Benzene Structure
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What is an arene
Hydrocarbons with a ring of carbon atoms in which there are delocalised electrons
Molecular formula of benzene
C6H6
What did Kekulé propose about benzene
That it is an unsaturated compound with three double bonds
Problems with Kekulé’s structure
-absence of isomers of 1,2-dichlorobenzene, should be two with one having Cl linked to single bond and other to double bond but never been possible to separate these isomers—->suggested benzene molecules may alternate rapidly between the two possible structures -X-ray diffraction data shows all bonds are same length but single and double are dif lengths -does not decolourise bromine water or take part in addition reactions with normal electrophiles -study of enthalpy changes of hydrogenation show it is more stable than expected for Kekulé’s formula
How was it found that benzene is more stable than expected by its enthalpy change of hydrogenation
-enthalpy of hydrogenation of cyclohexene x 3 is -360kJ/mol which is expected for benzene but is -208kJ/mol so is less exo
What does infrared data for benzene show
Does not have typical strong absorptions for alkanes or alkenes, has one different to both
What structure of benzene was later proposed
Six p electrons are delocalised above and below the ring by overlapping p orbitals, gives rise to circular clouds of negative charge above and below ring of carbon atoms
Why is benzene more stable than expected and what does this mean
Delocalisation of p orbitals above and below the plane of the ring stabilise it meaning more energy required to break the bonds in benzene compared with three separate pi bonds
How many sigma bonds does each carbon atom form and what happens to the 4th
-each forms 3 sigma bonds -fourth electron in p orbital which overlaps with those of the other carbon atoms to form delocalised pi ring system
Is electron density of benzene ring system higher or lower than alkene’s and what does this mean
Lower, does not attract electrophiles as easily as electron density not exposed so undergoes electrophilic substitution rather than addition
Why do arenes have high mps but low bps
High mps due to high stability of delocalised benzene ring, low bps as they are non-polar so only weak London forces between molecules