Benzene and Aromatic Compounds

Question 1

what was the experimental evidence for a delocalised model rather than Kekule?

Answer 1

• Kekule’s model was less reactive than alkenes
• length of carbon-carbon bonds in benzene are the same
• benzene’s enthalpy change of hydrogenation is less exothermic than expected

Question 2

explain how benzene is less reactive than alkenes.

Answer 2

• kekule’s model contains 3 C=C bonds, which should decolourise bromine in an electrophilic addition
• but benzene doesn’t undergo electrophilic additions
• benzene doesn’t decolourise bromine
• bromination of benzene requires a halogen carrier

= led scientists to believe that benzene cannot contain C=C bonds

Question 3

explain why the length of C-C bonds in benzene(Kekule) led to it being disapproved.

Answer 3

• all C-C bonds in benzene were the same length of 0.139nm
• bond length was between C-C which 0.153nm and C=C bonds whcih is 0.134nm

Question 4

explain why the hydrogenation enthalpies of benzene(Kekule) led to it being disapproved.

Answer 4

• if benzene was kekules model then we’d expect the enthalpy change of hydrogenation to be 3 times that of cyclohexene as it contains 3 double bonds
• enthalpy change of hydrogenation of cyclohexene = -119kJmol-1
• Kekule’s predicted = -357kJmol-1
• actual benzene structure was more stable and less exothermic and the enthalpy change of hydrogenation of benzene is only -208kJmol-1

Question 5

what is the delocalised model of benzene?

Answer 5

• contains 6 hydrogen and carbon atoms
• carbon has 4 outer shell electrons = 3 of these form sigma bonds, other 1 exists in a p-orbital at right angles to the plane of the ring and above and below the plane of the ring, this forms a ring of electron density
• trigonal planar, 120

Question 6

what does the overlapping of p-orbitals in delocalised model of benzene create?

Answer 6

a system of pi bonds which is spread over all of the 6 carbon atoms
- six electrons occupying this system of pi bonds are delocalised

Question 7

what do the delocalised electrons cause in the delocalised model?

Answer 7

• stablility
• as charge is spread over the entire benzene molecule