IB DP Chemistry: HL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