IB DP Chemistry: SL复习笔记10.1.12 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