Benzene

(a) Structure of and Bonding in Benzene and Other Arenes

Structure of Benzene:

• Benzene (C₆H₆) is a planar, cyclic molecule with six carbon atoms arranged in a hexagonal ring.

• Each carbon atom forms three sigma bonds: two with adjacent carbon atoms and one with a hydrogen atom.

• The remaining p-orbitals on each carbon overlap to form a delocalized π-electron system above and below the plane of the ring.

Bonding in Benzene:

• The delocalization of π-electrons results in a ring of electron density, which makes the bond lengths between carbon atoms equal (1.39 Å).

• The delocalized electron system provides stability, known as resonance stabilization.

• Benzene does not exhibit alternating single and double bonds; instead, it has a continuous π-system.

Structure of Other Arenes:

• Arenes are aromatic hydrocarbons that include one or more benzene rings (e.g., toluene, naphthalene).

• Substituents can be attached to the benzene ring, influencing its chemical properties.

(b) Resistance to Addition Reactions Shown by Aromatic Compounds Such as Benzene

Resistance to Addition Reactions:

• Benzene resists addition reactions, which would disrupt the delocalized π-electron system and reduce its stability.

• Instead, benzene undergoes substitution reactions, where a hydrogen atom is replaced by another group, preserving the aromatic ring.

Comparison with Alkenes:

• Alkenes readily undergo addition reactions due to localized C = C double bonds.

• Benzene's delocalized electrons make the ring more stable and less reactive toward addition reactions.

Energy Considerations:

• The enthalpy change for hydrogenation of benzene is less exothermic than expected for a molecule with three double bonds, indicating the extra stability from delocalization.

(c) Mechanism of Electrophilic Substitution in Benzene

General Mechanism:

1. Formation of an Electrophile: The attacking species must be a strong electrophile to disrupt the stable benzene ring.

2. Attack on the Benzene Ring: The electrophile attacks the delocalized π-electron cloud, forming a carbocation intermediate (arenium ion).

3. Restoration of Aromaticity: The carbocation loses a proton, and aromaticity is restored.

See physical notes for the diagram

Examples of Electrophilic Substitution Reactions:

1. Nitration of Benzene:

• Reagents: Concentrated HNO₃ and H₂SO₄.

• Conditions: 50^oC

• Electrophile: Nitronium ion (NO₂⁺).

• Overall Equation: C₆H₆ + HNO₃ —> C₆H₅NO₂ + H₂O

2. Halogenation of Benzene:

• Reagents: Halogen (e.g., Cl₂) and a halogen carrier (e.g., FeCl₃).

• Conditions: 25^oC

• Electrophile: Cl⁺ (chloronium ion).

• Overall Equation: C₆H₆ + X₂ —> C₆H₅X + HX

3. Friedel-Crafts Alkylation:

• Reagents: Alkyl halide (e.g., CH₃Cl) and AlCl₃ catalyst.

• Conditions: 0 - 25^oC (to prevent further substitution)

• Electrophile: CH₃⁺.

• Overall Equation: C₆H₆ + RCl —> C₆H₅R + HCl

(d) Interaction Between Benzene and Substituent Groups

Effect of Substituents on Benzene:

• Substituents can be electron-donating or electron-withdrawing, affecting the reactivity of the benzene ring.

• Electron-donating groups (e.g., OH, CH₃) activate the ring, making it more reactive toward electrophilic substitution.

• Electron-withdrawing groups (e.g., NO₂, Cl) deactivate the ring, making it less reactive.

C—Cl Bond Strength in Chlorobenzene vs. Chloroalkane:

• In chlorobenzene, the lone pairs on chlorine interact with the delocalized π-electron system of the benzene ring.

• This interaction increases the strength of the C—Cl bond compared to a chloroalkane.

• The partial double bond character in the C—Cl bond of chlorobenzene makes it stronger and less reactive in nucleophilic substitution reactions.

Comparison: