M6 Aromatic Compounds Q&A
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Describe, using diagrams, the bonding and structure of benzene
What is the evidence for the delocalised model of benzene structure?
• The delocalised electrons make benzene resistant to reaction; when it does react it is by substitution rather than addition • It only reacts with bromine (or chlorine) in the presence of a halogen carrier • in benzene all the carbon-carbon bond lengths are equal and intermediate between single and double bonds • the enthalpy of hydrogenation is less exothermic than expected
Why is benzene less reactive than cyclohexene?
• Benzene has (6) delocalised pi electrons spread over the six carbon atoms • This has lower electron density • Cyclohexene has (2) localised pi electrons spread over the two carbon atoms • This has higher electron density • Electrophiles are less readily polarised and less strongly attracted to benzene
Why is phenol more reactive than benzene?
• In phenol the lone pair of electrons on the oxygen atom (p orbital) is donated • Becomes (partially) delocalised into the pi-system (8 electrons) • This has higher electron density (than benzene) • Electrophiles are more readily polarised and more strongly attracted to phenol • The pi-system is sufficiently electron dense to induce a dipole in electrophiles more (e.g. in the Br2 molecule) and so attracts electrophiles more strongly (more susceptible to electrophilic attack)
What is the mechanism of arenes?
• Electrophilic substitution