Aromatic

Question 1

Describe the shape of, and bonding in, a molecule of benzene

Answer 1

• Planar (Flat) ring of six C atoms
• Each carbon atom is bonded to one hydrogen atom and two other carbon atoms.
• The angle between all the bonds is 120.
• The unused 4th C electron is in the p orbitals, perpendicular to the plane of the ring
• The p orbitals overlap and the 6 electrons move freely within the overlapping orbitals
• there are delocalised electrons above and below the plane of benzene

Question 2

Explain why benzene does not readily undergo addition reactions

Answer 2

• -The delocalised electrons give stability to the benzene molecule that normal “double bonds” do not.
• -Addition reactions break the delocalised ring, so are not favoured.

Question 3

Explain why phenylamine is a weaker base than ethylamine

Answer 3

• The lone pair
• on the N is spread into the ring
• So it is less available for bonding with a proton

Question 4

Why does methylbenzene undergo faster substitution reactions than benzene

Answer 4

• 1 methyl group has (positive) inductive effect as it increases electron density on benzene ring
• 2 this makes the electrophile more attracted, making the Benzene ring a better nucleophile

Question 5

Why doesn’t benzene undergo nuc sub

Answer 5

• Delocalised structure repels nucleophile

Question 6

Explain the stability of benzene compared to hypothetical cyclohexatriene

• ΔH for cyclohexene= -120
• ΔH for Benzene= -208

Answer 6

• Benzene is more stable than cyclohexatriene
• Expected ΔH hydrogenation of triene is 3(–120) = –360 kJ mol
• Actual ΔH hydrogenation is 152 kJ mol-1 different from expected
• This means benzene is lower in energy
• Difference is due to delocalisation