1. A guide for A level students KNOCKHARDY PUBLISHING
THE CHEMISTRY
OF ARENES
A guide for A level students
2015 SPECIFICATIONS
KNOCKHARDY PUBLISHING

2. 12.2C: Aromatic compounds - understand electrophilic reactions - understand and draw the bonding in benzene - understand using thermochemical evidence, the additional stability conferred by the structure - understand and be able to explain why the structure causes benzene and associated compounds to undergo electrophilic reactions - know and understand a general mechanism for electrophilic substitution - understand the mechanism of nitration and the importance of nitration in synthesis and know some compounds for which nitration is an important precursor - understand Friedel-Crafts acylation and its importance is synthesis

3. STRUCTURE OF BENZENE
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.

4. THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED
When unsaturated hydrocarbons are reduced (hydrogen added) 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

5. HYBRIDISATION OF ORBITALS - REVISION1 1s 2 2s 2p
The electronic configuration of a carbon atom is 1s22s22p2
If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 1 1s 2 2s 2p
The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

6. HYBRIDISATION OF ORBITALS - REVIEW
The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.
sp3 HYBRIDISATION
2s22p2
2s12p3
4 x sp3
PROMOTE
HYBRIDISE

7. HYBRIDISATION OF ORBITALS - REVIEW
Alternatively, only three orbitals (an s and two p’s) HYBRIDISE to give three new orbitals. The remaining 2p orbital is unchanged.
sp2 HYBRIDISATION
2s22p2
2s12p3
3 x sp2 2p
UNHYBRIDISED
PROMOTE
HYBRIDISE

8. DELOCALISATION
Instead of three localised (in one position) and 3 double bonds, the p electrons making up
those double 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.

9. STRUCTURE OF BENZENE
ANIMATION

10. WHY SUBSTITUTION?
Theory Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Overall there is ELECTROPHILIC SUBSTITUTION
STABLE DELOCALISED SYSTEM
ELECTRONS ARE NOT DELOCALISED
AROUND THE WHOLE RING - LESS STABLE

11. 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

12. 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.

13. 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

14. 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.

15. 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

16. 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)

17. FURTHER SUBSTITUTION OF ARENES
RELATIVE POSITIONS ON A BENZENE RING 1 1 1 2 3 4
1,2-DICHLOROBENZENE
ortho dichlorobenzene
1,3-DICHLOROBENZENE
meta dichlorobenzene
1,4-DICHLOROBENZENE
para dichlorobenzene
The compounds have similar chemical properties but different physical properties
They are STRUCTURAL ISOMERS

18. 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 6 2 6 2 5 3 5 3 4 4

19. FURTHER SUBSTITUTION OF ARENES
Examples Substitution of nitrobenzene is...
• more difficult than with benzene
• produces a 1,3 disubstituted product

20. FURTHER SUBSTITUTION OF ARENES
Examples Substitution of methylbenzene is…
• easier than with benzene
• produces a mixture of 1,2 and 1,4
isomeric products

21. FURTHER SUBSTITUTION OF ARENES
Examples Some groups (OH) make substitution so much
easier that multiple substitution takes place

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THE CHEMISTRY
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