Topic 18 – Organic Chemistry III
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include investigating the reactions of different functional groups, preparing an aromatic ester such as methyl benzoate, making nylon,
purifying an organic solid. Mathematical skills that could be developed in this topic include calculating the resonance stability of benzene from
thermodynamic data, calculating percentage yields. Within this topic, students can consider how the model for benzene structure has developed in
response to new evidence. By this stage, their continuing practical experience should enable them to use techniques to carry out reactions and purify
products efficiently and safely.
Topic 18A: Arenes - benzene
1. understand that the bonding in benzene has been represented using the Kekulé and the delocalised
model, the latter in terms of overlap of p-orbitals to form π-bonds
Describe how the orbitals from carbon atoms interact to form the bonds in a benzene ring.
Head-on overlap of orbitals of neighbouring carbon atoms forms the sigma bonds. The remaining p orbitals overlap
sideways, causing electrons to delocalise around the ring.
2. understand that evidence for the delocalised model of the bonding in benzene is provided by data from
enthalpy changes of hydrogenation and carbon-carbon bond lengths
Just a note from Edexcel – Students may represent benzene’s structure as: or: as
appropriate in equations and mechanisms.
Benzene is sometimes represented by the Kekulé structure, shown on the right above. What structural feature of
benzene shows that this is not an accurate representation?
In this diagram, all carbon-carbon bonds are the same length. Longer C-C and shorter C=C bonds are not present.
Suggest why the ‘delocalised’ model of benzene is more common among scientists, given that ΔH cyclohexene = -
0120kJ/mol and ΔHbenzene = -208kJ/mol.
Kekulé’s benzene, with three C=C bonds, should have a ΔHhydrogenation that is three times the value of cyclohexane’s,
since it has to react wth 3 moles of H2 rather than just 1 in cyclohexene’s case. However, it is 152kJmol -1 less than
expected (calculated as 360) – and so is 152kJmol-1 more stable.
3. understand why benzene is resistant to bromination, compared with alkenes, in terms of delocalisation of
π-bonds in benzene and the localised electron density of the π-bond in alkenes
Explain why benzene will not willingly decolourise bromine water (as other alkenes would).
The delocalisation of the pi-bond electrons in benzene make a particularly stable structure, giving an even electron
density above and below the planar carbon structure – hence bromine will not readily react with benzene in
electrophilic addition. Conversely, alkene pi-bonds cause a high local electron density that is susceptible to attack
from electrophiles.
4. understand the reactions of benzene with:
i oxygen in air (combustion with a smoky flame)
State the reaction of the complete combustion of benzene. Explain why a smoky flame is observed.
C6H6 + (15/2)O2 ---> 6CO2 + 3H2O The flame is smoky due to the high proportion of carbon
needing a high proportion of oxygen for complete combustion.
, ii bromine, in the presence of a catalyst
Give the reaction between benzene and bromine in the presence of AlBr 3.
Br2 + AlBr3 ----> Br+ + AlBr4-; the electrophile Br+ now reacts with benzene as below:
The AlCl3
then regenerates
as follows; H+ +
AlBr4- ----> AlBr3 +
HBr.
iii a mixture of concentrated nitric and sulfuric acids
State the reaction between nitric and sulfuric acid to form the nitronium ion (NO 2+).
HNO3 + 2H2SO4 ----> NO2+ + 2HSO4- + H3O+
Hence, draw the mechanism for the reaction between the nitronium ion and benzene.
In the reaction between nitric + sulfuric acid, which acid acts as a base? State the reaction conditions.
The sulfuric acid protonates the nitric acid, which acts as a base. Heat at 50oC.
iv halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts reaction)
What conditions are required for alkylation and acylation?
Alkylation occurs at rtp. Acylation needs to be heated under reflux at 60 oC.
5. understand the mechanism of the electrophilic substitution reactions of benzene
(halogenation, nitration and Friedel-Crafts reactions), including the generation of the electrophile (out/in)
One method for the production of phenylethanone involves the reaction of benzene with ethanoyl chloride.
Write the equation for the formation of the electrophile using a suitable catalyst.
CH3COCl + AlCl3 ----> AlCl4- + CH3CO+
Hence show the mechanism for the reaction, including curly arrows, between this electrophile and benzene to
produce phenylethanone. Include the regeneration of the catalyst.
6. understand the reaction of phenol with bromine water
2
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include investigating the reactions of different functional groups, preparing an aromatic ester such as methyl benzoate, making nylon,
purifying an organic solid. Mathematical skills that could be developed in this topic include calculating the resonance stability of benzene from
thermodynamic data, calculating percentage yields. Within this topic, students can consider how the model for benzene structure has developed in
response to new evidence. By this stage, their continuing practical experience should enable them to use techniques to carry out reactions and purify
products efficiently and safely.
Topic 18A: Arenes - benzene
1. understand that the bonding in benzene has been represented using the Kekulé and the delocalised
model, the latter in terms of overlap of p-orbitals to form π-bonds
Describe how the orbitals from carbon atoms interact to form the bonds in a benzene ring.
Head-on overlap of orbitals of neighbouring carbon atoms forms the sigma bonds. The remaining p orbitals overlap
sideways, causing electrons to delocalise around the ring.
2. understand that evidence for the delocalised model of the bonding in benzene is provided by data from
enthalpy changes of hydrogenation and carbon-carbon bond lengths
Just a note from Edexcel – Students may represent benzene’s structure as: or: as
appropriate in equations and mechanisms.
Benzene is sometimes represented by the Kekulé structure, shown on the right above. What structural feature of
benzene shows that this is not an accurate representation?
In this diagram, all carbon-carbon bonds are the same length. Longer C-C and shorter C=C bonds are not present.
Suggest why the ‘delocalised’ model of benzene is more common among scientists, given that ΔH cyclohexene = -
0120kJ/mol and ΔHbenzene = -208kJ/mol.
Kekulé’s benzene, with three C=C bonds, should have a ΔHhydrogenation that is three times the value of cyclohexane’s,
since it has to react wth 3 moles of H2 rather than just 1 in cyclohexene’s case. However, it is 152kJmol -1 less than
expected (calculated as 360) – and so is 152kJmol-1 more stable.
3. understand why benzene is resistant to bromination, compared with alkenes, in terms of delocalisation of
π-bonds in benzene and the localised electron density of the π-bond in alkenes
Explain why benzene will not willingly decolourise bromine water (as other alkenes would).
The delocalisation of the pi-bond electrons in benzene make a particularly stable structure, giving an even electron
density above and below the planar carbon structure – hence bromine will not readily react with benzene in
electrophilic addition. Conversely, alkene pi-bonds cause a high local electron density that is susceptible to attack
from electrophiles.
4. understand the reactions of benzene with:
i oxygen in air (combustion with a smoky flame)
State the reaction of the complete combustion of benzene. Explain why a smoky flame is observed.
C6H6 + (15/2)O2 ---> 6CO2 + 3H2O The flame is smoky due to the high proportion of carbon
needing a high proportion of oxygen for complete combustion.
, ii bromine, in the presence of a catalyst
Give the reaction between benzene and bromine in the presence of AlBr 3.
Br2 + AlBr3 ----> Br+ + AlBr4-; the electrophile Br+ now reacts with benzene as below:
The AlCl3
then regenerates
as follows; H+ +
AlBr4- ----> AlBr3 +
HBr.
iii a mixture of concentrated nitric and sulfuric acids
State the reaction between nitric and sulfuric acid to form the nitronium ion (NO 2+).
HNO3 + 2H2SO4 ----> NO2+ + 2HSO4- + H3O+
Hence, draw the mechanism for the reaction between the nitronium ion and benzene.
In the reaction between nitric + sulfuric acid, which acid acts as a base? State the reaction conditions.
The sulfuric acid protonates the nitric acid, which acts as a base. Heat at 50oC.
iv halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts reaction)
What conditions are required for alkylation and acylation?
Alkylation occurs at rtp. Acylation needs to be heated under reflux at 60 oC.
5. understand the mechanism of the electrophilic substitution reactions of benzene
(halogenation, nitration and Friedel-Crafts reactions), including the generation of the electrophile (out/in)
One method for the production of phenylethanone involves the reaction of benzene with ethanoyl chloride.
Write the equation for the formation of the electrophile using a suitable catalyst.
CH3COCl + AlCl3 ----> AlCl4- + CH3CO+
Hence show the mechanism for the reaction, including curly arrows, between this electrophile and benzene to
produce phenylethanone. Include the regeneration of the catalyst.
6. understand the reaction of phenol with bromine water
2
