26.1    Introduction to Aromatic Compounds

Aliphatic and aromatic compounds

• 1. Aliphatic compounds are organic compounds that have carbon atoms joined together in straight chains, branched chains or non-aromatic rings.
  2. Aromatic compounds are organic compounds that contain one or more benzene ring(in A-level). Examples of aromatic compounds:

The structure of benzene

• Benzene is a hexagonal and planar molecule with the formula C6H6.

• In benzene, each carbon atom undergoes sp² hybridisation to give three hybrid orbitals and an unchanged p orbital which is perpendicular to the plane containing the three hybrid orbitals

• Two of the hybrid orbitals overlap with two other hybrid orbitals of carbon atoms and the other overlaps with the orbital of The unchanged p orbitals overlap with one another via a system of π bond to give a giant molecular orbital. This orbital is above and below the plane containing the carbon atoms.

• The six electrons from each p orbital are then delocalised around the A delocalised electron does not belong to any carbon atom. This delocalisation of electrons gives benzene a relatively stable structure.

• In a benzene molecule, all the C-C bonds are identical, with a bond length of 140 pm. This bond length is longer than a double bond but shorter than a single bond. The bond angle between carbon atoms is 120º.

• The stability of benzene is shown in its reluctance to undergo addition reactions. This is because when electrons are added, the delocalisation will be broken and so does the  stability.

• There are two common symbols for benzene, the right one is generally preferred over the left one because it indicates that the electrons are delocalised

• Before this modern structure was discovered, the Kekulé structure(right) was used. However, the Kekulé structure could not account for the unexplained properties of benzene
  1. The structure has three double bonds, so it should undergo addition reactions like other alkenes do. However for real benzene, it does not undergo addition reactions easily.
  2. Single and double bonds have different bond length, therefore Kekulé’s benzene should have a distorted hexagonal shape. However in real benzene, all the C-C bonds are identical and it is a perfect hexagon

Introduction to arenes

• Arenes are aromatic hydrocarbons, that is, hydrocarbon that contains at least one benzene ring
• Arenes have higher melting and boiling points than aliphatic hydrocarbons.
  1. This is because in aromatic hydrocarbons, temporary dipoles can be easily set up between the delocalised electrons. Hence the van der Waals’forces are stronger.
  2. Benzenes with side chains also have permanent dipole forces between This is because the presence of side chains makes them a polar molecule.