A-level Chemistry/OCR (Salters)/Benzene

Benzene is an aromatic hydrocarbon or arene.

Structure

The usual structural representation for benzene is a six carbon ring (represented by a hexagon) which includes three double bonds. Each of the carbons represented by a corner is also bonded to one other atom. In benzene itself, these atoms are hydrogens. The double bonds are separated by single bonds so we recognize the arrangement as involving conjugated double bonds. An alternative symbol uses a circle inside the hexagon to represent the six pi electrons.

Instead of having three short double bonds and three long single bonds (above, left), it has six medium-length bonds (above, right).

A double bond is really two bonds, with two electrons in each bond, so four electrons in total in a double bond. Two electrons from each double bond in benzene (if the double bonds really existed, which they don't) are smeared over all six carbon atoms.

Whenever electrons are not confined to a bond between just two atoms, they are called declocalised electrons.

• 
  the full displayed structural formula of benzene
• 
  Kékulé proposed this structure for benzene
• 
  Benzene rings are commonly represented like this
• 
  Space-filling models like this show all C−C and C−H distances are equal
• 
  This electrostatic potential surface shows greater electron density inside the ring
• 
  The π electrons in benzene occupy molecular orbitals like this one, which are delocalised over the entire molecule

Pic

Physical Properties

Boiling point: 80 °C

Melting point: 6 °C

Relative density (water = 1): 0.88

Solubility in water, g/100 ml at 25 °C: 0.18

Vapour pressure, kPa at 20 °C: 10

Relative vapour density (air = 1): 2.7

Relative density of the vapour/air-mixture at 20 °C (air = 1): 1.2

Flash point: -11 °C c.c.

Auto-ignition temperature: 498 °C

Explosive limits, vol% in air: 1.2-8.0

Octanol/water partition coefficient as log Pow: 2.13