5.4Unit 5. Transition Metals and Organic Nitrogen Chemistry
Organic Chemistry: Arenes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
10 min read 21 parts Lesson 4 of 43 2 quizzes
Curriculum Equation Key Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Lesson summary
5.4 Organic Chemistry: Arenes — structure of benzene, electrophilic substitution and phenol — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Compare the Kekulé and delocalised models of benzene using evidence, describe the electrophilic substitution reactions and mechanisms of benzene, an…
- Saturation test
- Halogen carriers
- Alkylation
- Sulfonation
- Bromination
- Delocalisation
Part 1 of 10Structure of benzene
1
Kekulé proposed a hexagonal ring of six carbons with alternating single and double bonds,
- which suggests benzene should react like an alkene; it does not.
2
Delocalised model
- Each carbon uses sp2 orbitals to form three σ bonds (to two carbons and one hydrogen).
- The remaining p orbital on each carbon overlaps sideways with its neighbours to form a delocalised π system of six electrons.
- The π system is two ring-shaped clouds of electron density, one above and one below the plane.
3
Benzene is planar with bond angles of 120°, and all the C-C bonds are identical with character between single and double.
4
Arenes are aromatic hydrocarbons containing one or more benzene rings.
Part 2 of 10Evidence for delocalisation
Enthalpy of hydrogenation
- Cyclohexene (one C=C)C6H10 + H2 → C6H12, ΔH = −120 kJ mol−1.
- Kekulé benzene with three C=C would give 3 × (−120) = −360 kJ mol−1.
- Benzene actually gives −208 kJ mol−1, about 150 kJ mol−1 less exothermic, so it is more stable;
- this is the delocalisation energy.
Bond lengths from X-ray diffraction
- Cyclohexene has C-C 154 pm and C=C 134 pm, so a Kekulé ring would have two bond lengths.
- All benzene C-C bonds are about 140 pm, between the two.
Saturation test
- Saturation test benzene does not decolourise bromine water, so there are no localised C=C bonds.
Infrared spectroscopy
- Cyclohexene shows a C=C peak near 1650 cm−1.
- Benzene shows none there, but peaks near 1450, 1500 and 1580 cm−1, characteristic of arene C=C.
Only one isomer of 1,2-dibromobenzene exists, which a Kekulé structure would not predict.
Part 3 of 10Resistance to bromination
1
An alkene's localised π bond is an area of high electron density that polarises Br2, giving Brδ+ that attacks, then Br− adds: electrophilic addition.
2
Benzene has no localised high electron density, so it cannot polarise Br2 and does not react at room temperature.
3
Addition would destroy the delocalisation, so substitution is favoured.
4
A halogen carrier (e.g. AlBr3) must be present for substitution to occur.
Part 4 of 10Combustion and types of reaction
1
2C6H6 + 15O2 → 12CO2 + 6H2O.
2
Benzene burns with a smoky yellow flame because it has a high carbon:hydrogen ratio and oxygen is insufficient for complete combustion.
3
The main reactions replace one hydrogen: nitration, halogenation, Friedel-Crafts alkylation and acylation, and sulfonation.
4
These are electrophilic substitution reactions; electrophiles must be strong because delocalisation makes the ring less reactive than an alkene.
Part 5 of 10General mechanism of electrophilic substitution
- The delocalised π system is a region of high electron density attracting electrophiles, which are produced in situ.
1
Generate the electrophile.
2
Electrophilic attack: a pair of electrons from the ring forms a bond to the electrophile, leaving a cation intermediate with delocalisation partly lost.
3
Restore aromaticity: the C-H bond breaks heterolytically, its electrons returning to the ring, and H+ is lost.
4
Regenerate the catalyst.
- Drawing the mechanism
- Start the curly arrow from within the ring and point it to the electrophile, showing its + charge.
- Draw the intermediate as a horseshoe over at least three (ideally five) carbons, facing the tetrahedral carbon, with the + inside it.
- Start the final curly arrow from the C-H bond and point it into the ring.
Part 6 of 10Nitration
C6H6 + HNO3 → C6H5NO2 + H2O, giving nitrobenzene.
Reagents
- Reagents concentrated HNO3 and concentrated H2SO4, heated under reflux at 55–60 °C.
- (below 55 °C reduces multiple substitution)
Electrophile NO2+ (nitronium ion):
- HNO3 + H2SO4 → NO2+ + HSO4− + H2O.
- Or HNO3 + 2H2SO4 → NO2+ + 2HSO4− + H3O+.
The H+ lost from the intermediate reforms the catalyst: H+ + HSO4− → H2SO4.
Part 7 of 10Halogenation
C6H6 + Br2 → C6H5Br + HBr, using a halogen carrier and heat under reflux.
Halogen carriers
- AlBr3, AlCl3, FeCl3, or iron which forms FeBr3 with bromine; the carrier must match the halogen.
The halogen donates a lone pair into the empty 3p orbital of the carrier, generating the electrophile.
- Br2 + FeBr3 → Br+ + FeBr4−.
- Cl2 + AlCl3 → Cl+ + AlCl4−.
The catalyst is regenerated: H+ + FeBr4− → FeBr3 + HBr.
Part 8 of 10Friedel-Crafts reactions and sulfonation
Friedel-Crafts reactions substitute an alkyl or acyl group for a hydrogen, so arenes can be used as starting materials for synthesis.
Alkylation
- RCl with anhydrous AlCl3; the electrophile is R+, giving an alkylbenzene.
Acylation
- RCOCl with anhydrous AlCl3, heated under reflux; the electrophile is the acylium ion RCO+.
- C6H6 + CH3COCl → C6H5COCH3 + HCl.
- CH3COCl + AlCl3 → CH3CO+ + AlCl4−.
- H+ + AlCl4− → HCl + AlCl3.
- Acylation forms a C-C bond and gives a phenyl ketone.
Anhydrous conditions are needed because AlCl3 is hydrolysed by water.
Sulfonation
- Sulfonation warm benzene with fuming sulfuric acid (SO3 dissolved in concentrated H2SO4) at about 40 °C for about 30 minutes;
- an -SO3H group replaces a hydrogen.
| Reaction | Reagents / conditions | Electrophile | Product |
|---|---|---|---|
| Nitration | Conc. HNO3 + conc. H2SO4, 55-60 °C | NO2+ | Nitrobenzene |
| Halogenation | Br2 with FeBr3 (or Fe) | Br+ | Bromobenzene + HBr |
| Friedel-Crafts alkylation | RCl, AlCl3, anhydrous | R+ | Alkylbenzene |
| Friedel-Crafts acylation | RCOCl, AlCl3, anhydrous, reflux | RCO+ | Phenyl ketone |
Part 9 of 10Phenol
Phenol, C6H5OH, has an OH group attached directly to the ring.
Phenols react more readily with electrophiles than benzene.
- A lone pair on oxygen overlaps with the π system.
- The ring has higher electron density, so it polarises Br2 without a catalyst.
- The OH group is activating and directs incoming electrophiles to the 2, 4 and 6 positions.
Bromination
- Bromination phenol with bromine water at room temperature decolourises the orange solution and forms a white precipitate of 2,4,6-tribromophenol.
- C6H5OH + 3Br2 → C6H2Br3OH + 3HBr.
Part 10 of 10More from the specification
More from the specification
Delocalised Pi Bonding System in Benzene
- Benzene contains a planar ring of six sp2 hybridized carbon atoms where unhybridised p-orbitals overlap sideways to form a continuous delocalised pi bonding system above and below the ring plane.
Bond Equivalence and Length in Arenes
- Unlike Kekulé's alternating single and double bonds, all carbon-carbon bonds in aromatic compounds have an identical intermediate length of 0.139 nm, intermediate between single (0.154 nm) and double (0.134 nm) carbon-carbon bonds.
Thermodynamic Stability and Enthalpy of Hydrogenation
- The experimental enthalpy change of hydrogenation of benzene (-208 kJ mol-1 is significantly less exothermic than the theoretical value for cyclohexa-1,3,5-triene (-360 kJ mol-1, proving benzene has 152 kJ mol-1 of delocalisation resonance stability.
Electrophilic Substitution in Aromatic Compounds
- Aromatic compounds preferentially undergo electrophilic substitution reactions rather than addition, maintaining the stable delocalised pi electron cloud while replacing a hydrogen atom on one of the ring carbon atoms.
Compound
- Two or more elements chemically combined in a fixed ratio.
Bonding
- The joining of atoms.
Enthalpy change
- The thermal energy transferred during a reaction, ΔH.
Aromatic compound
- A compound with a benzene-type ring
Key terms
| Term | Meaning |
|---|---|
| Arene | An aromatic hydrocarbon containing one or more benzene rings. |
| Delocalisation | The spreading of π electrons over several atoms, here all six carbons of the ring. |
| Delocalisation energy | The extra stability of benzene compared with the Kekulé structure, about 150 kJ mol−1. |
| Electrophile | A species that accepts a pair of electrons to form a covalent bond. |
| Electrophilic substitution | A reaction in which an electrophile replaces a hydrogen on an aromatic ring. |
| Halogen carrier | A Lewis acid such as FeBr3 or AlCl3 that generates the halogen electrophile. |
| Nitronium ion | The electrophile NO2+, formed from concentrated nitric and sulfuric acids. |
| Friedel-Crafts reaction | Alkylation or acylation of an arene using AlCl3. |
Avoid these mistakes
1
Saying benzene contains three localised C=C bonds — the π electrons are delocalised over the ring.
2
Drawing the horseshoe too small or not facing the tetrahedral carbon — it must cover at least three carbons and face the carbon bearing H and the substituent.
3
Saying benzene undergoes addition — substitution preserves the delocalisation.
4
Using wet conditions for Friedel-Crafts — AlCl3 is hydrolysed by water.
5
Using the wrong halogen carrier — it must match the halogen.
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