Benzene

Introduction to benzene (A2)=> It is an aromatic compound.=> Phenyl = Benzene group and used as the prefix.=> Benzene is used as the suffix.=> Made up of C6H6.=> CH is the empirical formula. => There are two ways of representing benzene: The Kekule model and the delocalised model.August KekuleIn 1865 he proposed that benzene was made up of a planar (flat) ring of carbon atoms with alternating single and double bonds. Each carbon atom is bonded to one hydrogen atom. However he later adapted what he suggested to the benzene molecule constantly flipping between two forms by switching over the double and single bonds. FOR Kekule It can be hydrogenated to form cyclohexane. This requires a platinum catalyst. Single and double bonds change rapidly and continuously. Electrophilic substitution occurs. Against Kekule Benzene reacts with chlorine in an addition reaction. However benzene doesn't undergo addition reactions. Addition reactions would reduce its stability as it would disrupt delocalisation. However due to the double bonds in Kekule's structure it would suggest it does. Benzene doesn't decolourise bromine water under normal conditions. The lengths of the C-C bonds and C=C bonds are different meaning it is not a perfect hexagon. However X-ray diffraction has shown that all the carbon-carbon bonds in benzene are the same length of 140 pm, meaning they are between the lengths of C-C and C=C. If Kekule's model is correct you would expect it to have an bond enthalpy change of hydrogenation 3 times greater than that of cyclohexene. For cyclohexene it is -120. But for benzene it is -208. Therefore we know it can't have this structure. The delocalised modelThis suggests that the p orbitals of all six carbon atoms overlap to create a pi-system. This means that there are single bonds with delocalised electrons in the ring. They are delocalised as they don't belong to a specific carbon atom. The evidence from hydrogenation indicates that benzene is more stable than Kekule's model. Benzene's resistance to reaction gives more evidence for it being more stable than Kekule's model. This stability is thought to be created by the delocalised ring of electrons.

Nitration of Benzene=> Substitution reaction not addition as addition would de-stablise benzene. There are two steps to the process: Addition and elimination.Addition Electrophile E+ is attracted to the delocalised electron cloud. A co-ordinate bond forms between one of the carbon atoms in the benzene ring and the electrophile. An unstable positively charged intermediate is formed. Elimination C-H bond breaks Its pair of shared electrons restores the stable delocalised electron structure. A hydrogen ion is eliminated. Nitration=> Nitro group=> When NO2 is substituted into benzene in place of one of the hydrogens.=> Benzene becomes the yellow oil liquid.=> C6H6 + HNO3 ---> C6H5NO2 +H2O=> C6H6 + NO2+ ---> C6H5NO2 + H+=> We make the nitronium ion by using the nitrating mixture.=> This consists of concentrated nitric acid and concentrated sulphuric acid. Reaction to produce nitrobenzene:=> 50 degrees celsius=> Nitrating mixture of concentrated sulphuric acid and concentrated nitric acid.=> Reagents: Benzene and nitric acid (concentrated)=> The catalyst is considered to be concentrated sulphuric acid as it is reformed at the end. Halogenation of benzene Works with AlCl3, FeCl3, AlBr3, and FeBr3 To make mix halogen with metal. Halogen carrier acts as the catalyst. Example overall equation: H+ + FeBr4- ---> FeBr3 and HBr