Aromaticity and Electrophilic Aromatic Substitution (EAS)

Core Review

Aromaticity Criteria (All four must be met)

1. Cyclic: The molecule must be a ring.
2. Planar: All atoms in the ring system must be sp² hybridized (or nearly so) for continuous p-orbital overlap.
3. Fully conjugated: Every atom in the ring must have a p-orbital participating in the π system (no sp³ carbons interrupting conjugation).
4. Hückel's rule: The ring must have 4n+2 π electrons, where n = 0, 1, 2, 3…

Examples of aromatic systems: Benzene (6 π e⁻, n=1), cyclopentadienyl anion (6 π e⁻, n=1), cycloheptatrienyl cation (tropylium, 6 π e⁻, n=1), naphthalene (10 π e⁻, n=2), pyridine (6 π e⁻, n=1).

Antiaromaticity

A compound that is cyclic, planar, fully conjugated, but has 4n π electrons (n = 1, 2, 3…) is ANTIaromatic. Antiaromatic compounds are DESTABILIZED relative to their open-chain counterparts and are highly reactive.

Example: Cyclobutadiene (4 π e⁻, n=1). It distorts to a rectangular shape to avoid antiaromaticity.

Cyclooctatetraene (8 π e⁻) avoids antiaromaticity by adopting a tub-shaped (nonplanar) conformation — it is NONaromatic, not antiaromatic.

Nonaromatic Compounds

Fail any one of the four criteria: not cyclic, not planar, not fully conjugated, or wrong π electron count. Most organic molecules are nonaromatic.

Electrophilic Aromatic Substitution (EAS) — General Mechanism

1. Electrophile generation: The electrophile (E⁺) is generated (often by a Lewis acid catalyst).
2. Electrophilic attack (rate-determining): The π system attacks E⁺, forming a carbocation intermediate (sigma complex, arenium ion). The positive charge is delocalized over three carbons (resonance-stabilized).
3. Deprotonation: A base abstracts the proton from the sp³ carbon, restoring aromaticity.

The Six EAS Reactions

Directing Effects of Substituents

Activating (electron-donating) — Ortho/Para Directors

These groups donate electron density into the ring, making it MORE reactive than benzene. They direct to ortho and para positions.

Halogens are a special case: Deactivating (inductive withdrawal outweighs resonance donation) BUT ortho/para directors (resonance donation creates o/p selectivity). The halogens: -F, -Cl, -Br, -I.

Deactivating (electron-withdrawing) — Meta Directors

These groups withdraw electron density, making the ring LESS reactive. They direct to the meta position.

Understanding Directing Effects

• Ortho/para directors stabilize the sigma complex when the electrophile adds ortho or para. This happens through resonance (lone pair donation) or inductive stabilization by alkyl groups.
• Meta directors destabilize the sigma complex MOST at the ortho and para positions (adjacent positive charge next to electron-withdrawing group). The meta position avoids this destabilization.
• Halogens: Resonance donation (lone pair) → ortho/para preference. Inductive withdrawal (electronegativity) → deactivation. The inductive effect dominates overall rate; the resonance effect controls regiochemistry.

Friedel-Crafts Limitations

1. Alkylation: Carbocation rearrangements are possible (primary RCl may rearrange via hydride shift).
2. Alkylation: Polyalkylation is hard to avoid (product is more reactive than starting material).
3. Both: No reaction on strongly deactivated rings (if the ring has a meta director, especially NO₂, no FC reaction).
4. Both: No reaction with aromatic amines (the NH₂ group complexes with AlCl₃, deactivating the ring).
5. Acylation: NO rearrangements (acylium ion is resonance-stabilized).

Substituent Effects on Acidity

Electron-withdrawing groups on phenols INCREASE acidity (stabilize phenoxide by delocalizing charge). p-Nitrophenol (pKa ~7.2) is far more acidic than phenol (pKa ~10). Electron-donating groups DECREASE acidity.

Common Traps

• Cyclooctatetraene is NOT antiaromatic — it's nonaromatic because it's tub-shaped (nonplanar).
• Halogens are the exception to the "deactivating = meta" rule. They deactivate but direct ortho/para.
• Friedel-Crafts alkylation with primary alkyl halides can rearrange. Acylation never rearranges.
• Counting π electrons in charged rings: Cyclopentadienyl anion has 6 π e⁻ (each C contributes 1, plus 2 from the lone pair on the anionic carbon). Cyclopentadienyl cation has 4 π e⁻ (antiaromatic).