Topic 18 - Organic Chemistry 3

compounds containing a benzene ring eg. phenol, nitrobenzene, etc

C6H6

6 carbon atoms in a ring with alternating single and double carbon-carbon bonds

• x-ray diffraction shows that benzene only has 1 carbon-carbon bond length but with the Kekulé model, you would expect two • IR spectrum has fewer peaks than expected for the Kekulé model • benzene does not undego electrophilic addition reactions, which you would expect it to if it had C=C bonds • enthalpy change of hydrogenation is much lower than expected for the Kekulé model (benzene is more stable)

• 6 carbon atoms in a ring where each carbon atom forms a sigma bond with its hydrogen and each neighbouring carbon and the remaining p-orbital forms a ring of delocalised electrons • p orbitals overlap above and below the molecule, forming a delocalised pi-electron system

carbon dioxide + water

bromobenzene

• dark - AlCl3/FeBr3 catalyst (halogen carrier)

nitrobenzene + water

conc nitric acid and conc sulfuric acid

• conc sulfuric acid catalyst - 55°C

to avoid multiple substitution of benzene

it produces a stronger electrophile -nitronium ion

HNO3 + H2SO4 → NO2+ + HSO4- + 2H2O

H+ + HSO4- → H2SO4

alkyl benzene + hydrogen halide

• aluminium chloride catalyst - heat under reflux

phenyl ketone + hydrogen chloride

• aluminium chloride catalyst - heat under reflux

C6H6OH

• slightly acidic • not soluble • solid at room temp • more reactive than benzene

-phenol’s oxygen activates the benzene ring -oxygen’s lone pair of e- are incorporated into the delocalised pi-electron system -this increases electron density in the ring -it can attract electrophiles more easily (catalysts/halogen carriers not needed)

2,4,6 tribromophenol + hydrogen bromide

• fishy smell • generally soluble (can H-bond with water) • have higher mp/bp than respective hydrocarbons • have basic nature (nitrogen’s lone pair of e- can accept H+)