Benzene (C6H6) - Definition, Discovery, Structure, FAQs

Q: What is the chemical name of C6H6?

C 6 H 6 chemical name is benzene.

Benzene (C6H6) - Definition, Discovery, Structure, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:57 PM IST

Benzene is the parent member of the hydrocarbon family which is composed of a highly unique unsaturated structural unit C₆H₆. C₆H₆ is a chemical formula of Benzene.

Discovery of Benzene:

The majority of hydrocarbons possessed a sweet and pleasant odor and hence they were named aromatic compounds. Benzene was discovered by Michael Faraday of the Royal Institution Back in 1825.

What is benzene?

Michael Faraday was the first person to come up with the name “bicur-buret of hydrogen” for this aromatic compound which is now known as Benzene. Michael was successful in isolating benzene from the compressed gas of whale oil. In the wake of Benzene's curiosity in 1834, Eilhardt Mitscherlick heated benzoic acid with calcium oxide which led to the successful synthesis of benzene.

C₆H₆COOH+CaO ∆→ C₆H₆+CaCO₃

The Molecular formula of benzene was quite shocking at that time because this was for the first time when a hydrocarbon was discovered with an index of hydrogen deficiency of. Whereas, the other compounds had twice the number of Hydrogen. This meant that C₆H₆ would be a highly unsaturated compound. However, the properties of benzene were highly unexpected as it was expected to show properties of unsaturated compounds.

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C₆H₆Structure:

In 1865, August Kekulé was the first person to put forward the structure of benzene which satisfied the four bonds of Carbon and one bond of hydrogen. The structure has six carbons and six hydrogens bonded through alternating single and double bonds.

In the beginning, Kekulé suggested that the two forms of benzene are in resonance with each other and this equilibrium is established so rapidly that it is not possible to separate the two forms. However, now we know that no such equilibrium exists. The unique stability of Benzene is due to aromaticity. Even though the compound is saturated it gives a substitution reaction in place of addition reactions.

Stability of Benzene-

Resonance in benzene is a major reason for the high stability of benzene.

For an unsaturated compound, it is quite unexpected to not show any addition reactions. Benzene is a compound which undergoes substitution reactions but does not undergo addition reactions.

Cyclohexene cyclohexane ∆H= - 119.5 kJmol^-1

Cyclohexatriene cyclohexane ∆H= -119.5 ✕ 3= -358.5 kJmol^-1

Experimental value of enthalpy of hydrogenation of benzene

Benzene cyclohexane ∆H= -208.1 kJmol^-1

Enthalpy of hydrogenation for Cyclohexene having one single bond is 119.5 kJ mol^-1. For benzene, the enthalpy of hydrogenation containing three double bonds should be three times the enthalpy of hydrogenation of cyclohexene i.e -358.5 kJ mol^-1

However, experimental value for enthalpy of hydrogenation of benzene is -208.1 kJ mol^-1

This means that benzene is more stable than the hypothetical cyclohexene molecule by 150.4 kJ mol^-1. This value depicts the Resonance energy in Benzene. The difference between the energy of hypothetical cyclohexatriene and that of benzene is called Resonance energy of benzene.

Kekulé structure of benzene:

The stability of benzene can be explained by the resonance hybrid structure of benzene. Number of resonating structures of Benzene are two and one resonance hybrid of the two resonating structures. Actual Benzene molecule is comparatively more stable than either of the resonating structures given by kekule.

Kekulé structures of benzene Resonance hybrid

It is the resonance structure of benzene.

Bond Length of benzene C-C bond-

All C-C bonds in benzene are equal in length due to resonance. Bond length of the carbon-carbon single bond is 139 pm and this value is the same for all the bonds. ∏ e^-1 charge density is spread over a large area which leads to delocalization. Delocalization leads to a decrease in energy. This resonance energy stabilizes the benzene ring.

If the two resonating structures of the benzene truly existed then there would be two isomers of 1,2-dichlorobenzene. However, it is not true. Only one such structure exists, double bonds keep switching between C=C double bonds and hence only one product is formed.

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Orbital picture of benzene:

According to molecular Orbital theory of benzene, all the bond angles of carbon atoms in the benzene ring are 180⁰ which indicates the sp² hybridization of all the six carbon atoms. All the carbon atoms are sp² hybridized except one orbital which remains unhybridized. The six electrons of the pi bond give the electronic configuration of the ground state.

Out of the three orbitals of carbon, two orbital overlap axially with neighbouring carbon atoms to form a sigma bond and the third hybrid orbital of carbon overlaps with the orbital of hydrogen atom to form carbon and hydrogen sigma bonds. The unhybridized orbitals on each of the carbon atoms form a pi bond through sidewise overlapping. There is a pi cloud on the benzene ring due to continuous rings above and below the carbon atoms .

Fig1 -Unhybridized 2p orbital of benzene ring

Fig2 -Sidewise overlap of the Unhybridized p orbitals of benzene ring

Following are some of the Physical Characteristics of Benzene-

Smell of benzene	Aromatic
Density of benzene	0.87 gm/cm³
Color	Colorless
Boiling Point	80.1⁰C
Point group	D6h
Flash Point	12⁰F

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Chemistry of Benzene and its Derivatives-

Chlorobenzene-

Chlorobenzene can be synthesized from benzene in the presence of a Lewis acid such as AlCl₃ or ferric halide at a temperature between 310-320 K

Benzene Sulphonic acid-

-SO₃H group can be substituted with the hydrogen atom via Sulphonation. Benzene is heated with sulfuric acid at 330K.

Acetophenone-

A very famous method to prepare acetophenone is by treating benzene with ethanoyl chloride or ethanoic anhydride in the presence of lewis acid. This reaction is famously known as Friedel Craft Acylation.

Nitrobenzene-

Substituting the Hydrogen atom with the nitro (NO₂) group in the presence of sulphuric acid yields nitrobenzene. Attacking species is an electrophile NO⁺₂

Toluene-

Benzene with CH₃is known as toluene. When an alkyl halide reacts with Benzene in the presence of Lewis acid such as anhydrous aluminium chloride toluene is formed and this reaction is famously known as Friedel- Craft alkylation.

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Polynuclear Hydrocarbons-

Hydrocarbons containing two or more than two benzene rings are known as polycyclic aromatic compounds. Some common examples of such compounds are anthracene, phenanthrene, naphthalene. Most of the polynuclear hydrocarbons are carcinogenic in nature and are also toxic.

Anthracene

Napthalene

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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Is Benzene an aromatic compound?

In order for an aromatic compound to be aromatic it has to fulfil the certain criteria-

Planarity- Molecules should be planar so that delocalization of pi electron density can occur. If the ring of the compound is planar only then this delocalization can occur.
Delocalization - Molecules must contain a cyclic pi electron cloud above and below the ring which is formed by the overlap of p orbitals. This ensures the delocalization of electron density.
Huckel rule- The molecule must contain (4n+2)∏ electrons where n= 0,1,2,3.. so on.

Benzene is a planar molecule which has delocalized electron density and also obeys Huckel’s rule as it contains a total of 6∏ electrons where n=1

2. Is Benzene a planar molecule?

From the orbital picture of Benzene the carbon atoms of the benzene ring are sp² hybridized and one p orbital is unhybridized. Two hybridized orbitals form a sigma bond with the neighbouring hybridized orbital of carbon atoms and one hybrid orbital of carbon atom overlaps with the s orbital of Hydrogen atoms. As a result there is a pi electron cloud spread over the six carbon atoms.

This makes benzene a plane and flat molecule and electron density is delocalized over a larger area which increases the stability of the benzene ring.

3. What is base resonance in benzoic acid?

Conjugate base of benzoic acid is benzoate.

4. What is the chemical name of C6H6?

C₆H₆ chemical name is benzene.

5. Why is benzene's structure considered unique compared to other hydrocarbons?

Benzene's structure is unique because it consists of a planar, hexagonal ring of six carbon atoms with delocalized electrons. Unlike typical alkenes, benzene's double bonds don't behave as isolated double bonds. Instead, the electrons are spread evenly around the ring, giving benzene enhanced stability and distinct chemical properties.

6. How does the structure of benzene contribute to its stability?

Benzene's stability comes from its delocalized π electrons. The six π electrons are spread evenly around the ring, creating a cloud of electrons above and below the plane of the molecule. This electron delocalization lowers the overall energy of the molecule, making it more stable than a hypothetical cyclohexatriene structure with localized double bonds.

7. What is resonance, and how does it apply to benzene?

Resonance is a concept in chemistry where a molecule's structure is best described as a hybrid of multiple valid Lewis structures. In benzene, two equivalent resonance structures with alternating single and double bonds contribute equally to the actual structure. This resonance hybrid better represents benzene's true electronic distribution and explains its enhanced stability.

8. Why is benzene represented by a circle inside a hexagon?

The circle inside a hexagon is a simplified representation of benzene that emphasizes its aromatic nature and electron delocalization. This symbol, introduced by Sir Robert Robinson in 1925, conveys that the six π electrons are evenly distributed around the ring rather than existing as three fixed double bonds.

9. What is aromaticity and how does it relate to benzene?

Aromaticity is a property of cyclic, planar molecules with delocalized electrons that follow Hückel's rule (4n+2 π electrons). Benzene is the quintessential aromatic compound, exhibiting enhanced stability, a tendency to undergo substitution rather than addition reactions, and a characteristic ring current in NMR spectroscopy.

10. What is benzene and why is it important in organic chemistry?

Benzene (C6H6) is a cyclic hydrocarbon with a unique ring structure. It's important in organic chemistry because it's the simplest aromatic compound and serves as the basis for many other aromatic molecules. Benzene's stability and reactivity patterns have led to the development of aromatic chemistry, a crucial area in organic synthesis and materials science.

11. How was benzene discovered and who is credited with its discovery?

Benzene was discovered in 1825 by Michael Faraday, who isolated it from oil gas. However, its structure remained a mystery until 1865 when Friedrich August Kekulé proposed the cyclic structure with alternating single and double bonds. Kekulé's work laid the foundation for understanding aromatic compounds.

12. How does benzene's structure affect its reactivity?

Benzene's aromatic structure makes it less reactive than typical alkenes. It tends to undergo electrophilic aromatic substitution reactions rather than addition reactions. This is because substitution reactions preserve the aromatic system, while addition reactions would disrupt the stable aromatic π electron cloud.

13. How does benzene's structure explain its unusually low heat of hydrogenation?

Benzene's low heat of hydrogenation is due to its aromatic stability. The heat released when hydrogenating benzene to cyclohexane is less than expected for three double bonds because benzene is more stable than a hypothetical cyclohexatriene. This difference in energy is called the resonance energy or aromatic stabilization energy.

14. How does benzene's structure affect its physical properties?

Benzene's planar, symmetrical structure and delocalized electrons influence its physical properties. It has a relatively high boiling point for its molecular weight due to stronger intermolecular forces (π-π stacking). Its planarity and symmetry also contribute to its low dipole moment, making it a non-polar solvent.

15. What is Hückel's rule, and how does benzene satisfy it?

Hückel's rule states that planar, cyclic molecules with (4n+2) π electrons (where n is a non-negative integer) are aromatic. Benzene has 6 π electrons (4(1)+2), satisfying this rule for n=1. This explains benzene's aromatic stability and serves as a criterion for predicting aromaticity in other cyclic compounds.

16. How does benzene's structure relate to its UV-Vis spectrum?

Benzene's UV-Vis spectrum shows strong absorption bands due to π→π* transitions of its conjugated π system. The main absorption band at around 255 nm is characteristic of aromatic compounds. This spectral feature is useful for identifying aromatic structures and studying electronic transitions in conjugated systems.

17. What is the relationship between benzene's structure and its NMR spectrum?

In benzene's 1H NMR spectrum, all hydrogen atoms appear as a single peak due to their chemical equivalence. The 13C NMR shows a single peak for all carbon atoms. The aromatic ring current causes these protons to be more deshielded than expected, resulting in a characteristic downfield shift (around 7.3 ppm in 1H NMR).

18. How does benzene's structure contribute to its ability to conduct electricity?

While benzene itself is not a good conductor, its delocalized π electron system makes it a semiconductor. This property is crucial in organic electronics, where benzene rings are often incorporated into larger conjugated systems to create materials with tunable electronic properties.

19. How does the concept of aromaticity extend beyond benzene to other compounds?

While benzene is the prototypical aromatic compound, aromaticity extends to many other molecules. Heterocyclic compounds like pyridine and furan, polycyclic aromatics like naphthalene, and even some inorganic compounds can exhibit aromaticity. The key requirements are a cyclic, planar structure with (4n+2) π electrons.

20. How does the concept of benzene extend to more complex aromatic systems?

The concept of benzene extends to more complex aromatic systems through the idea of fused rings and extended conjugation. Polycyclic aromatic hydrocarbons (PAHs) like naphthalene and anthracene consist of multiple fused benzene rings. These compounds retain many of benzene's characteristics but can exhibit unique properties based on their extended π systems.

21. How does benzene's structure influence its infrared (IR) spectrum?

Benzene's IR spectrum shows characteristic bands due to its symmetric structure. The C-H stretching vibrations appear around 3030 cm^-1, while ring vibrations produce bands at about 1500 and 1600 cm^-1. The pattern of these bands, especially in the fingerprint region, is useful for identifying aromatic compounds.

22. How does benzene's structure influence its behavior in mass spectrometry?

In mass spectrometry, benzene's molecular ion (M+) at m/z 78 is relatively stable due to its aromatic structure. The fragmentation pattern often shows peaks at m/z 52 (C4H4+) and m/z 39 (C3H3+), which are characteristic of aromatic compounds. This stability and fragmentation pattern help in identifying benzene and its derivatives.

23. What is the significance of benzene's π electron count in relation to other aromatic systems?

Benzene's six π electrons set the standard for aromatic systems. According to Hückel's rule, cyclic compounds with (4n+2) π electrons are aromatic. Benzene represents the case where n=1. This concept extends to other aromatic systems: cyclopentadienyl anion (6π), pyridine (6π), and larger systems like [18]annulene (18π, n=4).

24. How does benzene's structure influence its behavior in photochemical reactions?

Benzene's aromatic structure affects its photochemistry. Upon UV irradiation, benzene can undergo electrocyclic reactions, forming Dewar benzene or benzvalene. These photoproducts are less stable than benzene and can revert back. Understanding these processes is important in studying the photostability of aromatic compounds and in developing photochemical synthetic methods.

25. How does the concept of benzene extend to organometallic chemistry?

Benzene's structure extends to organometallic chemistry through compounds like bis(benzene)chromium, where benzene acts as a ligand. The aromaticity and π electron system of benzene allow it to bond to metals in a η6 (eta-6) fashion, where all six carbon atoms interact with the metal. This has led to the development of sandwich compounds and other metal-arene complexes with unique properties.

26. How does benzene's structure relate to its role in environmental chemistry?

Benzene's stable aromatic structure contributes to its persistence in the environment. It's a common pollutant due to its use in industry and its presence in petroleum products. The study of benzene's environmental fate, including its volatilization, biodegradation, and potential for bioaccumulation, is crucial for assessing environmental risks and developing remediation strategies.

27. What is Kekulé's dream, and how does it relate to benzene's structure?

Kekulé's dream refers to the story of how Friedrich August Kekulé supposedly conceived the cyclic structure of benzene. He claimed to have dreamt of a snake biting its own tail, which inspired him to propose the ring structure. While the veracity of this story is debated, it's often used to illustrate the creative process in scientific discovery.

28. What is the hybridization of carbon atoms in benzene?

The carbon atoms in benzene are sp2 hybridized. This hybridization results in trigonal planar geometry around each carbon atom, with bond angles of approximately 120°. The remaining p orbital on each carbon atom overlaps to form the delocalized π system above and below the ring plane.

29. What is the bond length between carbon atoms in benzene, and why is this significant?

The carbon-carbon bond length in benzene is approximately 1.39 Å, which is intermediate between typical single (1.54 Å) and double (1.34 Å) bond lengths. This uniform bond length supports the concept of electron delocalization and resonance in benzene, as all carbon-carbon bonds are equivalent.

30. How does the molecular orbital theory explain benzene's structure?

Molecular orbital theory describes benzene as having six π molecular orbitals formed from the overlap of p orbitals. Three of these are bonding orbitals (occupied) and three are antibonding (unoccupied). The electrons in the highest occupied molecular orbital (HOMO) are delocalized over all six carbon atoms, contributing to benzene's stability and aromatic character.

31. How does benzene's structure influence its reactivity in electrophilic aromatic substitution?

Benzene's aromatic structure makes it prone to electrophilic aromatic substitution reactions. The delocalized π electrons can attack electrophiles, forming a resonance-stabilized carbocation intermediate (arenium ion). This process preserves the aromatic system, making substitution more favorable than addition reactions.

32. What is the significance of benzene's D6h symmetry?

Benzene's D6h symmetry refers to its high degree of molecular symmetry, including a 6-fold rotational axis and six mirror planes. This symmetry contributes to benzene's unique properties, such as its lack of dipole moment, equivalent carbon atoms, and uniform reactivity around the ring.

33. What is the significance of benzene as a parent compound in organic chemistry nomenclature?

Benzene serves as the parent compound for naming many aromatic derivatives. Substituents on the benzene ring are named using prefixes, and their positions are indicated by numbers. This system forms the basis for naming more complex aromatic compounds, emphasizing benzene's fundamental role in organic chemistry.

34. What is the relationship between benzene's structure and its reactivity towards oxidation?

Benzene is relatively resistant to oxidation due to its aromatic stability. Unlike alkenes, which readily undergo oxidation, benzene requires harsh conditions (like high temperature and strong oxidizing agents) to break its aromatic system. This stability contributes to benzene's persistence in the environment.

35. How does benzene's structure influence its role as a solvent?

Benzene's planar, non-polar structure makes it an excellent solvent for non-polar organic compounds. Its ability to engage in π-π interactions also allows it to dissolve some polar aromatic compounds. However, its use as a solvent is limited due to its carcinogenicity, and safer alternatives are preferred in most applications.

36. What is the significance of benzene's resonance energy?

Benzene's resonance energy, also known as aromatic stabilization energy, is the additional stability gained from electron delocalization. It's quantified as the difference between benzene's actual heat of hydrogenation and the theoretical value for three isolated double bonds. This energy (about 36 kcal/mol) explains benzene's unique stability and reactivity.

37. What is the significance of benzene's planarity?

Benzene's planarity is crucial for its aromatic character. The planar structure allows for maximum overlap of p orbitals, creating the delocalized π electron system. This planarity also influences benzene's ability to participate in π-π stacking interactions, which are important in crystal packing and some biological processes.

38. How does benzene's structure relate to its carcinogenicity?

Benzene's carcinogenicity is linked to its metabolism in the body. The aromatic structure can be oxidized to form reactive epoxide intermediates, which can bind to DNA and proteins, leading to mutations and cellular damage. Understanding this process has been crucial in developing safety protocols for handling benzene and related compounds.

39. What is the significance of benzene's bond angles?

Benzene's bond angles are approximately 120°, consistent with sp2 hybridization of the carbon atoms. This geometry is crucial for the planarity of the molecule and the effective overlap of p orbitals to form the delocalized π system. The 120° angles also contribute to benzene's hexagonal symmetry.

40. What is the relationship between benzene's structure and its dipole moment?

Benzene has a dipole moment of zero due to its highly symmetrical structure. The six C-H bond dipoles cancel each other out, resulting in a non-polar molecule. This property influences benzene's solubility, boiling point, and its behavior in electric fields.

41. How does the concept of benzene relate to the development of molecular orbital theory?

Benzene played a crucial role in the development of molecular orbital theory. The inability of simple valence bond theory to fully explain benzene's properties led to the refinement of molecular orbital concepts. Hückel's molecular orbital treatment of benzene was a significant step in understanding aromatic systems and conjugated π electrons.

42. What is the significance of benzene's high C:H ratio?

Benzene's formula (C6H6) gives it a high carbon-to-hydrogen ratio compared to saturated hydrocarbons. This high C:H ratio contributes to benzene's tendency to burn with a sooty flame, a characteristic of many aromatic compounds. It also relates to benzene's relatively high density and its potential as a precursor for carbon-rich materials.

43. What is the relationship between benzene's structure and its role in organic synthesis?

Benzene's unique structure makes it a versatile starting material in organic synthesis. Its aromatic system can be functionalized through various reactions (e.g., electrophilic aromatic substitution, nucleophilic aromatic substitution), allowing the creation of diverse benzene derivatives. These derivatives serve as building blocks for more complex molecules in pharmaceuticals, materials science, and other fields.

44. How does the concept of benzene relate to the development of aromaticity indices?

The study of benzene led to the development of various aromaticity indices, which attempt to quantify the degree of aromaticity in molecules. These include geometric indices (like HOMA), electronic indices (like NICS), and energetic indices (like ASE). These tools help chemists understand and predict the behavior of aromatic systems beyond benzene.

45. What is the relationship between benzene's structure and its role in supramolecular chemistry?

Benzene's planar, aromatic structure makes it important in supramolecular chemistry. It can participate in π-π stacking interactions, which are crucial in molecular recognition, crystal engineering, and the design of self-assembled structures. These interactions are utilized in creating materials with specific properties, such as liquid crystals or molecular machines.

46. What is the significance of benzene's resonance structures in understanding other conjugated systems?

Benzene's resonance structures serve as a model for understanding electron delocalization in other conjugated systems. The concept of multiple contributing structures that better describe a molecule's true electronic distribution extends to many organic and inorganic compounds. This approach helps explain stability, reactivity, and spectroscopic properties of various conjugated molecules.

47. What is the relationship between benzene's structure and its role in materials science?

Benzene's structure is fundamental to many advanced materials. Its aromatic system is