benzene and aromatic compounds

Compare the kekule model with the delocalised model (what type of molecule is the delocalised model? What is the bonding angle?)

Kekule- 6 membered ring of carbon atoms joined by alternate single and double bonds

Benzene - planar,120, cyclic, hexagonal hydrocarbon C6H6.

• Each carbon uses 3 of its available 4 electrons in bonding to two other carbon atoms and one hydrogen atom.

• Each carbon atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms

• Adjacent p orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms- forming a ring of electron density

• The overlapping of the p orbitals creates a system of pi bonds which spread over all six of the carbon atoms in the ring structure- which are said to be delocalised

Draw the delocalised structure (show the p orbitals)

Evidence for the delocalised model rather than kekule model

• The enthalpy change for hydrogenation is less exothermic than expected when comparing to the enthalpy change for hydrogenation for kekules structure

• The bond length was suggested to be different between double C bonds and single C bonds, but in reality the carbon bond lengths are all the same

• Kekules model would’ve been expected to react with bromine readily because of the C double bonds, but benzene in reality does not undergo electrophilic addition reactions or decolourise bromine under normal conditions

What is the substituent group name for benzene and when does this happen

Phenyl

When benzene is attached to an alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atoms