Arenes: properties, reactions

Arenes: Properties, Reactions and Aromaticity

1. Aromaticity – definition and criteria

• Aromatic compound: a cyclic, planar, fully conjugated system that follows Hückel’s rule.
• Hückel’s rule: the molecule must contain 4n + 2 π‑electrons (n = 0, 1, 2,…).
• Structural requirements:
  1. Cyclic – atoms form a closed ring.
  2. Planar – all ring atoms lie in the same plane so that each p‑orbital can overlap with its neighbours, giving a continuous delocalised π‑system.
  3. Conjugated – a continuous chain of p‑orbitals around the ring (every atom in the ring contributes one p‑orbital).
• Applying Hückel’s rule (examples):
  • Benzene (C₆H₆): 6 π e⁻ → 4(1)+2 → aromatic.
  • Pyridine (C₅H₅N): 6 π e⁻ (5 C π + 1 N π) → aromatic.
  • Pyrrole (C₄H₅N): 6 π e⁻ (4 C π + 2 N π) → aromatic.
  • Furan (C₄H₄O) and Thiophene (C₄H₄S): 6 π e⁻ → aromatic.
  • Cyclooctatetraene (C₈H₈): 8 π e⁻ → 4(2) → anti‑aromatic (but adopts a non‑planar “tub” conformation to avoid anti‑aromaticity).

2. Aromatic Stabilisation Energy (ASE)

• ASE = the extra stability of an aromatic system compared with a hypothetical non‑conjugated reference.
• How it is measured: compare the experimental heat of hydrogenation of the aromatic with the sum of three isolated C=C hydrogenations (≈ ‑120 kJ mol⁻¹). The difference is the ASE.
• Typical values (derived from hydrogenation data):

  Compound	ΔHhydrogenation (kJ mol⁻¹)	ASE (kJ mol⁻¹)
  Benzene	‑208 ± 2	≈ ‑36
  Naphthalene	‑332 ± 3	≈ ‑61
  Anthracene	‑425 ± 4	≈ ‑73
  Phenanthrene	‑421 ± 4	≈ ‑71

• ASE increases with the size of the conjugated system but not linearly – the extra rings share the delocalisation.

3. Anti‑aromatic compounds

• Must be cyclic, planar and fully conjugated **and** contain 4n π‑electrons.
• Result: severe destabilisation; compounds are usually highly reactive and difficult to isolate.
• Typical examples:
  • Cyclobutadiene (C₄H₄) – 4 π e⁻, n = 1.
  • Pentalene (C₈H₆) – 8 π e⁻, n = 2 (exists only as a transient species).
• Destabilisation is explained by the inability to achieve a closed‑shell, fully delocalised π‑system; the molecules adopt distorted geometries or react instantly to relieve anti‑aromaticity.

4. Other aromatic systems

4.1 Poly‑cyclic aromatic hydrocarbons (PAHs)

• Reactivity trend: the most reactive positions are the “outer” (peri) carbons of the terminal rings because they possess the highest electron density in the resonance forms.
• Physical properties (representative):

  Compound	Melting point (°C)	Boiling point (°C)	Solubility in water
  Benzene	5.5	80.1	~1.8 g L⁻¹
  Naphthalene	80.2	218	≈ 0.03 g L⁻¹
  Anthracene	216	340	≈ 0.001 g L⁻¹

  The increase in melting/boiling points reflects stronger London dispersion forces as the π‑surface grows.

4.2 Hetero‑aromatics

• The orientation of the hetero‑atom lone pair determines whether it participates in the aromatic sextet:
  • In pyridine the lone pair lies in the ring plane → it does **not** contribute to the π‑system, making the ring electron‑deficient and meta‑directing.
  • In pyrrole, furan and thiophene the lone pair occupies a p‑orbital → it **does** contribute, donating electron density and giving ortho/para activation.

5. Physical properties of simple arenes (benzene and related)

• Low polarity of benzene explains its limited solubility in water and its relatively low boiling point compared with more polar arenes.
• Hydrogenation requires harsh conditions (high temperature/pressure) because of aromatic stabilisation.

6. Electronic effects and directing groups

• Why –NO₂ is meta‑directing:
  1. The nitro group withdraws electron density by –I and –R.
  2. If electrophilic attack occurs at the ortho or para position, a resonance form places a positive charge directly on the nitrogen, which is highly destabilising.
  3. Attack at the meta position avoids this unfavorable resonance, so the σ‑complex is lower in energy.

7. Electrophilic Aromatic Substitution (EAS)

7.1 General mechanism (curved‑arrow notation)

1. Generation of the electrophile (e.g., NO₂⁺ from HNO₃/H₂SO₄, Cl⁺ from Cl₂/FeCl₃, acylium ion R‑C≡O⁺ from RCOCl/AlCl₃).
2. π‑Complex formation (σ‑complex or arenium ion):
   • The aromatic π‑bond donates a pair of electrons to the electrophile (curved arrow from the ring to the electrophile).
   • The resulting σ‑complex bears a positive charge delocalised over three carbon atoms (draw all resonance forms).
3. De‑protonation:
   • A base (often the conjugate base of the acid catalyst, e.g., HSO₄⁻) removes the hydrogen attached to the carbon that received the electrophile (curved arrow from C‑H bond to the base).
   • Aromaticity is restored.

Rate‑determining step (RDS): formation of the σ‑complex. Electron‑donating groups stabilise this intermediate, accelerating the reaction; electron‑withdrawing groups destabilise it, slowing the reaction.

7.2 Comparison: Friedel–Crafts Alkylation vs. Acylation

• Alkylation generates a carbocation (R⁺) which can undergo rearrangements (hydride or alkyl shifts) before attacking the ring, often giving a mixture of products.
• Acylation forms an acylium ion (R‑C≡O⁺) that is resonance‑stabilised (R‑C⁺≡O ↔ R‑C=O⁺). It does **not** rearrange, giving a single ketone product. The carbonyl group is deactivating, preventing poly‑substitution.

7.3 Representative EAS reactions (balanced equations)

• Halogens are deactivating because the –I effect outweighs the weak +R donation, yet they are ortho/para‑directing because the lone pair can donate after the σ‑complex is formed, stabilising the intermediate.
• Strong deactivators (e.g., –NO₂) normally prevent further EAS unless a more powerful electrophile (e.g., nitronium ion generated in super‑acidic media) is used.

8. Regio‑selectivity in substituted aromatic rings

• Activating, ortho/para directors (–OH, –OCH₃, –NH₂, –CH₃):
  • Electrophilic attack occurs preferentially at the positions that give the most stabilised σ‑complex (ortho and para).
  • Steric hindrance may suppress ortho substitution, giving a higher para proportion.
• Deactivating, meta directors (–NO₂, –CN, –COOH, –SO₃H):
  • Electrophilic attack at ortho or para positions would place a positive charge adjacent to the electron‑withdrawing group, which is highly destabilising.
  • Meta attack avoids this, so meta products dominate.
• Halogen substituents:
  • Deactivating by –I, but ortho/para directing because the halogen’s lone pair can donate to the σ‑complex after the electrophile has attached.

9. Reduction of arenes

9.1 Catalytic hydrogenation

• Typical conditions: Pd/C, PtO₂ or Raney Ni; 150–200 °C; 5–10 atm H₂ (higher pressures for less activated rings).
• Reaction: C₆H₆ + 3 H₂ → C₆H₁₂ (cyclohexane).
• Requires harsh conditions because the aromatic ASE must be overcome.

9.2 Birch reduction (dissolving‑metal reduction)

• Reagents & conditions: Na (or Li) metal in liquid NH₃ (–78 °C) with a proton donor (usually EtOH or t‑BuOH).
• Outcome: 1,4‑dihydro‑benzene (non‑conjugated diene). Example:

  C₆H₆ + 2 Na + 2 EtOH → C₆H₈ (1,4‑dihydrobenzene) + 2 NaOEt

• Regio‑selectivity:
  • Electron‑donating groups (–OR, –R) direct the double bonds **away** from the substituent; the reduced positions are meta to the EDG.
  • Electron‑withdrawing groups (–COOR, –CN, –NO₂) direct the double bonds **toward** the substituent; the reduced positions are ortho and para to the EWG.

  Example: p‑methoxy‑benzene → double bonds appear at the 3‑ and 5‑positions (meta to –OCH₃); p‑nitro‑benzene → double bonds at the 2‑ and 4‑positions (ortho/para to –NO₂).

10. Industrial and practical relevance

• Aniline production – Nitration of benzene → nitrobenzene → catalytic reduction (Fe/HCl) → aniline. Aniline is the precursor for dyes, rubber chemicals and nylon‑6,6.
• Cumene process – Friedel‑Crafts alkylation of benzene with propylene → cumene; oxidation of cumene gives phenol and acetone (major industrial route to phenol).
• Sulfonated polymers – Benzenesulfonic acid is a key intermediate for detergents, sulfonated polystyrene resins and ion‑exchange membranes.
• Pharmaceuticals & agro‑chemicals – Many active ingredients (e.g., paracetamol, herbicide atrazine) contain hetero‑aromatic rings; controlling directing effects is essential for multi‑step syntheses.
• Petrochemical aromatics – Catalytic reforming of n‑alkanes produces a mixture of benzene, toluene and xylenes (BTX) that serve as feedstocks for countless downstream processes.
• Environmental relevance – PAHs are persistent pollutants; understanding their reactivity helps in designing remediation strategies.

11. Summary – key take‑aways

• Aromaticity requires a cyclic, planar, fully conjugated system with 4n + 2 π‑electrons; it confers a measurable stabilisation energy (ASE).
• Anti‑aromatic compounds contain 4n π‑electrons and are highly unstable.
• PAHs and hetero‑aromatics obey the same rules, but the orientation of hetero‑atom lone pairs determines whether they donate to the π‑system and how they direct electrophilic substitution.
• Electronic effects (inductive and resonance) control both the rate of EAS and the orientation of new substituents; –NO₂ is meta‑directing because ortho/para σ‑complexes place a positive charge on the nitro nitrogen.
• EAS proceeds via electrophile generation → σ‑complex formation (rate‑determining) → de‑protonation; alkylation can give carbocation rearrangements, whereas acylation gives a stable acylium ion and avoids poly‑substitution.
• Reduction of aromatic rings is difficult: catalytic hydrogenation needs high temperature/pressure; Birch reduction offers a milder, regio‑selective alternative.
• Industrial processes exploit these reactions for large‑scale manufacture of aniline, phenol, detergents, polymers and many pharmaceuticals.