Benzene
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
