Organic chemistry recognizes benzene (C6H6) as a key aromatic hydrocarbon. While it naturally appears in volcanic emissions, forest fires, and even in small quantities in plants and animals, industries primarily extract it from coal and petroleum.
This colourless liquid is easily identified by its distinct sweet smell. While benzene is indispensable to modern manufacturing, serving as a key building block for countless products, particularly plastics such as polystyrene, its hazardous nature cannot be overlooked.
Benzene is both highly toxic and a known carcinogen; prolonged exposure carries a significant risk, including a strong link to leukaemia. Consequently, stringent safety protocols are paramount whenever handling or utilising this substance.
The journey of benzene's discovery began in 1825 when Michael Faraday first identified it as an illuminating gas.
Nearly a decade later, in 1834, German chemist Eilhard Mitscherlich successfully synthesised benzene by heating benzoic acid with lime. A pivotal moment in the industrial application of benzene arrived in 1845 when another German chemist, A.W. von Hofmann, successfully isolated benzene from coal tar.
Benzene (C₆H₆), isolated by Michael Faraday in 1825, exhibits high unsaturation and unique stability. August Kekulé proposed its cyclic structure in 1865, with six carbon atoms arranged alternately with single and double bonds, each bonded to a hydrogen atom.
The structure suggested two isomeric 1,2-dibromo benzenes, but benzene forms only one ortho-disubstituted product. Kekulé resolved this by proposing oscillating double bonds.
Later, resonance theory explained the unusual stability of benzene and its preference for substitution over addition reactions. Benzene’s stability is also confirmed by its ability to form a triozonide, which indicates the presence of three double bonds.
Benzene is a colourless, highly flammable liquid with a distinct, sweet, aromatic smell. It is less dense than water, with a density of approximately 0.87 g/cm³.
Benzene’s resonance, explained by Valence Bond Theory, is depicted as a hybrid of Kekulé’s structures (A and B), represented by a hexagon with a circle denoting delocalized electrons. All six sp²-hybridized carbon atoms form sigma bonds in a hexagonal plane, with unhybridised p orbitals perpendicular to the ring, enabling electron delocalisation and stability.
X-ray diffraction reveals that benzene’s six C–C bonds are equal in length (139 pm) between a single and double bond. This indicates delocalised π electrons, making benzene more stable than cyclohexatriene. The delocalised electron cloud, formed above and below the plane of the ring, accounts for benzene’s resistance to addition reactions and its unusual stability.
In 1931, Eric Hückel developed a theory to determine which compounds exhibit aromaticity. According to his theory, a compound’s aromatic nature is governed by the electronic structure of its ring system.
The aromaticity of benzene is due to the six delocalized π electrons that are evenly spread across its six carbon atoms, which form a planar hexagonal ring.
In simpler terms:
To check if a ring is aromatic, Huckel gave a simple formula:(4n + 2) π electrons
Where n = ( 1, 2, 3…)
If a compound has (4n + 2) delocalised π electrons, it is aromatic.
If it has 4n π electrons, it is not aromatic.
Examples:
Benzene (C6H6) is an aromatic hydrocarbon renowned for its high stability, attributed to its resonance. This resonance stability makes benzene behave differently from typical unsaturated compounds, such as alkenes.
It predominantly undergoes electrophilic substitution reactions rather than addition reactions (which would disrupt its aromaticity). Electrophilic substitution reactions preserve the aromatic ring, which is why they are preferred.
Major Electrophilic Substitution Reactions of Benzene
When benzene is monosubstituted (i.e., it has one substituent group), the nature of that group significantly influences where a new substituent enters during electrophilic aromatic substitution reactions like nitration, halogenation, sulfonation, or Friedel-Crafts reactions.
Substituents on the benzene ring effect:
Activating and Deactivating Groups
Activating groups increase the reactivity of the benzene ring by donating electrons (usually via resonance or induction).
Deactivating groups decrease reactivity by withdrawing electrons.
Benzene is a key industrial chemical used in the production of various materials. However, because it is toxic and potentially cancer-causing, its usage is now strictly controlled and monitored.
Industrial Uses of Benzene:
While useful, benzene is highly toxic, especially with prolonged or high exposure.
Acute Exposure Effects:
Chronic Exposure Risks:
(Session 2025 - 26)