25.1 Benzene

Question 1

what is Benzene’s molecular formula?

Answer 1

C6H6

Question 2

what are the features of the kekule model?

Answer 2

ring of 6 carbon atoms joined together by alternating double and single bonds

Question 3

what evidence is there to disprove the kekule model?

Answer 3

• lack of reactivity of Benzene
• lengths of C-C bonds
• hydrogenation enthalpies

Question 4

lack of reactivity of Benzene

Answer 4

according to Kekule’s model, Benzene has C=C bonds. therefore it should be able to decolourise bromine water

however it does not
it does not undergo electrophilic addition

Question 5

lengths of C-C bonds are the same

Answer 5

in Benzene the lengths of the bonds between carbon atoms are the same therefore it is not possible to have 2 different bond types

Question 6

hydrogenation enthalpies

Answer 6

if we follow Kekule’s model, Benzene should have a hydrogenation enthalpy which is 3 times greater than the enthalpy for cyclohex-1-ene

however Benzene’s enthalpy is less exothermic than this value

Question 7

why is Benzene stable?

Answer 7

it has delocalised pi system which is spread across all 6 carbon atoms

Question 8

what is the delocalised model?

Answer 8

p orbitals of all 6 carbons overlap to create a delocalised pi system above and below the plane

Question 9

what are the exceptions to the Benzene naming system?

Answer 9

• benzoic acid
• benzaldehyde
• phenylamine