Topic 11.4 · Arenes

Arenes

1

Structure of Benzene

1.1Kekulé structure and the real structure

Kekulé structures (do not exist)≠real structure
ShapeC₆H₆: planar regular hexagon, all bond angles 120°
Bond lengthsAll C–C bonds are equal in length (0.139 nm), between C–C and C=C
Drawing itThe Kekulé structure (cyclohexatriene) does not exist, the real molecule is drawn with a circle

1.2Delocalisation in benzene

  1. Each C is sp² hybridised: it forms three σ bonds (two C–C and one C–H)
  2. The unhybridised p orbital on each C, containing one electron, is perpendicular to the plane of the ring
  3. Each p orbital overlaps side-on with both neighbours → a continuous ring of 6 delocalised π electrons above and below the ring
π cloud above the ringand below

1.3Why is benzene less reactive than an alkene towards electrophiles?

  1. In benzene, the π electrons are delocalised over six C atoms, so the electron density is lower than in the localised π bond of C=C
  2. → electrophiles are less strongly attracted, reaction also needs the loss of delocalisation in the intermediate → higher Ea, so a catalyst or stronger electrophile is needed
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2

Evidence for Delocalisation

2.1Enthalpy change of hydrogenation

Expected vs actual

Expected for 3 C=C: 3 × (−120) = −360 kJ mol⁻¹

Actual: −208 kJ mol⁻¹

Delocalisation energy

Benzene is 152 kJ mol⁻¹ more stable than cyclohexatriene.

−360expectedbenzene + 3H₂−208+152energycyclohexatriene + 3H₂cyclohexane

2.2Bond lengths

BondLength / nm
C–C in cyclohexane0.154
C–C in benzene0.139
C=C in cyclohexene0.133

The C–C bond length in benzene lies between a single and a double bond: each bond has partial double bond character, as the π electrons are delocalised over all six bonds.

2.3Substitution, not addition

ReagentCyclohexeneBenzene
Br₂ in CCl₄, darkadditionno reaction
HBr(g)additionno reaction

Addition would destroy the delocalised π system and its extra stability, substitution retains it.

3

Naming Arenes

NO₂nitrobenzene
CH₃methylbenzene
OHphenol
CO₂Hbenzoic acid
CHObenzaldehyde
CH₃NO₂2-nitro­methylbenzene
  • C₆H₅– is phenyl, C₆H₅CH₂– is benzyl. An alkyl group with more than 6 C: name as phenylalkane (2-phenylheptane).
  • Traditional name: some groups give the compound a special name, e.g. phenol (–OH), benzoic acid (–CO₂H), benzaldehyde (–CHO), methylbenzene (–CH₃). That group is at C1, number the other substituents from it and list them alphabetically.
Topic 11.4 · Arenes67
4

Electrophilic Substitution

The delocalised π cloud is a region of high electron density → attacked by electrophiles, substitution keeps the ring's stability.

4.1Mechanism: electrophilic substitution (nitration of benzene)

Reagents and conditions: conc. HNO₃, conc. H₂SO₄, 50 °C

HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺

NO₂⁺slow+HNO₂arenium ionfastNO₂+ H⁺nitrobenzene

H⁺ + HSO₄⁻ → H₂SO₄

Answer structureDescribe the mechanism Name it (electrophilic substitution), write the equation that forms the electrophile, draw every step perfectly with reagents and conditions, curly arrows, the arenium ion and charges, then write the equation that regenerates the catalyst. Write the equations only: no headings such as "forming the electrophile" and no written explanation.
ReactionReagents and conditionsElectrophile formed
Nitrationconc. HNO₃, conc. H₂SO₄, 50 °C (methylbenzene 30 °C, deactivated 70 °C)NO₂⁺
HalogenationX₂, anhydrous FeX₃ or AlX₃ (or Fe), r.t., darkX₂ + FeX₃ → X⁺ + FeX₄⁻
Friedel–Crafts alkylationRCl, anhydrous AlCl₃ or FeCl₃, r.t.RCl + AlCl₃ → R⁺ + AlCl₄⁻
Friedel–Crafts acylationRCOCl, anhydrous AlCl₃, heat under refluxRCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
NO₂nitrobenzeneClchlorobenzeneCH₃methylbenzene (R = CH₃)COCH₃phenylethanone (RCO = CH₃CO)

FeX₃ and AlX₃ are Lewis acids, regenerated at the end: H⁺ + FeX₄⁻ → HX + FeX₃. Anhydrous, as they react with water.

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5

Effect of Substituents

5.1Effect on reactivity

G23432

Ring-activating group, e.g. –CH₃

  1. –CH₃ is electron-donating: increases the electron density of the ring → ring more attractive to electrophiles
  2. and stabilises the carbocation intermediate → faster than benzene

Ring-deactivating group, e.g. –NO₂

  1. –NO₂ is electron-withdrawing: decreases the electron density of the ring → ring less attractive to electrophiles
  2. and destabilises the carbocation intermediate → slower than benzene

5.2Directing effect: where the second group goes

CH₃methylbenzeneconc. HNO₃, 30 °Cconc. H₂SO₄CH₃NO₂2-nitromethylbenzene+CH₃NO₂4-nitromethylbenzeneNO₂nitrobenzeneconc. HNO₃, 70 °Cconc. H₂SO₄NO₂NO₂1,3-dinitrobenzene
Activating, 2,4-directing–CH₃, –OH, –NH₂, –OR. Electron-donating, e.g. phenol + dil. HNO₃ → 2- and 4-nitrophenol
Deactivating, 3-directing–NO₂, –CN, –CO₂H, –CHO, –COR. Electron-withdrawing, slower than benzene
Deactivating, 2,4-directingHalogens: withdraw by induction, but the lone pair stabilises the 2,4-intermediates
6

Methylbenzene: Ring vs Side Chain

Reaction of methylbenzeneReagents and conditionsProductNotes
Ring: electrophilic substitutionCl₂, anhydrous AlCl₃, r.t., darkCH₃Clat the 2- and 4- positions (–CH₃ is 2,4-directing)
Side chain: free radical substitutionCl₂, UV lightCH₂Clfurther substitution gives –CHCl₂ and –CCl₃
Side-chain oxidationKMnO₄, dil. H₂SO₄, heat
or KMnO₄, OH⁻, heat, then H⁺		CO₂Hbenzoic acid (white ppt on cooling), needs a benzylic H

Any alkyl side chain with a benzylic H is oxidised to –CO₂H (ethylbenzene also gives CO₂). tert-Butylbenzene has no benzylic H: no reaction. K₂Cr₂O₇ is too weak.

Topic 11.4 · Arenes69