Benzene Structure

Question 1

What is an arene

Answer 1

Hydrocarbons with a ring of carbon atoms in which there are delocalised electrons

Question 2

Molecular formula of benzene

Answer 2

C6H6

Question 3

What did Kekulé propose about benzene

Answer 3

That it is an unsaturated compound with three double bonds

Question 4

Problems with Kekulé’s structure

Answer 4

-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

Question 5

How was it found that benzene is more stable than expected by its enthalpy change of hydrogenation

Answer 5

-enthalpy of hydrogenation of cyclohexane x 3 is -360kJ/mol which is expected for benzene but is -208kJ/mol so is less exo

Question 6

What does infrared data for benzene show

Answer 6

Does not have typical strong absorptions for alkanes or alkenes, has one different to both

Question 7

What structure of benzene was later proposed

Answer 7

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

Question 8

Why is benzene more stable than expected

Answer 8

Delocalisation of p orbitals above and below the plane of the ring stabilise it

Question 9

How many sigma bonds does each carbon atom form and what happens to the 4th

Answer 9

-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

Question 10

Is electron density of benzene ring system higher or lower than alkene’s and what does this mean

Answer 10

Lower, does not attract electrophiles as easily as electron density not exposed so undergoes electrophilic substitution rather than addition

Question 11

Why do arenes have high mps but low bps

Answer 11

High mps due to high stability of delocalised benzene ring, low bps as they are non-polar so only weak London forces between molecules