Eduqas Chemistry for A Level Year 2: Student Bk

120 Eduqas A Level Chemistry Year 2: Component 2 If benzene existed as the Kekule forms above then there would be two different bond lengths between carbon atoms in the molecule. In the early 20th century, x-ray crystallography showed that each carbon-to-carbon bond was the same length at 0.140 nm. This distance is between a carbon-to-carbon double bond at 0.135 nm and a carbon-to-carbon single bond at 0.147 nm. When cyclohexene is hydrogenated the energy released is 120 kJ mol –1 . + H 2 ∆H = –120 kJ mol –1 If benzene existed as the Kekule structure and all three double bonds are hydrogenated, then the enthalpy change would be –360 kJ mol –1 . However, when benzene is fully hydrogenated the enthalpy change is –208 kJ mol –1 . This is 152 kJ mol –1 less than expected, suggesting that benzene is more stable than the Kekule structure, and it suggests that the Kekule structure is incorrect. This difference in energy values is called the resonance energy. The delocalised bonding model of benzene Modern studies of benzene have shown that it is a planar molecule and that the angles between three adjacent carbon atoms are 120 ° . In an ethene molecule, C 2 H 4 , each carbon atom is bonded to two hydrogen atoms and a carbon atom by sigma ( σ ) bonds. The remaining outer p electrons of each carbon atom, overlap above and below the plane of the molecule, giving a localised pi (π) orbital. In benzene each carbon atom is bonded to two other carbon atoms and a hydrogen atom by sigma bonds. The fourth outer shell electron of a carbon atom is in a 2p orbital, above and below the plane of the carbon atoms. These p-orbitals overlap to give a delocalised electron structure, above and below the plane of carbon atoms. This delocalised electron π structure is often represented by a circle inside the ring hexagon. H H H H H H C C C C C C H H H H H H C C C C C C Compounds containing a single benzene ring represent the simplest type of aromatic system but there are a number of other aromatic ring systems, some with fused sides. These include naphthalene and anthracene. There are also aromatic compounds whose rings also contain atoms other than carbon, for example pyridine, C 5 H 5 N. naphthalene anthracene N pyridine If benzene underwent an addition reaction, the process would disrupt the stable delocalised electron system and the resulting product would be less stable. Benzene can be made to react by addition but more forcing conditions are needed. We have already seen that benzene can be hydrogenated to give cyclohexane but the reaction needs higher temperatures and a nickel or platinum catalyst. Benzene will also react with chlorine in an addition reaction to give hexachlorocyclohexane, C 6 H 6 Cl 6 . This is a radical reaction that needs bright sunlight to be effective. One isomer of formula C 6 H 6 Cl 6 , & S C Stretch & Challenge Ethene reacts with cold dilute aqueous potassium manganate(VII) to produce ethane-1,2-diol, CH 2 OHCH 2 OH. Suggest why benzene cannot react in a similar way to give the diol shown below. OH OH ethene