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BENZENE

 

IUPAC NAME: BENZENE

MOLECULAR FORMULA:

MOLAR MASS: 78.11g/mol

BOILING POINT: 176.2°F

MELTING POINT: 41.9°F

Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a ring with one hydrogen atom attached to each. As it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon.

Benzene is one of the most fundamental compounds used in the manufacturing of various plastics, resins, synthetic fibers, rubber lubricants, dyes, detergents, drugs, and pesticides. Naturally, benzene is produced through volcanoes and forest fires. There are various laboratories and industrial techniques existing for the preparation of benzene. Commercially, benzene is chiefly obtained from coal tar. Some of the laboratory techniques for the preparation of benzene are discussed below:

 

Preparation of benzene from alkynes

Benzene is prepared from ethyne by the process of cyclic polymerization. In this process, Ethyne is passed through a red-hot iron tube at 873 K. The ethyne molecule then undergoes cyclic polymerization to form benzene.

Preparation of benzene from aromatic acids

Benzene can be prepared from aromatic acids through decarboxylation reaction. In this process, sodium salt of the benzoic acid (sodium benzoate) is heated with soda lime to produce benzene along with sodium carbonate.

Preparation of benzene from phenol

Benzene can be prepared from phenols too through their reduction. In this process vapors of phenol are passed over heated zinc dust. Zinc dust reduces them to form benzene.

Preparation of benzene from sulphonic acids

Benzene can be prepared from sulphonic acids through their hydrolysis. In this process Benzene sulphonic acid is exposed to superheated steam leading to the formation of benzene.

C₆H₅-SO₃H + H₂O → C₆H₆ + H₂SO₄

 

Discussion of Physical/ Chemical property

• Acidity/ Basicity

According to Lewis theory of acids and bases, an acid accepts a pair of electrons and base donates the pair electrons, so benzene is a base because it donates a pair of electrons.

• Nucleophilic/ electrophilic sites

Benzene is a nucleophile because of its delocalized electrons. The molecule has electron rich areas which allow it to donate them to electrophiles.

 

• Chirality

Even first condition isn’t satisfied in benzene, there are no two groups joined to any carbon in benzene thus no stereocenter. Hence, no stereocenter is equivalent to achiral benzene. Also, benzene has multiple mirror planes.

 

• Intermolecular/ Intramolecular forces

Intermolecular forces are much weaker than the intramolecular forces of attraction but are important because they determine the physical properties of molecules like their boiling point, melting point, density, and enthalpies of fusion and vaporization. Benzene possess intermolecular force.

 

• Label functional group

A benzene ring with a single substituent is called a phenyl group (Ph)

 

 

Detailed Interesting Physical Property of Benzene

• Benzene appears as a clear colorless liquid with a petroleum-like odor. Flash point less than 0°F. Less dense than water and slightly soluble inwater therefore floats on  Vapors are heavier than air. (PubChem)
• It has aromatic odor- similar to gasoline which is an odor rather than pleasant aromatic odor.

• Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5°C, Melting point: 5.5°C)
• Benzene shows resonance.
• It is highly inflammable (auto-ignition) and burns with a sooty flame. It is stable if followed recommended storage condition and instruction.

 

Uses of Benzene

• Benzene is used mainly as an intermediate to make other chemicals, above all ethylbenzene, cumene, cyclohexane, nitrobenzene, and alkylbenzene.
• Benzene is used in very large quantities to make other chemicals that are used to make plastics, resins,nylon and synthetic fibers. (PB)
• It is used to make some types of rubbers, lubricants, dyes, detergents, drugs, and pesticides. It is also used in paints, glues, and furniture wax. (PB)

Interesting Facts

 

 

• In 2013, the biggest consumer country of benzene was China, followed by the USA. Benzene production is currently expanding in the Middle East and in Africa, whereas production capacities in Western Europe and North America are stagnating.
• In 1903, a German coffee merchant, Ludwig Roselius, used benzene to decaffeinate coffee. Since we’ve come to learn that benzene is carcinogenic, this use for benzene is thankfully no longer the case.
• In the late 19th and early 20th centuries, benzene was actually used as an ingredient for after shave because of its aroma. However, much like coffee decaffeination, it is no longer used for this purpose.
• Severe exposure to benzene can prevent cells from working properly. It can, for example, cause bone marrow to stop producing enough red blood cells and whereas long term exposure could lead to blood-related cancer like leukemia. It can enter the body either through direct absorption via the skin, through inhalation, or consumption.

History of Benzene

Benzene was first discovered by the English scientist Michael Faraday in 1825 in illuminating gas. In 1834 German chemist Eilhardt Mitscherlich heated benzoic acid with lime and produced benzene. In 1845 German chemist A.W. von Hofmann isolated benzene from coal tar. The structure of benzene has been of interest since its discovery. German chemists Joseph Loschmidt (in 1861) and August Kekule von Stradonitz (in 1866) independently proposed a cyclic arrangement of six carbons with alternating single and double bonds. Kekule subsequently modified his structural formula to one in which oscillation of the double bonds gave two equivalent structures in rapid equilibrium. In 1931 American chemist Linus Pauling suggested that benzene had a single structure, which was a resonance hybrid of the two Kekule structures.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Citations

https://byjus.com/chemistry/preparation-of-benzene/ (Whole synthesis)

https://pubchem.ncbi.nlm.nih.gov/compound/Benzene#section=Experimental-Properties

https://interestingengineering.com/what-is-benzene-and-why-is-it-so-useful (facts 2 on)

https://www.britannica.com/science/benzene (History)