene From Cyclohexane via Dehydrogenation

Benzene can be prepared from cyclohexane through a dehydrogenation process. However, this method is not as widely used as other methods due to the lower availability of cyclohexane, the higher cost of the catalysts and the need for high temperatures, which can lead to side reactions and the formation of undesired products. The dehydrogenation of cyclohexane to form benzene can be summarized as follows:

C₆​H₁₂ ​→ C₆​H₆ ​+ 3H_2​

This process typically involves high temperatures (around 500-800°C) and a catalyst on an alumina support, such as platinum, palladium, or nickel.

Laboratory Synthesis of Benzene

In the laboratory, benzene can be prepared through several methods. Some of the standard laboratory techniques for the preparation of benzene are discussed below:

Synthesis of Benzene from Ethynes by Cyclic Polymerization

One of the preparation method of benzene is through the cyclic polymerization of ethyne, where ethyne is passed through a red-hot iron tube at 873 K, and the ethyne molecule undergoes cyclic polymerization to form benzene.

3C₂H₂ –––– (red hot tube /873K, catalyst)–––› C₆H₆

Synthesis of Benzene from Sulphonic Acids

Benzene can be prepared from sulphonic acids through hydrolysis, specifically by exposing benzene sulfonate (C₆H₅-SO₃H) to superheated steam. This process leads to benzene and sulfuric acid (H₂SO₄) formation. The hydrolysis of sulfonates is not a standard laboratory method for benzene production, but it is mentioned as an alternative to other more common methods, such as the decarboxylation of aromatic acids or the reduction of phenols.

C₆H₅-SO₃H + H₂O → C₆H₆ + H₂SO₄

Synthesis of Benzene from Phenol

Benzene can be prepared from phenols through a reduction process. In this method, phenol vapors are passed over heated zinc dust, which reduces the phenol to form benzene. This laboratory technique is not commonly used for large-scale production but helps demonstrate the reduction of phenols to benzene

Phenol + Zn –––––› Benzene + ZnO

Industrial Production of Benzene

The industrial production of benzene primarily relies on petroleum and coal via catalytic reforming, steam cracking, and toluene disproportionation processes. These methods account for most global benzene production, with catalytic reforming being the most common, contributing to about 50% of the worldwide output.

Steam cracking process

It is a petrochemical process that breaks down saturated hydrocarbons into smaller, often unsaturated hydrocarbons, primarily ethylene and propylene. The process occurs in steam-cracking furnaces at high temperatures (around 850°C) and very short residence times (milliseconds). Steam cracking of hydrocarbons, such as naphtha or ethane, produces ethylene, propylene, and other lighter hydrocarbons.

Steam cracking is less common for benzene due to its lower reactivity than other hydrocarbons. However, it can be coproduced with ethylene by steam-cracking refinery sources that contain benzene, such as naphtha.

Key points for steam cracking of benzene:

• Benzene is less reactive than other hydrocarbons in steam cracking due to its lower reactivity.
• The process results in the slow deposition of coke, which must be minimized to maintain furnace efficiency.
• The workup and separation of the different fractions and products are complex.
• Undesired fractions like alkanes can be recycled to the furnace, and fractions like methane can be used to fire the furnaces.
• Pygas can be blended with other hydrocarbons as a gasoline additive or distilled to separate BTX (benzene, toluene, xylene).

Refining process

In the refining process, benzene is separated from the reformate by solvent extraction techniques, and it can also be prepared by cracking, a multistep process where crude oil is heated, steam is added, and the mixture is then briefly passed through a furnace at temperatures of 700–900°C. Benzene reduction in gasoline is a current focus in the refining industry, with many refiners adjusting the C₆ content of the naphtha feed to their reformer by prefractionation or installing hydrogenation facilities.

Benzene production in petrochemical industries

More than 98% of the benzene produced in the United States is derived from the petrochemical and petroleum refining industries.

• Some chemical processes contribute to industrial benzene production: toluene hydrodealkylation, catalytic reforming, steam cracking and toluene disproportionation.
• Catalytic reforming is the most common method, accounting for about 50% of global production.
• In catalytic reforming, a mixture of hydrocarbons with boiling points between 60 and 200°C is treated with hydrogen gas and then exposed to a catalyst at high temperatures and pressures.
• Toluene hydrodealkylation and toluene disproportionation processes are used to produce benzene from toluene.
• Steam cracking is less common for benzene production due to its lower reactivity than other hydrocarbons. Still, it can be coproduced with ethylene by steam-cracking refinery sources that contain benzene, such as naphtha.
• Benzene is separated from the reformate by solvent extraction techniques in refining.

Properties of Benzene

The properties of Benzene can be given as follows:

• Benzene has a planar hexagonal structure with all carbon atoms being sp² hybridized, and all the carbon-carbon bonds are of equal length.

• It is immiscible in water but soluble in organic solvents and has a density of 0.87 g/cm³, making it lighter than water.

• Benzene has a boiling point of 80.5°C and a melting point of 5.5°C.

• Benzene is highly inflammable and burns with a sooty flame.

• Benzene molecule comprises six carbon atoms joined in a planar hexagonal ring with one hydrogen atom attached to each carbon atom, making it a hydrocarbon.

• Benzene is highly toxic in nature and can lead to unconsciousness if inhaled.

Application of benzene

The applications of benzene can be summarized as follows:

• Production: Benzene is primarily used to produce polystyrene, plastics, synthetic fibers, and rubber.
• Solvent: It is widely used as a solvent in various industrial processes, such as in the manufacturing of adhesives, cleaning agents, and asphalt.
• Chemical Intermediates: Benzene produces detergents, pesticides, insecticides, herbicides, and dyes.
• Fuel: It forms a significant part of gasoline and is used as a fuel.
• Other Uses: Benzene is found in products used in the printing industry, including inks, spray paints, sealers, lacquers, and stains.