Aromatic chemistry

-each carbon atom in the ring forms three σ bonds using the sp2 orbitals -remaining p orbitals overlap laterally with p orbitals of neighbouring carbon atoms -forms a π system -results in the electrons being delocalised -can freely spread over the entire ring -π system is made up of two ring shaped clouds of electron density -one above the plane and one below it -aromatic compounds are regular and planar compounds with bond angles of 120 o -delocalisation of electrons means all of the C-C bonds are identical -have both single and double bond character -evidence for the delocalised ring structure of benzene

Delocalisation of p electrons

Problem with reactivity: -benzene undergoes substitution reactions, whereas other alkenes undergo addition reactions Problem with shape: -benzene = planar molecule -C-C single and double bonds should be different lengths C-C 0.154 nm C=C 0.134 nm -in benzene all the bonds are exactly the same (intermediate length between C-C and C=C at 0.139 nm) -hence it’s a perfectly regular hexagon Problems with stability: -benzene is stable due to its delocalised ring of electrons -forms due to C-C bonds being neither single nor double bonds -instead an intermediate length with electrons in the p-orbitals sticking out above and below the carbon ring -means that, compared to cyclohexa-1,3,5-triene, benzene has a much less exothermic enthalpy of hydrogenation (-208kJmol-1) -predicted: -360kJmol-1 -more energy is needed to break benzene’s bonds than cyclohexa-1,3,5-triene -hence more stable compound use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability

-structure is too stable -ring of delocalised electrons is electron-rich -attracts electron-deficient species