aromatic chemistry

Question 1

What is the molecular formula for benzene

Answer 1

C6H6

Question 2

Describe benzene’s bonding, shape & stability

Answer 2

1a) Each C has three (covalent) bonds
1b) Spare electrons (in a p orbital) overlap (to form a π cloud)
1c) delocalisation

2a) Planar
2b) Hexagon/6 carbon ring/120o bond angle
2c) C–C bonds equal in length / C–C bond lengths between single and double bond

3a) Expected ΔH hydrogen of cyclohexatriene = –360 kJ mol–1
3b) ΔH hydrogen benzene (is less exothermic) by 152 kJ mol–1
3c) Benzene lower in energy than cyclohexatriene / Benzene is more stable

Question 3

What is a phenyl group

Answer 3

In some compounds, the benzene ring is regarded as a substituent group

Question 4

Why is benzene not an alkene

Answer 4

• Bond lengths are an intermediate between (short) C=C and (long) C–C.
• ΔH hydrogenation less exothermic than expected (when
compared to ΔH hydrogenation for cyclohexene) because benzene is more stable.
• Benzene is less reactive than an alkene. It doesn’t undergo an addition reaction to decolourise bromine.
• Benzene undergoes substitution reactions in order to maintain the ring of delocalised electrons.

Question 5

Hydrogenase of benzene

Answer 5

If we do hydrogenate benzene only about -208 KJmol-1 is released, about 150 Kjmol-1 less exothermic than expected.

Benzene is about 150Kjmol-1 more stable than expected – this is what we call the delocalisation stabilisation of benzene.

Question 6

What are the electrons within benzene able to do

Answer 6

The electrons are free to move around the ring, and are said to be delocalised.