8.2 The Structure of Benzene

Benzene is one of the most important and fascinating molecules in organic chemistry. Although its molecular formula, C_6H_6, suggests a high degree of unsaturation (like a molecule with three double bonds), benzene does not behave like a typical alkene. It is unusually stable and undergoes substitution reactions rather than addition reactions. Understanding the true structure of benzene—a resonance hybrid with delocalized pi electrons—was a major milestone in chemistry and is essential for understanding all aromatic compounds.

Main Idea

The main idea is that benzene is a planar, hexagonal ring of six carbon atoms, each bonded to one hydrogen atom. The carbon atoms are sp² hybridized, forming sigma bonds to each other and to the hydrogen atoms. Each carbon atom also has an unhybridized p orbital containing one electron. These six p orbitals overlap sideways to form a continuous, delocalized pi electron cloud above and below the plane of the ring. This delocalization gives benzene its exceptional stability.

Core Concept

The core concept is that benzene is a resonance hybrid of two equivalent Kekulé structures. The true structure is not a single Lewis structure with fixed double and single bonds, but an average of the two. The pi electrons are delocalized over all six carbon atoms, meaning they are not localized between any two specific carbons. This delocalization (resonance) lowers the energy of the molecule, making it more stable than a hypothetical molecule with three localized double bonds.

Detailed Explanation

Historical Background and the Kekulé Structure

After the discovery of benzene in 1835, the structure of the benzene molecule puzzled nineteenth-century chemists for forty years. Elemental analysis proved that benzene has an amazingly low hydrogen-to-carbon (H:C) ratio, with molecular formula C_6H_6 and empirical formula "CH," suggesting a high degree of unsaturation. Considering the unsaturated nature of the benzene molecule, several chemists proposed different structures for it having carbon-carbon double and triple covalent bonds. However, the observed properties of benzene could not align with these structures because they imagined benzene as an open molecule.

In 1865, a German chemist named August Kekulé proposed the ring (cyclic) structure of benzene molecule, having six carbon atoms for the first time. He proposed that benzene is a cyclic molecule with six carbon atoms in the ring, each bonded to one hydrogen atom. Kekulé further added that the benzene ring contains three carbon-carbon double bonds which alternate between their positions so rapidly that the two structures he suggested cannot be distinguished. The rapid alternation of double bonds is because pi (π) electrons are free to move over the six carbon atoms of the entire benzene molecule.

• One with double bonds between C1-C2, C3-C4, and C5-C6.
• The other with double bonds between C2-C3, C4-C5, and C6-C1.

The real structure of benzene is a resonance hybrid of the two Kekulé structures. This implies that Kekulé's structures cannot represent the actual structure of benzene.

The Atomic/Molecular Orbital Model of Benzene

In the 1930s, Linus Pauling developed the concept of hybridization and the theory of resonance. These two concepts gave the first adequate explanation of the structure of benzene.

• Shape: The carbon skeleton of benzene forms a regular hexagon with C-C-C and H-C-C bond angles of 120^°.
• Hybridization: Each carbon of the ring exhibits sp² hybridization.
• Sigma Bonds: Each carbon forms sigma (σ) bonds to two adjacent carbon atoms (by the overlap of sp²-sp² hybrid orbitals) and one σ bond to hydrogen (by the overlap of sp²-1s orbitals).
• Bond Length: It was determined experimentally that the bond length between carbon atoms in the benzene molecule is 1.39 Å, which falls in between the bond lengths of a carbon-carbon single bond (1.54 Å) and a carbon-carbon double bond (1.30 Å). This also shows that the carbon-carbon bonds are neither double nor single, rather partial double bonds.
• Pi Electron Cloud: Each carbon atom also has a single unhybridized 2p orbital that contains one electron. These six 2p orbitals lie perpendicular to the plane of the ring and overlap to form a continuous pi (π) electron cloud encompassing all six carbon atoms. The electron density of the pi electron (π) system of a benzene ring lies in one torus (a doughnut-shaped region) above the plane of the ring and in a second torus below.

The Resonance Model of Benzene

A molecule that can be represented by more than one Lewis structure shows resonance. Resonance is the delocalization of electron pairs across atoms in a molecule. The true structure for such a molecule is a resonance hybrid of its possible Lewis structures. The different possible Lewis structures for such molecules are called resonance contributing structures or canonical structures.

The actual structure of the benzene molecule is a resonance hybrid, representing an average of all the possible structures.

Interesting Information: The resonance structures are hypothetical, so they cannot exist.

Evidence for the Delocalized Structure: Hydrogenation

The unusual stability of benzene is demonstrated by its enthalpy of hydrogenation. When cyclohexene (which has one double bond) is hydrogenated to cyclohexane, the enthalpy change is about -120 kJ/mol. If benzene had three localized double bonds (like a hypothetical 1,3,5-cyclohexatriene), we would expect its enthalpy of hydrogenation to be 3 × (-120) = -360 kJ/mol.

However, the actual enthalpy of hydrogenation of benzene is only -208 kJ/mol. The difference between the expected value and the experimentally observed value is 152 kJ/mol. This difference is the resonance energy of benzene, which is the extra stability gained from the delocalization of the pi electrons.

Intermediate Level

The structure of benzene is best described as a resonance hybrid of two Kekulé structures. Each carbon atom is sp² hybridized, forming a planar hexagonal ring with 120^° bond angles. The unhybridized p orbitals on each carbon overlap to form a delocalized pi electron cloud above and below the plane of the ring. This delocalization gives benzene its characteristic stability, known as resonance energy. The carbon-carbon bonds in benzene are all identical in length (1.39 Å), intermediate between a single and a double bond. This is why benzene undergoes electrophilic substitution rather than addition reactions—the delocalized pi system is very stable and is not easily disrupted.

Important Points

• - Benzene Structure: Planar hexagonal ring, sp² hybridized carbons, 120^° bond angles.
• - Delocalized Pi System: Six p orbitals overlap to form a continuous pi electron cloud above and below the ring.
• - Resonance Hybrid: The true structure is an average of the two Kekulé structures.
• - Bond Length: 1.39 Å (between single 1.54 Å and double 1.30 Å).
• - Resonance Energy: The extra stability gained from delocalization, about 152 kJ/mol.
• - Hydrogenation: Enthalpy of hydrogenation is -208 kJ/mol, less than the expected -360 kJ/mol for three localized double bonds.
• - Kekulé Structures: Two hypothetical resonance structures with alternating double bonds.

Remember

• - Benzene: C_6H_6, hexagonal, planar.
• - sp² hybridization, bond angles 120^°.
• - Delocalized pi electrons above and below the ring.
• - Resonance hybrid of two Kekulé structures.
• - Bond length: 1.39 Å (partial double bonds).
• - Resonance energy: 152 kJ/mol (extra stability).
• - Hydrogenation: Actual Δ H = -208 kJ/mol, expected = -360 kJ/mol.
• - Kekulé structures are hypothetical.

Common Mistakes

• - Thinking that benzene has three localized double bonds. The double bonds are delocalized, making benzene a resonance hybrid.
• - Believing that benzene undergoes addition reactions like alkenes. It primarily undergoes substitution reactions because its delocalized pi system is very stable.
• - Forgetting that the carbon-carbon bonds in benzene are all equal in length. They are partial double bonds (1.39 Å), not alternating single and double bonds.
• - Confusing resonance energy with bond energy. Resonance energy is the extra stability gained from delocalization, not the energy of a single bond.
• - Assuming that the Kekulé structures represent the real molecule. They are just hypothetical contributing structures.

Quick Revision

The structure of benzene is best described as a resonance hybrid of two Kekulé structures. It is a planar, hexagonal ring of six sp²-hybridized carbon atoms, with bond angles of 120^°. Each carbon atom has an unhybridized p orbital, and these p orbitals overlap to form a delocalized pi electron cloud above and below the plane of the ring. This delocalization gives benzene its exceptional stability (resonance energy of about 152 kJ/mol) and is why its carbon-carbon bonds are all equal in length (1.39 Å), intermediate between single and double bonds. This stability is also why benzene undergoes substitution reactions rather than addition reactions.

60-Second Revision

Benzene is a planar, hexagonal ring of six sp²-hybridized carbons with 120^° bond angles. Its true structure is a resonance hybrid of two Kekulé structures, with delocalized pi electrons above and below the ring. This delocalization (resonance energy ≈ 152 kJ/mol) makes benzene very stable, so it undergoes substitution reactions rather than addition. Its carbon-carbon bonds are all equal in length (1.39 Å), intermediate between single and double bonds.

Frequently Asked Questions (FAQs)

Q1. Why does benzene not react in a way alkenes do, if it has double bonds like alkenes? Answer: Benzene does not react like alkenes because its pi electrons are delocalized over the entire ring, forming a resonance hybrid. This delocalization gives benzene extra stability (resonance energy), making its pi electrons less available for addition reactions. The delocalized pi system is not easily broken, so benzene prefers substitution reactions, which preserve the aromatic ring.

Q2. If benzene has three double bonds, why can it not add three moles of bromine? Answer: Because the pi electrons in benzene are delocalized, not localized as three separate double bonds. The delocalization (resonance) stabilizes the molecule, so it does not readily undergo addition reactions like alkenes. Adding bromine would require breaking the aromatic delocalization, which is energetically unfavorable. Benzene undergoes substitution reactions with bromine instead, in the presence of a catalyst.

Q3. Why does benzene favor substitution reactions over addition reactions? Answer: Benzene favors substitution reactions because the aromatic ring is very stable due to the delocalization of its pi electrons. In a substitution reaction, the aromatic ring is preserved, so the molecule retains its stability. In an addition reaction, the delocalized pi system would be disrupted, losing the aromatic stability. Therefore, substitution is thermodynamically favored.

Q4. What is the resonance hybrid of benzene? Answer: The resonance hybrid is the true structure of benzene, representing an average of its two Kekulé structures. In the resonance hybrid, the pi electrons are delocalized over all six carbon atoms, so the carbon-carbon bonds are all partial double bonds (bond length 1.39 Å) and are identical. The resonance hybrid is more stable than either Kekulé structure alone.

• Equal bond lengths: All C-C bonds in benzene are 1.39 Å, intermediate between single (1.54 Å) and double (1.30 Å) bonds.
• Resonance energy: The enthalpy of hydrogenation of benzene is less exothermic than expected for three localized double bonds, indicating extra stability.
• Reactivity: Benzene undergoes substitution rather than addition reactions, preserving the aromatic system.
• Planarity and 120^° bond angles: Consistent with sp² hybridization and a delocalized pi system.

Q6. What is resonance energy, and what is its value for benzene? Answer: Resonance energy is the extra stability gained by a molecule due to the delocalization of electrons (resonance). For benzene, the resonance energy is about 152 kJ/mol. It is calculated as the difference between the expected enthalpy of hydrogenation for a hypothetical molecule with three localized double bonds (-360 kJ/mol) and the actual enthalpy of hydrogenation of benzene (-208 kJ/mol).

Q7. Explain the atomic orbital treatment of benzene. Answer: In the atomic orbital treatment, each carbon atom in benzene is sp² hybridized, forming three sigma bonds (two to adjacent carbons and one to hydrogen) in a planar hexagonal arrangement with 120^° bond angles. Each carbon has one unhybridized p orbital perpendicular to the plane. These six p orbitals overlap sideways to form a continuous, delocalized pi electron cloud above and below the ring. This delocalization is the basis of the resonance hybrid model.

Q8. How can unhybridized 2p orbitals of benzene adopt a doughnut shape? Answer: The six unhybridized 2p orbitals, one on each carbon atom, are all perpendicular to the plane of the ring. They overlap sideways (laterally) to form a continuous loop of electron density. This electron density is concentrated in two torus-shaped (doughnut-shaped) regions, one above and one below the plane of the carbon ring.

Q9. What are the two Kekulé structures of benzene? Answer: The two Kekulé structures are hypothetical resonance structures of benzene with alternating single and double bonds. In one structure, the double bonds are between C1-C2, C3-C4, and C5-C6. In the other, the double bonds are between C2-C3, C4-C5, and C6-C1. The real structure is a resonance hybrid of these two.

Multiple Choice Questions (MCQs)

Q1. Which one of the following challenges was faced by Kekulé's structure of benzene? A. Benzene is immiscible with water. B. Benzene is hexagonal. C. Benzene is less reactive than alkenes. D. Benzene is planar. Correct Answer: C Explanation: Kekulé's structure (with alternating double and single bonds) could not explain why benzene is much less reactive than alkenes towards addition reactions.

• Q2. The C-C-C bond angle in benzene molecule is:
• A. 180^°
• B. 120^°
• C. 90^°
• D. 109.5^°
• Correct Answer: B
• Explanation: The bond angle in benzene is 120^° due to sp² hybridization.

• Q3. Delocalization of pi (π) electrons in benzene molecule is because:
• A. Each carbon is sp² hybridized.
• B. It is hexagonal.
• C. It is unsaturated.
• D. It has 5 unhybrid 2p orbitals.
• Correct Answer: A
• Explanation: Each carbon in benzene is sp² hybridized, leaving one unhybridized p orbital to form the delocalized pi system.

Q4. What is the experimentally determined C-C bond length in benzene? A. 1.54 Å B. 1.30 Å C. 1.39 Å D. 1.20 Å Correct Answer: C Explanation: The C-C bond length in benzene is 1.39 Å, intermediate between a single bond (1.54 Å) and a double bond (1.30 Å).

• Q5. The resonance energy of benzene is approximately:
• A. 120 kJ/mol
• B. 152 kJ/mol
• C. 208 kJ/mol
• D. 360 kJ/mol
• Correct Answer: B
• Explanation: The resonance energy is 152 kJ/mol, the difference between the expected and actual enthalpies of hydrogenation.

• Q6. What is the expected enthalpy of hydrogenation of benzene if it had three localized double bonds?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: C
• Explanation: If benzene had three localized double bonds, the enthalpy of hydrogenation would be 3 × (-120) = -360 kJ/mol.

• Q7. What is the actual enthalpy of hydrogenation of benzene?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: B
• Explanation: The actual enthalpy of hydrogenation of benzene is -208 kJ/mol.

• Q8. What is the shape of the pi electron cloud in benzene?
• A. Linear
• B. Spherical
• C. Two torus shapes above and below the ring
• D. A single torus above the ring
• Correct Answer: C
• Explanation: The pi electron cloud forms two torus (doughnut-shaped) regions, one above and one below the plane of the ring.

• Q9. Which of the following is NOT a resonance structure of benzene?
• A. A structure with alternating single and double bonds.
• B. A structure with a circle inside a hexagon.
• C. The two Kekulé structures.
• D. A structure with a triple bond.
• Correct Answer: D
• Explanation: Benzene does not have any triple bonds. Its resonance structures have only single and double bonds.

• Q10. The true structure of benzene is a:
• A. Single Lewis structure with fixed double bonds.
• B. Resonance hybrid of two Kekulé structures.
• C. Mixture of two Kekulé structures.
• D. Structure with localized double bonds.
• Correct Answer: B
• Explanation: The true structure is a resonance hybrid, an average of the two Kekulé structures.

Multiple Choice Questions (MCQs)

• Q1. Which one of the following challenges was faced by Kekulé's structure of benzene?
• A. Benzene is immiscible with water.
• B. Benzene is hexagonal.
• C. Benzene is less reactive than alkenes.
• D. Benzene is planar.
• Correct Answer: C
• Explanation: Kekulé's structure could not explain the low reactivity of benzene compared to alkenes.

• Q2. The C-C-C bond angle in benzene molecule is:
• A. 180^°
• B. 120^°
• C. 90^°
• D. 109.5^°
• Correct Answer: B
• Explanation: The bond angle in benzene is 120^°.

• Q3. Delocalization of pi (π) electrons in benzene molecule is because:
• A. Each carbon is sp² hybridized.
• B. It is hexagonal.
• C. It is unsaturated.
• D. It has 5 unhybrid 2p orbitals.
• Correct Answer: A
• Explanation: Each carbon is sp² hybridized, allowing for delocalization.

• Q4. What is the experimentally determined C-C bond length in benzene?
• A. 1.54 Å
• B. 1.30 Å
• C. 1.39 Å
• D. 1.20 Å
• Correct Answer: C
• Explanation: The C-C bond length in benzene is 1.39 Å.

• Q5. The resonance energy of benzene is approximately:
• A. 120 kJ/mol
• B. 152 kJ/mol
• C. 208 kJ/mol
• D. 360 kJ/mol
• Correct Answer: B
• Explanation: The resonance energy is 152 kJ/mol.

• Q6. What is the expected enthalpy of hydrogenation of benzene if it had three localized double bonds?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: C
• Explanation: The expected value is -360 kJ/mol.

• Q7. What is the actual enthalpy of hydrogenation of benzene?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: B
• Explanation: The actual enthalpy of hydrogenation of benzene is -208 kJ/mol.

• Q8. What is the shape of the pi electron cloud in benzene?
• A. Linear
• B. Spherical
• C. Two torus shapes above and below the ring
• D. A single torus above the ring
• Correct Answer: C
• Explanation: The pi electron cloud forms two torus shapes.

• Q9. Which of the following is NOT a resonance structure of benzene?
• A. A structure with alternating single and double bonds.
• B. A structure with a circle inside a hexagon.
• C. The two Kekulé structures.
• D. A structure with a triple bond.
• Correct Answer: D
• Explanation: Benzene does not have triple bonds.

• Q10. The true structure of benzene is a:
• A. Single Lewis structure with fixed double bonds.
• B. Resonance hybrid of two Kekulé structures.
• C. Mixture of two Kekulé structures.
• D. Structure with localized double bonds.
• Correct Answer: B
• Explanation: The true structure is a resonance hybrid.

Multiple Choice Questions (MCQs)

• Q1. What is the molecular formula of benzene?
• A. C_6H_{12}
• B. C_6H_6
• C. C_6H_{10}
• D. C_5H_6
• Correct Answer: B
• Explanation: The molecular formula of benzene is C_6H_6.

• Q2. What is the hybridization of carbon atoms in benzene?
• A. sp
• B. sp²
• C. sp³
• D. sp³d
• Correct Answer: B
• Explanation: The carbon atoms in benzene are sp² hybridized.

• Q3. How many pi electrons are delocalized in benzene?
• A. 3
• B. 6
• C. 12
• D. 4
• Correct Answer: B
• Explanation: There are 6 pi electrons, one from each unhybridized p orbital.

• Q4. What is the bond angle in benzene?
• A. 90^°
• B. 109.5^°
• C. 120^°
• D. 180^°
• Correct Answer: C
• Explanation: The bond angle is 120^°.

• Q5. What is the resonance energy of benzene?
• A. 152 kJ/mol
• B. 208 kJ/mol
• C. 360 kJ/mol
• D. 120 kJ/mol
• Correct Answer: A
• Explanation: The resonance energy is 152 kJ/mol.

• Q6. What is the expected enthalpy of hydrogenation of benzene if it had three localized double bonds?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: C
• Explanation: The expected value is -360 kJ/mol.

• Q7. What is the actual enthalpy of hydrogenation of benzene?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: B
• Explanation: The actual enthalpy of hydrogenation is -208 kJ/mol.

• Q8. How many Kekulé structures does benzene have?
• A. 1
• B. 2
• C. 3
• D. 4
• Correct Answer: B
• Explanation: Benzene has two Kekulé structures.

• Q9. What is the shape of the pi electron cloud in benzene?
• A. Linear
• B. Spherical
• C. Two torus shapes above and below the ring
• D. A single torus above the ring
• Correct Answer: C
• Explanation: The pi electron cloud forms two torus shapes.

• Q10. Which of the following is a correct statement about the structure of benzene?
• A. It has three localized double bonds.
• B. It is a resonance hybrid of two Kekulé structures.
• C. It is a non-planar molecule.
• D. It has sp³ hybridized carbons.
• Correct Answer: B
• Explanation: The true structure is a resonance hybrid.

Multiple Choice Questions (MCQs)

• Q1. What is the molecular formula of benzene?
• A. C_6H_{12}
• B. C_6H_6
• C. C_6H_{10}
• D. C_5H_6
• Correct Answer: B
• Explanation: The molecular formula of benzene is C_6H_6.

• Q2. What is the hybridization of carbon atoms in benzene?
• A. sp
• B. sp²
• C. sp³
• D. sp³d
• Correct Answer: B
• Explanation: The carbon atoms in benzene are sp² hybridized.

• Q3. How many pi electrons are delocalized in benzene?
• A. 3
• B. 6
• C. 12
• D. 4
• Correct Answer: B
• Explanation: There are 6 pi electrons.

• Q4. What is the bond angle in benzene?
• A. 90^°
• B. 109.5^°
• C. 120^°
• D. 180^°
• Correct Answer: C
• Explanation: The bond angle is 120^°.

• Q5. What is the resonance energy of benzene?
• A. 152 kJ/mol
• B. 208 kJ/mol
• C. 360 kJ/mol
• D. 120 kJ/mol
• Correct Answer: A
• Explanation: The resonance energy is 152 kJ/mol.

• Q6. What is the expected enthalpy of hydrogenation of benzene if it had three localized double bonds?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: C
• Explanation: The expected value is -360 kJ/mol.

• Q7. What is the actual enthalpy of hydrogenation of benzene?
• A. -120 kJ/mol
• B. -208 kJ/mol
• C. -360 kJ/mol
• D. -152 kJ/mol
• Correct Answer: B
• Explanation: The actual enthalpy of hydrogenation is -208 kJ/mol.

• Q8. How many Kekulé structures does benzene have?
• A. 1
• B. 2
• C. 3
• D. 4
• Correct Answer: B
• Explanation: Benzene has two Kekulé structures.

• Q9. What is the shape of the pi electron cloud in benzene?
• A. Linear
• B. Spherical
• C. Two torus shapes above and below the ring
• D. A single torus above the ring
• Correct Answer: C
• Explanation: The pi electron cloud forms two torus shapes.

• Q10. Which of the following is a correct statement about the structure of benzene?
• A. It has three localized double bonds.
• B. It is a resonance hybrid of two Kekulé structures.
• C. It is a non-planar molecule.
• D. It has sp³ hybridized carbons.
• Correct Answer: B
• Explanation: The true structure is a resonance hybrid.