1. Benzene
Part 1

2. Starter: name the functional group.1 2 3 4 5

3. Learning outcomes Define arene and aromatic Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions occurs with different electrophiles

4. Arene and aromatic Look at page 4
Write your definition of Arene and Aromatic

5. Benzene Isolated by Faraday in 1825 Physical properties:
Clear, colourless, hydrocarbon
92.3% Carbon, 7.7% hydrogen.
0.250g was vaporised at 100oC had a volume of 98cm3
Boiling point 80oC, melting point 5oC

6. Calculate the empirical formula
92.3% Carbon, 7.7% hydrogen.

7. Element C H % Ar
Moles
Ratio
empirical formula CH

8. Calculate the molecular formula
1 mole of gas is 24dm3 at RTP
0.250g was vaporised at 100oC had a volume of 98cm3.
From this it was calculated that the molecular mass was 78g
So what is the molecular formula?

9. The relative molecular mass of the sample is 78
The molecular formula78/13 = 6
Therefore the molecular formula is C6H6

10. Molymods Find as many structures as possible for C6H6
Draw them in your notes

11. Problem Doesn’t react like an alkene- no reaction with HBr
Doesn’t undergo electrophilic addition
Enthalpy change should be about 800 kJmol-1, where as it is only 207kJmol-1

12. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

13. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l) 2 3
- 120 kJ mol-1

14. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
- 120 kJ mol-1

15. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

16. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
It is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

17. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
It is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

18. Kekulé 1865

19. To explain the above, it was suggested that the structure oscillated
STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected
To explain the above, it was suggested that the structure oscillated
between the two Kekulé forms but was represented by neither of
them. It was a RESONANCE HYBRID.

20. Next: X-ray diffraction
Bond type
Structure
Bond length
C C
Cyclohexane
0.15
Benzene
0.14
cyclohexene
0.13

21. STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system
This final structure was particularly stable and
resisted attempts to break it down through normal
electrophilic addition. However, substitution of any
hydrogen atoms would not affect the delocalisation.

22. STRUCTURE OF BENZENE

23. Exam question
In this question, one mark is available for the quality of spelling, punctuation and grammar.
Describe with the aid of suitable diagrams the bonding and structure of a benzene molecule.

26. Homework
Produce a leaflet or a poster showing where benzene is used and hence why it is important. Due Friday 10th September

27. Learning outcomes Define arene and aromatic Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions occurs with different electrophiles

28. Naming aromatic compounds
Phenol
Chlorobenzene
Methylbenzene
Many of these compounds are foul smelling and toxic, they are still called aromatic

29. When a long alkyl chain with other substitutions is present, think of the benzene as substituted onto the chain, using phenyl and a number to position the chain.
CH3– CH – CH – CH3 Cl
2-Chloro-3-phenylbutane

30. Naming dominos

31. Identify the following molecules as alkene, arene or cycloaklane
CH3CH2CH2CH2CH3
C6H5CH3
CH3CH=CHCH2CH3
CH3CH(CH3)CH2CH3

32. Identify the following molecules as alkene, arene, alkane or cycloalkane
CH3CH2CH2CH2CH3 alkane
C6H5CH3 arene
CH3CH=CHCH2CH3 alkene
CH3CH(CH3)CH2CH3 alkane
cycloalkane

33. Learning outcomes Define arene and aromatic Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions occurs with different electrophiles

34. ELECTROPHILIC SUBSTITUTION
Theory The high electron density of the ring makes it open to attack by electrophiles
Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Because the mechanism involves an initial disruption to the ring,
electrophiles must be more powerful than those which react with alkenes
Overall there is ELECTROPHILIC SUBSTITUTION

35. ELECTROPHILIC SUBSTITUTION
Theory The high electron density of the ring makes it open to attack by electrophiles
Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Because the mechanism involves an initial disruption to the ring,
electrophiles must be more powerful than those which react with alkenes
Overall there is ELECTROPHILIC SUBSTITUTION
Mechanism
• a pair of electrons leaves the delocalised system to form a bond to the electrophile
• this disrupts the stable delocalised system and forms an unstable intermediate
• to restore stability, the pair of electrons in the C-H bond moves back into the ring
• overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION

36. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene

37. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene
Mechanism

38. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene
Mechanism
Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO HNO HSO4¯ H3O NO2+
acid base

39. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene
Mechanism
Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO HNO HSO4¯ H3O NO2+
acid base
Use The nitration of benzene is the first step in an historically important chain of
reactions. These lead to the formation of dyes, and explosives.

40. ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION
Reagents chlorine and a halogen carrier (catalyst)
Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3)
chlorine is non polar so is not a good electrophile
the halogen carrier is required to polarise the halogen
Equation C6H Cl2 ———> C6H5Cl HCl
Mechanism
Electrophile Cl+ it is generated as follows...
Cl FeCl FeCl4¯ Cl+ a
Lewis Acid

41. Now your turn Write the mechanisims for the following electrophiles:
CH3CO+

42. Where does the electrophile end up?
Groups with +I
Groups with a - I
Mostly 2, 4 and 6 positions
Mostly with 3 and 5 positions OH Cl
CH3
COOH
NH2
NO2

43. + I effect
- I effect

44. Learning outcomes Define arene and aromatic Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions occurs with different electrophiles

45. Diploma students only

46. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Overview Alkylation involves substituting an alkyl (methyl, ethyl) group
Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3
Conditions room temperature; dry inert solvent (ether)
Electrophile a carbocation ion R+ (e.g. CH3+)
Equation C6H C2H5Cl ———> C6H5C2H HCl
Mechanism
General A catalyst is used to increase the positive nature of the electrophile
and make it better at attacking benzene rings.
AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.

47. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Catalyst anhydrous aluminium chloride acts as the catalyst
the Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS ACID
it increases the polarisation of the C-Cl bond in the haloalkane
this makes the charge on C more positive and the following occurs
RCl AlCl AlCl4¯ R+

48. FRIEDEL-CRAFTS REACTIONS - INDUSTRIAL ALKYLATION
Industrial Alkenes are used instead of haloalkanes but an acid must be present
Phenylethane, C6H5C2H5 is made by this method
Reagents ethene, anhydrous AlCl3 , conc. HCl
Electrophile C2H5+ (an ethyl carbonium ion)
Equation C6H C2H4 ———> C6H5C2H5 (ethyl benzene)
Mechanism the HCl reacts with the alkene to generate a carbonium ion
electrophilic substitution then takes place as the C2H5+ attacks the ring
Use ethyl benzene is dehydrogenated to produce phenylethene (styrene);
this is used to make poly(phenylethene) - also known as polystyrene

49. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATION
Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) group
Reagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3
Conditions reflux 50°C; dry inert solvent (ether)
Electrophile RC+= O ( e.g. CH3C+O )
Equation C6H CH3COCl ———> C6H5COCH HCl
Mechanism
Product A carbonyl compound (aldehyde or ketone)

50. FURTHER SUBSTITUTION OF ARENES
Theory It is possible to substitute more than one functional group.
But, the functional group already on the ring affects...
• how easy it can be done • where the next substituent goes
Group ELECTRON DONATING ELECTRON WITHDRAWING
Example(s) OH, CH NO2
Electron density of ring Increases Decreases
Ease of substitution Easier Harder
Position of substitution 2,4,and and 5