Eduqas Chemistry for A Level Year 2: Student Bk

119 OA1.2 Aromaticity The structure and bonding in benzene and other arenes, and their resistance to addition reactions In 1825 Michael Faraday isolated a colourless flammable liquid from whale oil, whilst obtaining a suitable gas for street lighting. He found that this flammable liquid had the empirical formula CH. Nine years later the liquid, now called benzene, was found to have a relative molecular mass of 78 and the molecular formula C 6 H 6 . Later workers found that benzene was present in coal tar, from which it was obtained for many years. In 1865 Kekule suggested that the structure of benzene was a six-membered ring, containing alternating single and double carbon-to-carbon bonds. Unfortunately, the Kekule model for the structure of benzene does not explain some of benzene’s reactions. For example, it should react as an alkene and easily undergo addition reactions, such as the reaction with aqueous bromine, where the bromine is decolourised. However, it does not react in this way.To explain this discrepancy, Kekule proposed that benzene had two forms and suggested that one form changed to the other so quickly that an approaching molecule would have no time to react with it by addition. YOU SHOULD KNOW › › › ››› the structure and bonding in benzene ››› why benzene is resistant to addition reactions 6 Knowledge check Look at the Kekule structure of benzene below. Why might it lose a mark in the examination? 7 Knowledge check Choose the correct statement about the structure of benzene. (a) Each carbon atom is bonded to two hydrogen atoms and one carbon atom. (b) The C–C–C bond angle is 120 ° . (c) The C=C bond length is longer than the C–C bond length. (d) The smaller the resonance energy, the more stable the molecule. H C C H H C C H H C C H H C C H C C C C H H H H August Kekule