Benzene v Alkenes

Structure and Bonding in Benzene:

• Hexagonal Ring: Benzene (C₆H₆​) consists of a planar, cyclic structure with six carbon atoms forming a regular hexagon.

• Delocalized Electrons: Each carbon atom forms three sigma (σ) bonds (two with adjacent carbons and one with hydrogen), leaving one unused p-orbital per carbon.

• π-Electron Cloud: The unused p-orbitals overlap sideways, forming a delocalized π-electron system above and below the ring, rather than localized double bonds.

• Equal Bond Lengths: The delocalization causes all C–C bonds to be equal in length (0.139 nm), intermediate between single (0.154 nm) and double (0.134 nm) bonds, making benzene more stable than expected.

Why Benzene is Less Reactive to Addition Reactions than Alkenes:

• Stability Due to Delocalization: The π-electrons are spread out across the ring, making benzene more stable than alkenes, which have localized double bonds.

• High Activation Energy for Disrupting π-System: Addition reactions (e.g., with bromine) would require breaking the delocalized π-system, which is energetically unfavorable.

• Prefers Substitution Over Addition: Instead of undergoing addition, benzene undergoes electrophilic substitution (e.g., nitration, halogenation), which preserves the stable ring structure.

This stability explains why benzene does not decolorize bromine water, unlike alkenes, which readily undergo addition due to their localized double bonds.