Benzene (DP IB Chemistry: SL)

Kekulé structure for benzene

• Kekulé suggested that benzene was a hexagon with three double bonds
• It was therefore equivalent to three ethene molecules

Problems with Kekulé’s structure for benzene

• Since benzene has three double bonds, it should have similar reactivity to ethene
• However, this turned out not to be the case
  • Ethene undergoes addition reactions whereas benzene rarely does (only under very harsh conditions) and instead undergoes substitution reactions

The presence of three double bonds also suggested that benzene had shorter double and longer single bonds

• In fact, the bond lengths in benzene were exactly the same
• They were found to be an intermediate between single and double bonds

The benzene is also much more stable than Kekulé’s suggested structure for benzene

• Less energy was required to hydrogenate a benzene molecule compared to the hydrogenation of three ethene molecules
• This means that the bonds broken in benzene are stronger than the double bonds in ethene

The increase in stability of benzene is known as the delocalisation energy and is caused by the delocalised electrons in the benzene structure

The C-C in benzene are an intermediate between single and double bonds which is a result of these delocalised electrons

Shape of benzene

• Benzene is a planar regular hexagon with bond angles of 120º
  • All the bonds are identical due to the delocalization of electrons

Each sp² hybridised carbon atom in benzene forms:

• A σ bond with two other carbons
• A σ bond with one hydrogen atom

The remaining p orbital is overlapping with the p orbitals on both sides of it

• To achieve maximum overlap, the benzene ring must be planar

This results in the formation of a system of π bonds spread out over the whole ring

Due to this, the electrons are not bound to specific atoms but can instead freely move around the structure and are said to be delocalised

 

Benzene has a π system of delocalised electrons with carbon atoms that have bond angles of 120º