Benzene

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• Colourless liquid hydrocarbon that is a cyclic compound and an aromatic
• Molecular formula of $C_6{H}_6$
• A conjugated system is a system of connected p-orbitals
• Benzene is a planar molecule containing a ring of 6 carbon atoms, each with a hydrogen atom attached
• The 6 carbon atoms form a perfectly regular hexagon, and all carbon-carbon bonds have the same length
• Aromatics: Unsaturated hydrocarbons with ring/circle structure

Benzene has a conjugated system

1. These systems comprise of overlapping pi electron clouds
2. This allows for the delocalisation of pi electrons across all adjacent aligned p-orbitals, and thus the electrons do not belong to a single bond
3. There are exactly 6 (1 from each carbon atom) delocalised electrons above and below the plane of the ring. These electrons are delocalised within the benzene ring, and can move randomly

Stability

The presence of the delocalised electrons makes benzene particularly stable, as benzene’s conjugated pi electrons resonate freely within the cyclic ring, thus resulting in its 2 resonance forms of alternating single/double bonds. This resonance lowers the overall energy of the molecule, and thus makes it more stable

Reactions

• Undergoes complete and incomplete combustion
• No addition reactions
• Substitution: requires catalyst (UV light)
  • The benzene can easily undergo substitution reactions, as this does not involve breaking the conjugated pi electron system and losing its stability
  • Here, energy in the form of UV light is required to break the bonds between Carbon and Hydrogen in Benzene and between atoms in the reacting compound
  • Now the atoms are in a high energy state and can then react with benzene, substituting the hydrogen attached

Reactivity of Benzene

• Least reactive from alkanes, alkenes, cyclic compounds, and most other hydrocarbons
• This is because it is highly stable
• Benzene’s conjugated pi electrons resonate freely within the cyclic ring, thus resulting in its 2 resonance forms of alternating single/double bonds.
• This resonance lowers the overall energy of the molecule, and thus makes it more stable, and thus less reactive