1. 5.3.1 Benzene
C6H6

2. C H

3. Aromatic Hydrocarbons
Aromatic = Contains a Benzene ring in its structure
Hydrocarbon = Made only of carbon and hydrogen.
Aliphatic = chains of carbons. [opposite of aromatic] [branched or unbranched]
We have to know about
[a] benzene,
[b] methyl benzene
[c] ethyl benzene

4. Benzene C6H6 Discovered by Michael Faraday in 1825
Chemical formula suggested it had to be highly unsaturated - 3 double bonds
Does not behave like normal unsaturated compounds -
i.e. does not decolourise bromine
Kekule suggested resonating or rotating alternate double and single bonds.
but it did not fit the facts

5. Kekule’s Structure of Benzene
Each C has 4 bonds
Each H has 1 bond
Bonding Rules Satisfied
Double bond moves round or
Double bond moves back and forward i.e. resonates

6. Benzene Bonds Bonds length measured by X-ray diffraction
C - C single bond is nm
[ 1 nm = 1*10-9m]
C = C is nm
C  C in Benzene is nm
NOT [0.144] half way between single and double bond length so resonating or rotating double bonds do not work

7. Delocalised Pi bonds C H
The 6 valence electrons are shared between the whole molecule
not localised into 3 double bonds
For convenience the C and H atoms are not shown
Ring in centre essential

8. Sigma and Pi Bonding

9. Two S orbitals overlap
Sigma Bond ơ

10. Two px orbitals overlap
Sigma Bond

11. Sigma Bonds are sausage shaped They occur when Two s orbitals overlap
Two Px orbitals overlap
Head on

12. As the two px orbitals overlap so do the two py orbitals if they are occupied

13. 
Py or Pz orbital
Py or Pz orbital
Pi Bond

14. Pi Bonds As the two px orbitals join [forming a sigma bond]
The two py orbitals overlap [if they are occupied]
They also form a modified electron cloud.
Shaped vaguely like the symbol π

15. Pi Bonds are shaped like the symbol  Twp Py orbitals overlap
Occur when
Twp Py orbitals overlap
Two Pz orbitals overlap
side on

16. Bond Construction Single bonds are Sigma bonds
Double bonds consist of one Sigma bond and one Pi Bond
Triple bonds consist of one Sigma Bond and two Pi Bonds

17. Benzene Derivatives
In benzene the ring of carbon atoms is called the nucleus
Any group attached to the nucleus is called a side chain
Methylbenzene (toluene)
TNT - trinitrotoluene
Ethylbenzene

18. Other Points about benzene
Benzene both Toxic and Carcinogenic
(Methylbenzene useful substitute in lab.)
Liquid at room temperature
Soluble in organic [non-polar] solvents
e.g. cyclohexane, propanone
Insoluble in water
Good solvent for organic substances
and covalent substances e.g. iodine
Increases octane number

19. Test What does aromatic mean? [2]
Give the “opposite” of aromatic: what does it mean? [2]
What 3 aromatic compounds do you know? [3]
Draw Kekule’s structure of benzene. [2]
Draw the modern structure of benzene. [2]
Draw the structure of two other aromatic compounds.[4]
Give two dangers of benzene. [2]
Describe with reasons the solubility of benzene. [6]
Give a use of benzene. [1]
What do you call the CH3 in methyl benzene? [1]
What does hydrocarbon mean? [2]
What does the ring represent? [2]
How can you show that benzene is not unsaturated [3]

20. Test What does aromatic mean? [2] contains benzene ring
What is the “opposite” of aromatic and what does it mean? [2] aliphatic / chain
What 3 aromatic compounds do you know? [3] benzene / methyl benzene / ethylbenzene
Draw Kekule’s structure of benzene. [2] single and double bonds alternate
Draw the modern structure of benzene. [2] hexagon with circle inside
Draw the structure of the other two aromatic compounds. [4] Methyl and ethylbenzene structures
Give two dangers of benzene. [2] poisonous / carcinogenic / combustible

21. Give two dangers of benzene. [2] poisonous / carcinogenic / combustible
Describe with reasons the solubility of benzene. [4] insoluble in water / water polar / benzene not polar / soluble in cyclohexane / both non-polar
Give a use of benzene. [1] solvent / petrol additive
What do you call the CH3 in methyl benzene? [1] side chain
What does hydrocarbon mean? [2] carbon and hydrogen / only
What does the ring represent? [2] delocalised / Pi Bond
How do we know benzene is not unsaturated [3] doesn’t turn bromine from red/ brown to colourless [27]