Aromatic Chemistry

Question 1

Why does benzene undergo electrophillic substitution reactions most commonly

Answer 1

It has a high electron density and attracts electrophiles

Question 2

Why does benzene not undergo addition reactions

Answer 2

These would involve breaking up the delocalised system

Question 3

Describe the bonding in benzene

Answer 3

• each carbon atom forms 3 covalent bonds
• the remaining p orbitals overlap above and below the ring to form a pi system
• the electrons in these p orbitals are delocalised

Question 4

Describe the shape of benzene

Answer 4

• benzene is planar
• the bond angles in benzene are 120°
• the C-C bonds are equal in length

Question 5

Describe the problem with benzenes reactions in kekule’s model

Answer 5

• it suggests benzene should undergo addition reactions with bromine but it doesn’t

Question 6

Describe the problem with benzenes bond lengths in kekule’s model

Answer 6

• it says benzene has two different C-C bond lengths
• but they are actually all the same length

Question 7

Describe the problem with benzenes enthalpy of hydrogenation in kekule’s model

Answer 7

• it says it should be very exothermic (-360)
• but it’s actually much less exothermic than expected

Question 8

Why is the enthalpy of hydrogenation of benzene less exothermic than theoretically

Answer 8

• there are 6 pi electrons which are delocalised and not arranged in 3 double bonds
• this means that it is more thermodynamically stable and has a less exothermic value

Question 9

What is the equation for the formation of an electrophile (in nitration)

Answer 9

HNO3 + 2H2SO4 —> +NO2 + 2HSO4- + H3O+