Arenes

H2 Chemistry (9476), JC1

By the end of this topic, you can answer

1

On paper, benzene has three C=C bonds. So why does it not react like an alkene?

2

All six C–C bonds in benzene have the same length. What does that tell us?

3

How do you name a benzene ring that carries two or three groups?

4

How do chemists attach nitro groups to a benzene ring, as in making TNT?

5

Why does methylbenzene nitrate faster than benzene, and why does chlorine attack its ring with a catalyst but its methyl group in sunlight?

Recall: what you know already

1From Introduction to Organic Chemistry: what is the hybridisation of each C in benzene?

sp², trigonal planar, 120°.[1]

2From Alkenes: what does an alkene do with Br₂ in CCl₄?

Electrophilic addition: orange Br₂ decolourises.[1]

3From Introduction to Organic Chemistry: what does delocalisation do to a molecule?

Spreads the π electrons over a larger region, giving extra stability.[1]
1

Structure of Benzene

1.1Kekulé structure and the real structure

  • C₆H₆: planar regular hexagon, all bond angles 120°
  • All C–C bonds are equal in length (0.139 nm), between C–C and C=C
  • The Kekulé structure (cyclohexatriene) does not exist, the real molecule is drawn with a circle
1

Structure of Benzene

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
2

Evidence for Delocalisation

2.1Enthalpy change of hydrogenation

PredictCyclohexene + H₂: ΔH = −120 kJ mol⁻¹. What would you expect for a ring with three C=C?

Expected for 3 C=C: 3 × (−120) = −360. Actual: −208 kJ mol⁻¹.

Benzene is 152 kJ mol⁻¹ more stable than cyclohexatriene: the delocalisation energy.

2

Evidence for Delocalisation

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.

2

Evidence for Delocalisation

Why 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, and reaction also needs the loss of delocalisation in the intermediate → higher Ea, so a catalyst or stronger electrophile is needed

Quick checkWhich is evidence that benzene is not cyclohexatriene?

AIt burns with a smoky flame.
BAll six C–C bonds are 0.139 nm.
CIt is a liquid at r.t.
DIts formula is C₆H₆.
NoteB: cyclohexatriene would have three short C=C and three long C–C bonds.
3

Naming Arenes

nitrobenzene
methylbenzene
phenol
benzoic acid
benzaldehyde
2-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.
4

Electrophilic Substitution

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

4.1Nitration of benzene

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

HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺

H⁺ + HSO₄⁻ → H₂SO₄

Exam tipBoth equations are part of the exam answer for the mechanism. Write the equations only, without headings such as "forming the electrophile".
4

Electrophilic Substitution

4.1Mechanism: electrophilic substitution (nitration of benzene)

Answer structureDescribe the mechanism Name it (electrophilic substitution), then draw every step perfectly with reagents and conditions, the electrophile, curly arrows, the arenium ion and charges. No written explanation is needed.
4

Electrophilic Substitution

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⁻

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

4

Electrophilic Substitution

Your turnGive the reagents and conditions to make (a) bromobenzene, (b) ethylbenzene, from benzene.

(a)
Br₂, anhydrous FeBr₃, room temperature, dark[1]
(b)
CH₃CH₂Cl, anhydrous AlCl₃, room temperature[1]

ChallengeWrite the equation that forms the electrophile in (a).

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻[1]
5

Effect of Substituents

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

Effect of Substituents

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

Quick checkNitrobenzene is nitrated again. Where does the second –NO₂ go?

Aposition 2
Bposition 3
Cposition 4
Dpositions 2 and 4
NoteB: –NO₂ is deactivating and 3-directing, giving 1,3-dinitrobenzene (and it needs about 70 °C).
6

Methylbenzene: Ring vs Side Chain

Reaction of methylbenzeneReagents and conditionsProductNotes
Ring: electrophilic substitutionCl₂, anhydrous AlCl₃, r.t., darkat the 2- and 4- positions (–CH₃ is 2,4-directing)
Side chain: free radical substitutionCl₂, UV lightfurther substitution gives –CHCl₂ and –CCl₃
Side-chain oxidationKMnO₄, dil. H₂SO₄, heatbenzoic 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.

6

Methylbenzene: Ring vs Side Chain

Your turnMethylbenzene reacts with Br₂. Give the conditions and organic product for (a) substitution on the ring, (b) substitution on the side chain.

(a)
Br₂, anhydrous FeBr₃, room temperature, in the dark[1]
Products: 2- and 4-bromomethylbenzene[1]
(b)
Br₂, UV light[1]
Product: (bromomethyl)benzene, C₆H₅CH₂Br[1]

Exit ticket

1Why does benzene undergo substitution, not addition?

Substitution keeps the delocalised π system and its extra stability: addition would destroy it.[1]

2What is the electrophile in nitration, and how is it formed?

NO₂⁺[1]
HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺[1]

3Why is methylbenzene nitrated faster than benzene?

–CH₃ is electron-donating: it increases the ring's electron density[1]
and stabilises the carbocation intermediate.[1]

4Why doesn't tert-butylbenzene react with hot acidified KMnO₄?

It has no benzylic H.[1]