1 Introduction

There are two major classes of organic chemicals aliphatic : straight or branched chain organic substances aromatic or arene: includes one or more ring of six carbon atoms with delocalised bonding. All of the organic substances we have looked at so far have been aliphatic Benzene belongs to the aromatic class. Benzene’s Structure The simplest arene is benzene. It has the molecular formula C6H6 Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring. The Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms H H H H H H H H H H H H In 1865 Kekule suggested the following structure for Benzene consisting of alternate single and double covalent bonds between the carbon atoms C C C C C C H H H H H H This structure is not correct. Evidence suggests that all the C-C bonds are the same length. Benzene is a planar molecule.The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond In formulae we draw a circle to show this delocalised system Abbreviated formula Displayed formula The H-C-C bond angle is 120o in Benzene The six electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised means not attached to a particular atom. + H2
+ 3H2
+ 3H2
∆H = -120 kJ/mol ∆H = -360 kJ/mol ∆H = -208kJ/mol Enthalpies of Hydrogenation cyclohexene cyclohexane Non delocalised structure delocalised structure Theoretically because there are 3 double bonds one might expect the amount of energy to be 3 times as much. x3 However, the real amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bonds -360 kJ/mol Theoretical value ∆H = -208kJ/mol actual value enthalpy ∆H = -152kJ/mol delocalisation energy This when represented on an energy level diagram shows that the delocalised benzene is more thermodynamically stable. The increase in stability connected to delocalisation is called the delocalisation energy Summary of evidence for why benzene has a delocalised structure. • Bond length intermediate between short C=C and long C–C • ΔH hydrogenation less exothermic than expected when compared to ΔH hydrogenation for kekule structure • Only reacts with Br2 at high temp or in presence of a halogen carrier