Electrophilic Aromatic Substitution (EAS) and Benzene Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Benzene is aromatic: Planar, cyclic, conjugated ring with (4n+2) π electrons (Hückel’s Rule — n=1 gives 6 π electrons)
• EAS (Electrophilic Aromatic Substitution): The aromatic ring acts as a nucleophile; an electrophile (E⁺) attacks, then H⁺ is eliminated to restore aromaticity
• Directing Effects: Activating (ortho/para directors): -OH, -NH₂, -OCH₃ | Deactivating (meta directors): -NO₂, -CN, -COOH, -SO₃H
• Activating/Deactivating is measured by electron donation or withdrawal into/from the ring
• Halogenation: Br₂/FeBr₃ (or FeBr₃ alone) for bromination; Cl₂/AlCl₃ for chlorination
• ⚡ The key to mastering EAS is understanding why certain groups direct ortho/para vs meta — it always comes down to resonance stabilization of the carbocation intermediate

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Electrophilic Aromatic Substitution (EAS)

Aromatic compounds, particularly benzene and its derivatives, undergo characteristic electrophilic substitution reactions. Understanding EAS is crucial for pharmacy students — many drugs contain aromatic rings and undergo aromatic substitution in metabolism and synthesis.

Why Benzene is Unique — Aromaticity

Benzene’s stability and reactivity pattern stem from its aromatic character:

Hückel’s Rule: A cyclic, planar, conjugated molecule is aromatic if it has (4n+2) π electrons, where n = 0, 1, 2, 3…

For benzene:

• 6 carbon atoms in a planar ring
• Each carbon has one unhybridized p orbital perpendicular to the ring plane
• 6 π electrons fill the three bonding molecular orbitals (lowest energy)
• The result: exceptional stability (delocalization energy ~ 36 kcal/mol)

Key Consequences of Aromaticity:

• Benzene does NOT undergo addition reactions (would destroy aromaticity)
• Instead, benzene undergoes substitution reactions — electrophilic substitution preserves the aromatic ring
• When benzene is attacked by an electrophile, it temporarily loses aromaticity, but the subsequent elimination of a proton restores it — net result: substitution

The General EAS Mechanism

All EAS reactions follow the same two-step mechanism:

Step 1 — Electrophilic Attack (Rate-Determining): The π-electrons of the benzene ring attack the electrophile (E⁺), forming a resonance-stabilised arenium ion intermediate (sigma complex/whrilestone intermediate).

Step 2 — Deprotonation: A base (often the conjugate base of the Lewis acid catalyst) removes the proton from the sp³-hybridized carbon, restoring the aromatic sextet and forming the substituted benzene.

The Arenium Ion Intermediate

The sigma complex is the key intermediate in ALL EAS reactions. Its resonance stabilization determines how fast the reaction proceeds.

The positive charge in the arenium ion is delocalized over three positions — ortho and para positions relative to the site of attack bear partial positive charge.

Major EAS Reactions

1. Nitration

Reagents: Conc. HNO₃ + Conc. H₂SO₄ (creates the nitronium ion, NO₂⁺)

Mechanism: H₂SO₄ protonates HNO₃ → H₂O + NO₂⁺ (electrophile)

Product: Nitrobenzene; further nitration possible (dinitrobenzene)

Example Application: TNT (trinitrotoluene) synthesis; nitrobenzene in drug synthesis

2. Halogenation

Bromination: Br₂ + FeBr₃ (or just FeBr₃ catalyst)

• FeBr₃ acts as Lewis acid, polarising Br₂ to generate Br⁺

Chlorination: Cl₂ + AlCl₃

Fluorination: Requires special conditions (F₂ is very reactive) Iodination: I₂ + HNO₃ or peroxides; iodine itself is a weak electrophile

3. Friedel-Crafts Alkylation

Reagents: R–Cl + AlCl₃ (or other Lewis acid)

Electrophile: R⁺ (carbocation, generated by AlCl₃ extracting Cl⁻ from R–Cl)

Limitations:

• Rearrangements possible (1° → 2° or 3° carbocation)
• Polyalkylation common (once alkylated, benzene becomes more activated)
• Does not work with strongly deactivated rings (e.g., nitrobenzene)
• Anhydrous conditions essential

4. Friedel-Crafts Acylation

Reagents: (RCO)₂O or RCOCl + AlCl₃

Electrophile: RCO⁺ (acylium ion) — more stable than simple alkyl carbocation, so less rearrangement

Key Advantage: Only one acyl group adds (product is less activated than benzene itself, so no further acylation)

Product: Aromatic ketone — can be reduced to alkylbenzene using Clemmensen or Wolff-Kishner reduction

5. Sulfonation

Reagents: Conc. H₂SO₄ or SO₃/H₂SO₄

Product: Benzenesulfonic acid (–SO₃H)

Reversible: Can be removed by hydrolysis at high temperatures — useful for protecting positions in synthesis

6. Nitration (Detailed Mechanism)

Conc. H₂SO₄ protonates HNO₃: H₂SO₄ + HNO₃ → HSO₄⁻ + H₂O + NO₂⁺

NO₂⁺ (nitronium ion) attacks benzene to form the arenium ion → deprotonation → nitrobenzene.

Directing Effects of Substituents

When a benzene ring has an existing substituent, it determines where the new substituent attaches. This is the directing effect.

Activating Groups (Ortho/Para Directors)

Activating groups donate electron density into the ring, making it more reactive than benzene.

Strong Activators (–OH, –NH₂, –NHR, –OR):

• Direct ortho and para
• Resonance donation stabilizes the arenium ion intermediate when electrophile attacks ortho or para
• Example: Phenol → o-/p-bromination easily with Br₂ alone (no catalyst needed!)

Moderate Activators (–CH₃, –C₂H₅, alkyl groups):

• Direct ortho and para
• Hyperconjugation and inductive effect
• Example: Toluene nitrates faster than benzene; gives mostly p-nitrotoluene

Deactivating Groups (Meta Directors)

Deactivating groups withdraw electron density, making the ring less reactive than benzene.

Meta Directors:

• –NO₂, –CN, –COOH, –SO₃H, –CHO, –COR
• All electron-withdrawing by resonance and/or inductive effect
• Direct meta because ortho/para attack generates a resonance structure with the positive charge adjacent to the electron-withdrawing group (destabilizing!)
• Meta attack avoids this destabilization

Halogen as a Special Case

–Cl, –Br, –I are deactivating but ortho/para directing!

• They withdraw by induction (deactivating) but donate by resonance (ortho/para directing)
• Net effect: ring is less reactive but substitution occurs at ortho and para positions

Order of Directing Effects:

Ortho/Para Directors (activating): –O⁻ > –NR₂ > –OH > –OR > –NHR > –NH₂ > –R > –X Meta Directors (deactivating): –NO₂ > –CN > –SO₃H > –COOH > –CHO > –COR > –COOR

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Why Directing Effects Work — Resonance Explanation

Ortho/Para Direction by Activators (e.g., –OH on phenol):

When the electrophile attacks ortho or para to –OH, the resulting arenium ion has a resonance structure where the positive charge is on the oxygen — stabilized because oxygen can bear a positive charge (it’s the most electronegative atom). When the electrophile attacks meta, no such stabilization occurs. Hence ortho/para attack is favored.

Meta Direction by Deactivators (e.g., –NO₂ on nitrobenzene):

When the electrophile attacks ortho or para to –NO₂, one resonance structure places the positive charge on the carbon bearing the –NO₂ group. Since –NO₂ is strongly electron-withdrawing, this is highly destabilizing. Attack at meta avoids this — hence meta is favored.

Steric Effects

Even when ortho/para directing, bulky substituents favor para over ortho substitution:

• Toluene → mostly para (minor ortho) nitration
• tert-butylbenzene → almost exclusively para
• Rule of Thumb: Always some ortho/para mixture; para usually predominates due to steric hindrance at ortho

Poly-substitution — The Sequence Problem

When multiple substituents are present, priority rules apply:

1. Both directing same positions → reaction goes there
2. Directing to different positions → more activating group controls
3. Two activating groups: positions where both direct are favored
4. Activating + deactivating: Activating group controls, but deactivating group still influences (ortho/para to deactivating are disfavored)

Industrial and Pharmaceutical Applications

Aspirin (Acetylsalicylic Acid) Synthesis:

• Phenol → Salicylic acid (Kolbe-Schmitt carboxylation with NaOH + CO₂)
• Salicylic acid acetylated with acetic anhydride → Aspirin
• Note: The –OH activates ortho/para; Kolbe-Schmitt specifically gives ortho (major) due to specific reaction conditions

Sulfa Drugs:

• Sulfanilamide core involves aniline (–NH₂) chemistry
• –NH₂ is strongly activating and ortho/para directing
• Multiple steps involve EAS chemistry

Phenytoin, Phenobarbital: Contain aromatic rings that undergo Phase I metabolism via hydroxylation (not EAS, but understanding directing effects helps predict metabolic sites)

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