CD2 - Benzene + Delocalisation

Question 1

What evidence is the structure and bonding of benzene based from?

Answer 1

X-ray diffraction

produce contour map of electron density in benzene mol.

Question 2

What does anaysis of the contour map reveal about the features of benzene?

Answer 2

• regular, planar hexagon
• bond angles 120
• all C-C = same length

Question 3

Describe the delocalised model for benzene.

Answer 3

• each C atom = 4 outer electrons
• 3 electrons = single sigma-bonds to C or H - leaveing 1 p-electron on each C
• the 6 p-electrons delocalise + spread out evenly - shared by all C
• forms delocalised charge cloud ABOVE + BELOW

Question 4

As a general rule:

the more delocalised a structure is….

Answer 4

…the more stable the molecule becomes.

• delocalisation in benzene = strong –> stable (planar cyclic structure)

Question 5

Describe Kekule structure of benzene.

Answer 5

6 - C ring consisting of alternating double + single bonds

Question 6

Evidence to prove the Kekule structure wrong.

Answer 6

• 1 C=C bond - enthalpy change of hydrogenation = 120 kJ mol
• benzene expected to be 3x
• benzene actually = -208 kJ mol
• delocalisation in benzene = more stable –> so less energy released when it’s converted to cyclohexane
  ALSO
• single + double bonds = diff. lengths = irregular hexagon shape