Benzene

Question 1

give the formula for benzene

Answer 1

C6H6

Question 2

why was the bonding and structure of benzene a mystery to chemists (2 marks)

Answer 2

• it is unsaturated but doesn’t undergo addition reactions
• bond lengths are all equal due to delocalised system
• benzene is more thermodynamically stable than expected (enthalpy of hydrogenation less than expected so compound is more thermodynamically stable)

Question 3

how many delocalised pi orbitals are there in benzene

Answer 3

6

Question 4

describe the shape and bonding in a benzene molecule

Answer 4

• planar
• 120 degrees c-c bond angle
• c-c bond lengths are all the same
• single c-c (sigma) bonds
• delocalised Pi electrons above and below due to sideways overall of P orbitals

Question 5

Compare the stability of benzene with the hypothetical molecules hexa-1,3,5-triene. Use the data below in your answer (3 marks)

benzene
H = -120

hexa-1,3,5-triene
H = -208

Answer 5

predict 3 x -120 = -360
for benzene

benzene less exothermic because delocalised Pi electrons are stable and require more energy to break

Question 6

how is the delocalised ring in benzene formed

Answer 6

each carbon donates an electron which forms a delocalised ring