In the delocalised model, each of the six carbon atoms in benzene uses three of its four outer electrons to form σ bonds: two to neighbouring carbon atoms and one to a hydrogen atom. These σ bonds lie in one plane at 120° to each other, so the molecule is planar.The fourth electron of each carbon is in a p orbital, which sticks out above and below the plane of the ring. Each p orbital overlaps sideways with the p orbitals on both neighbouring carbons, not just one. This forms a continuous π system: rings of electron density above and below the plane of the carbon atoms.
• All six C-C bonds are identical, with a length between that of a single and a double bond.
• Delocalisation spreads out the electron density, which makes benzene much more stable than the Kekulé structure would be.
• The electron density between any two carbons is lower than in the C=C bond of an alkene, so benzene is less able to polarise molecules such as Br2 and does not readily undergo addition. It usually reacts by substitution, which keeps the stable ring intact.Worked example: how many bonding electrons hold the carbon ring together? There are six C-C σ bonds (6 × 2 = 12 electrons) plus the six delocalised π electrons, giving 18 electrons in total.
• All six C-C bonds are identical, with a length between that of a single and a double bond.
• Delocalisation spreads out the electron density, which makes benzene much more stable than the Kekulé structure would be.
• The electron density between any two carbons is lower than in the C=C bond of an alkene, so benzene is less able to polarise molecules such as Br2 and does not readily undergo addition. It usually reacts by substitution, which keeps the stable ring intact.Worked example: how many bonding electrons hold the carbon ring together? There are six C-C σ bonds (6 × 2 = 12 electrons) plus the six delocalised π electrons, giving 18 electrons in total.