STRUCTURE OF BENZENE

Question 1

Explain how the enthalpy change of hydrogenation disproves the kekules model and is evidence for the delocalised model

Answer 1

• when cyclohexane is hydrogenated its enthalpy change is 120kjmol
• So if benzene had 3 double bonds then you would expect the enthalpy change to be theoretically 360kjmol
• However the enthalpy change of hydrogenation of Benzene is actually 208kjmol
• This is far less exothermic than expected
• ​The real structure of benzene is 152kjmol more stable than the kekules structure containing C=C bonds.

Question 2

what was the problem with the kekules model of benzene

Answer 2

• C=C double bonds were present in the kekules model then Benzene would undergo electrophilic addition.
• Each C=C double bond would be expected to react with bromine water and decolourise it
• in kekules model the carbon to carbon bonds were different,single bonds had a bond length of 147pm and double bonds a bond length of 135pm
• This was disproved by X-ray diffraction studies which actually showed that the carbon to carbon bonds were all the same with a bond length of 140pm

Question 3

what type of reaction takes place between alkenes and bromine

Answer 3

electrophilic addition

Question 4

explain what you would expect to see if cyclohexane reacted with bromine

Answer 4

decolourise it

Question 5

Describe the kekules model of benzene

Answer 5

• Benzene is a 6 carbon hexagonal ring
• with alternating double and single bonds of different bond length
• he later suggested that the benzene was in equilibrium between two isomers