Benzene vs. Cyclohexene Reactivity

Overview

This lecture explains why benzene is less reactive than cyclohexene, focusing on electron delocalization, resonance, and molecular structure.

Benzene Structure and Reactivity

• Benzene contains six carbons arranged in a ring with alternating double bonds.
• The double bonds in benzene are not fixed; the electrons are delocalized around the ring.
• Resonance structures mean the double bonds can shift positions, making all C-C bonds equal.
• Each bond in benzene has a bond order of 1.5 (average of single and double bonds).
• Measurements show all benzene C-C bonds are the same length.

Electron Delocalization vs. Localization

• Delocalized electrons in benzene are spread over the entire ring and not confined to one bond.
• This delocalization makes benzene less chemically reactive.
• Cyclohexene has localized electrons in its single double bond, fixed between two carbons.
• Localized electrons can participate more easily in chemical reactions.

Reactivity with Bromine (Br₂)

• Benzene’s delocalized electrons do not strongly repel Br₂, so a catalyst is needed for reaction.
• Cyclohexene’s localized double bond electrons strongly interact with Br₂, allowing reaction without a catalyst.
• The difference in electron localization affects how easily these molecules react with Br₂.

Key Terms & Definitions

• Resonance Structure — alternate ways of drawing double bonds in a molecule, showing delocalization.
• Delocalized Electrons — electrons not confined to a specific bond, spread over multiple atoms.
• Localized Electrons — electrons confined to a specific bond or atom.
• Bond Order — the average number of bonds between two atoms, calculated from resonance structures.

Action Items / Next Steps

• Review resonance and delocalization concepts for benzene.
• Compare reactivity of benzene and cyclohexene with Br₂ in practice problems.