5.4Unit 5. Transition Metals and Organic Nitrogen Chemistry
Organic Chemistry: Arenes — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions
10 min read 17 parts Lesson 5 of 8 1 quiz
Lesson summary
5.4 Organic Chemistry: Arenes — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Explain the structure, stability and electrophilic substitution reactions of benzene and compare phenol with benzene.Kekulé structure — a hexagonal ring of six carbons with alternating single and double bonds,…
- Planar ring
- Identical bonds
- Enthalpy of hydrogenation
- Bond lengths (X-ray diffraction)
- Saturation test
- Infrared spectroscopy
Section 1 of 17Structure of benzene
Structure of benzene
Kekulé structure
- Kekulé structure a hexagonal ring of six carbons with alternating single and double bonds, which would suggest it reacts like an alkene.
- Benzene does not behave like an alkene.
Planar ring
- Planar ring each carbon is sp2 hybridised and forms three σ bonds with bond angles of 120°.
π system
- π system the remaining p orbitals overlap sideways around the ring to form delocalised electrons.
- They form two ring-shaped clouds of electron density, one above and one below the plane.
Identical bonds
- Identical bonds all C-C bonds are the same length with character between single and double.
Section 2 of 17Evidence for delocalisation
Evidence for delocalisation
Enthalpy of hydrogenation
- Enthalpy of hydrogenation cyclohexene gives -120 kJ mol-1.
- Kekulé benzene would give 3 x -120 = -360 kJ mol-1.
- Actual benzene gives -208 kJ mol-1, far less exothermic, so benzene is more stable than expected.
Bond lengths (X-ray diffraction)
- C-C is 154 pm and C=C is 134 pm in cyclohexene.
- All benzene C-C bonds are 140 pm, intermediate in length and equal.
Saturation test
- Saturation test cyclohexene decolourises bromine water by electrophilic addition but benzene does not, so it has no localised C=C bonds.
Infrared spectroscopy
- Infrared spectroscopy cyclohexene shows C=C near 1650 cm-1.
- Benzene shows no peak there but shows peaks near 1450, 1500 and 1580 cm-1 characteristic of aromatic C=C.
Resistance to bromination
- Resistance to bromination a localised π bond has high electron density that polarises Br2.
- Benzene has no localised region of high electron density, so it cannot polarise Br2 and needs a halogen carrier such as AlBr3.
Section 3 of 17Reactions of benzene
Combustion
- 2C6H6 + 15O2 → 12CO2 + 6H2O.
- Benzene burns with a smoky, sooty yellow flame because oxygen is insufficient for complete combustion.
Halogenation
- C6H6 + X2 → C6H5X + HX using a halogen carrier such as FeBr3 or AlCl3.
- The carrier acts as a catalyst and creates the electrophile X+, for example FeBr3 + Br2 → FeBr4- + Br+.
Nitration
- C6H6 + HNO3 → C6H5NO2 + H2O with concentrated HNO3 and concentrated H2SO4 at 25-60 °C.
- Under reflux at 55-60 °C.
Friedel-Crafts reactions
- Friedel-Crafts reactions substitute an alkyl or an acyl group for a hydrogen on the ring.
- Alkylation uses a halogenoalkane such as CH3Cl with AlCl3, giving methylbenzene.
- Acylation uses an acyl chloride with AlCl3.
- Substituted arenes are useful starting materials because aromatic stability makes benzene itself unreactive.
Sulfonation
- Sulfonation benzene is warmed at about 40 °C for about 30 minutes with fuming sulfuric acid (SO3 dissolved in concentrated H2SO4).
- It gives the sulfonic acid C6H5SO3H.
Section 4 of 17Electrophilic substitution: general mechanism
1Substitution
- Substitution one H on the ring is replaced by the electrophile, unlike alkene addition where the double bond breaks.
2Electrophile
- Electrophile a positive ion or the positive end of a polar molecule.
- The delocalised π system is a region of high electron density.
- The electrophile cannot simply be added, so it is generated in situ from reagents in the mixture.
3Three stages
- Three stages generation of the electrophile, electrophilic attack and regeneration of aromaticity.
4Electrophilic attack
- Electrophilic attack a pair of electrons from the ring forms a covalent bond to the electrophile.
- Aromaticity is lost, leaving four π electrons and a positive charge over five carbons.
5Restoring aromaticity
- Restoring aromaticity the C-H bond breaks heterolytically and the bonding pair returns to the π system as H+ leaves.
Section 5 of 17Drawing the mechanism correctly
1Initial arrow
- Initial arrow starts on or within the ring and points to the electrophile, which carries a + charge.
2Intermediate
- Intermediate drawn as a horseshoe facing the tetrahedral carbon bonded to H and the new group.
- (incomplete ring)
- The tetrahedral (sp3) carbon carries both H and the substituent.
3Horseshoe size
- Horseshoe size covers at least three, ideally the five, unsubstituted carbons.
4Charge
- Charge the + sign is inside the horseshoe.
5Final step
- Final step the arrow starts from the C-H bond and points into the ring.
Section 6 of 17Nitration and halogenation mechanisms
1Nitration electrophile
- NO2+ from concentrated HNO3 and concentrated H2SO4.
- HNO3 + H2SO4 → NO2+ + HSO4- + H2O.
- Or HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+.
2Nitration overall
- C6H6 + HNO3 → C6H5NO2 + H2O under reflux at 55-60 °C.
- H+ + HSO4- → H2SO4, so sulfuric acid is a catalyst.
3Halogenation electrophile
- X+ formed when a halogen donates a lone pair from one atom into the empty 3p orbital of the halogen carrier by a dative bond.
- Carriers are AlBr3, AlCl3 and FeCl3, or iron plus bromine, which forms FeBr3.
- The carrier must match the halogenBr-Br + AlBr3 → Br+ + AlBr4- and Cl-Cl + FeCl3 → Cl+ + FeCl4-.
4Halogenation overall
- C6H6 + Br2 → C6H5Br + HBr under reflux with the carrier.
- H+ + AlBr4- → HBr + AlBr3 regenerates the carrier.
Section 7 of 17Friedel-Crafts acylation mechanism
Friedel-Crafts acylation mechanism
Overall
- C6H6 + CH3COCl → C6H5COCH3 + HCl.
- Conditions are anhydrous AlCl3 and heating under reflux.
Electrophile
- Electrophile the acylium ion CH3CO+, formed when AlCl3 accepts a lone pair from chlorine.
- CH3COCl + AlCl3 → CH3CO+ + AlCl4-.
Attack
- Attack the curly arrow points to the positively charged carbon of the acylium ion.
Regenerating the catalyst
- H+ + AlCl4- → HCl + AlCl3.
Acyl group
- Acyl group an alkyl group containing a C=O group.
Section 8 of 17Phenol compared with benzene
Reactivity
- Reactivity phenol reacts more readily with electrophiles because a lone pair on the oxygen of the OH group overlaps with the π system.
- This raises the electron density of the ring.
Directing effect
- Directing effect the OH group is activating and directs incoming electrophiles to the 2, 4 and 6 positions.
| Feature | Benzene | Phenol |
|---|---|---|
| Ring electron density | delocalised ring only | raised by overlap of an O lone pair with the ring |
| Bromination | needs a halogen carrier | bromine water at room temperature |
| Product with bromine | bromobenzene | white precipitate of 2,4,6-tribromophenol and the orange colour is decolourised |
| Nitration | concentrated HNO3 / H2SO4 | dilute HNO3 |
Section 9 of 17Electrophilic substitution: general mechanism
1Substitution
- Substitution one H on the ring is replaced by the electrophile, unlike alkene addition where the double bond breaks.
2Electrophile
- Electrophile a positive ion or the positive end of a polar molecule.
- The delocalised π system is a region of high electron density.
- The electrophile cannot simply be added, so it is generated in situ from reagents in the mixture.
3Three stages
- Three stages generation of the electrophile, electrophilic attack and regeneration of aromaticity.
4Electrophilic attack
- Electrophilic attack a pair of electrons from the ring forms a covalent bond to the electrophile.
- Aromaticity is lost, leaving four π electrons and a positive charge over five carbons.
5Restoring aromaticity
- Restoring aromaticity the C-H bond breaks heterolytically and the bonding pair returns to the π system as H+ leaves.
Section 10 of 17Drawing the mechanism correctly
1Initial arrow
- Initial arrow starts on or within the ring and points to the electrophile, which carries a + charge.
2Intermediate
- Intermediate drawn as a horseshoe facing the tetrahedral carbon bonded to H and the new group.
- (incomplete ring)
- The tetrahedral (sp3) carbon carries both H and the substituent.
3Horseshoe size
- Horseshoe size covers at least three, ideally the five, unsubstituted carbons.
4Charge
- Charge the + sign is inside the horseshoe.
5Final step
- Final step the arrow starts from the C-H bond and points into the ring.
Section 11 of 17Nitration and halogenation mechanisms
1Nitration electrophile
- NO2+ from concentrated HNO3 and concentrated H2SO4.
- HNO3 + H2SO4 → NO2+ + HSO4- + H2O.
- Or HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+.
2Nitration overall
- C6H6 + HNO3 → C6H5NO2 + H2O under reflux at 55-60 °C.
- H+ + HSO4- → H2SO4, so sulfuric acid is a catalyst.
3Halogenation electrophile
- X+ formed when a halogen donates a lone pair from one atom into the empty 3p orbital of the halogen carrier by a dative bond.
- Carriers are AlBr3, AlCl3 and FeCl3, or iron plus bromine, which forms FeBr3.
- The carrier must match the halogenBr-Br + AlBr3 → Br+ + AlBr4- and Cl-Cl + FeCl3 → Cl+ + FeCl4-.
4Halogenation overall
- C6H6 + Br2 → C6H5Br + HBr under reflux with the carrier.
- H+ + AlBr4- → HBr + AlBr3 regenerates the carrier.
Section 12 of 17Friedel-Crafts acylation mechanism
Friedel-Crafts acylation mechanism
Overall
- C6H6 + CH3COCl → C6H5COCH3 + HCl.
- Conditions are anhydrous AlCl3 and heating under reflux.
Electrophile
- Electrophile the acylium ion CH3CO+, formed when AlCl3 accepts a lone pair from chlorine.
- CH3COCl + AlCl3 → CH3CO+ + AlCl4-.
Attack
- Attack the curly arrow points to the positively charged carbon of the acylium ion.
Regenerating the catalyst
- H+ + AlCl4- → HCl + AlCl3.
Acyl group
- Acyl group an alkyl group containing a C=O group.
Section 13 of 17Reactions of benzene
Combustion
- 2C6H6 + 15O2 → 12CO2 + 6H2O.
- Benzene burns with a smoky, sooty yellow flame because oxygen is insufficient for complete combustion.
Halogenation
- C6H6 + X2 → C6H5X + HX using a halogen carrier such as FeBr3 or AlCl3.
- The carrier acts as a catalyst and creates the electrophile X+, for example FeBr3 + Br2 → FeBr4- + Br+.
Nitration
- C6H6 + HNO3 → C6H5NO2 + H2O with concentrated HNO3 and concentrated H2SO4 at 25-60 °C.
- Under reflux at 55-60 °C.
Friedel-Crafts reactions
- Friedel-Crafts reactions substitute an alkyl or an acyl group for a hydrogen on the ring.
- Alkylation uses a halogenoalkane such as CH3Cl with AlCl3, giving methylbenzene.
- Acylation uses an acyl chloride with AlCl3.
- Substituted arenes are useful starting materials because aromatic stability makes benzene itself unreactive.
Sulfonation
- Sulfonation benzene is warmed at about 40 °C for about 30 minutes with fuming sulfuric acid (SO3 dissolved in concentrated H2SO4).
- It gives the sulfonic acid C6H5SO3H.
Section 14 of 17Evidence for delocalisation
Evidence for delocalisation
Enthalpy of hydrogenation
- Enthalpy of hydrogenation cyclohexene gives -120 kJ mol-1.
- Kekulé benzene would give 3 x -120 = -360 kJ mol-1.
- Actual benzene gives -208 kJ mol-1, far less exothermic, so benzene is more stable than expected.
Bond lengths (X-ray diffraction)
- C-C is 154 pm and C=C is 134 pm in cyclohexene.
- All benzene C-C bonds are 140 pm, intermediate in length and equal.
Saturation test
- Saturation test cyclohexene decolourises bromine water by electrophilic addition but benzene does not, so it has no localised C=C bonds.
Infrared spectroscopy
- Infrared spectroscopy cyclohexene shows C=C near 1650 cm-1.
- Benzene shows no peak there but shows peaks near 1450, 1500 and 1580 cm-1 characteristic of aromatic C=C.
Resistance to bromination
- Resistance to bromination a localised π bond has high electron density that polarises Br2.
- Benzene has no localised region of high electron density, so it cannot polarise Br2 and needs a halogen carrier such as AlBr3.
Section 15 of 17Phenol compared with benzene
Reactivity
- Reactivity phenol reacts more readily with electrophiles because a lone pair on the oxygen of the OH group overlaps with the π system.
- This raises the electron density of the ring.
Directing effect
- Directing effect the OH group is activating and directs incoming electrophiles to the 2, 4 and 6 positions.
| Feature | Benzene | Phenol |
|---|---|---|
| Ring electron density | delocalised ring only | raised by overlap of an O lone pair with the ring |
| Bromination | needs a halogen carrier | bromine water at room temperature |
| Product with bromine | bromobenzene | white precipitate of 2,4,6-tribromophenol and the orange colour is decolourised |
| Nitration | concentrated HNO3 / H2SO4 | dilute HNO3 |
Section 16 of 17Key terms
| Term | Meaning |
|---|---|
| Electrophile | A positive ion or the positive end of a polar molecule that accepts a pair of electrons. |
| Electrophilic substitution | A reaction in which an electrophile replaces a hydrogen atom on the ring. |
| Horseshoe | The incomplete ring drawn in the electrophilic substitution intermediate. |
| Halogen carrier | A catalyst such as AlCl3, FeCl3 or FeBr3 used to generate X+. |
| Acylium ion | The RCO+ electrophile in Friedel-Crafts acylation. |
| Delocalisation | Spreading of π electrons over the whole ring, giving equal bonds and extra stability. |
| Aromaticity | The special stability of a planar ring with delocalised π electrons. |
| Term | Meaning |
|---|---|
| Electrophile | A positive ion or the positive end of a polar molecule that accepts a pair of electrons. |
| Electrophilic substitution | A reaction in which an electrophile replaces a hydrogen atom on the ring. |
| Horseshoe | The incomplete ring drawn in the electrophilic substitution intermediate. |
| Halogen carrier | A catalyst such as AlCl3, FeCl3 or FeBr3 used to generate X+. |
| Acylium ion | The RCO+ electrophile in Friedel-Crafts acylation. |
| Delocalisation | Spreading of π electrons over the whole ring, giving equal bonds and extra stability. |
| Aromaticity | The special stability of a planar ring with delocalised π electrons. |
Section 17 of 17Exam technique
1
Drawing a complete circle with a + sign in the intermediate — use a horseshoe covering at least three carbons with the + inside.
2
Forgetting the + on the electrophile or starting the curly arrow outside the ring.
3
Not regenerating the catalyst at the end — H⁺ reforms H₂SO₄, AlBr₃ or AlCl₃.
4
Saying benzene undergoes addition like an alkene — it undergoes substitution to keep its delocalised ring.
1
Drawing a complete circle with a + sign in the intermediate — use a horseshoe covering at least three carbons with the + inside.
2
Forgetting the + on the electrophile or starting the curly arrow outside the ring.
3
Not regenerating the catalyst at the end — H⁺ reforms H₂SO₄, AlBr₃ or AlCl₃.
4
Saying benzene undergoes addition like an alkene — it undergoes substitution to keep its delocalised ring.
Marked practiceQuizzes for this lesson
Last updated: