DAT Review · Organic Chemistry
Aromaticity and Electrophilic Aromatic Substitution (EAS)
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Scope: Criteria for aromaticity/antiaromaticity/nonaromaticity, Hückel's rule, electrophilic aromatic substitution mechanisms and directing effects, and Friedel-Crafts reactions. Expect 3–5 questions. Directing effects (ortho/para vs. meta, activating vs. deactivating) are the single highest-yield subtopic.
The college version
Core Review
Aromaticity Criteria (All four must be met)
- Cyclic: The molecule must be a ring.
- Planar: All atoms in the ring system must be sp² hybridized (or nearly so) for continuous p-orbital overlap.
- Fully conjugated: Every atom in the ring must have a p-orbital participating in the π system (no sp³ carbons interrupting conjugation).
- 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
- Electrophile generation: The electrophile (E⁺) is generated (often by a Lewis acid catalyst).
- 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).
- Deprotonation: A base abstracts the proton from the sp³ carbon, restoring aromaticity.
The Six EAS Reactions
| Reaction | Reagents | Electrophile | Notes |
|---|---|---|---|
| Halogenation | Br₂/FeBr₃ or Cl₂/FeCl₃ | Br⁺ or Cl⁺ (as FeX₄⁻ complex) | FeX₃ is a Lewis acid catalyst |
| Nitration | HNO₃/H₂SO₄ | NO₂⁺ (nitronium ion) | H₂SO₄ protonates HNO₃, then water leaves |
| Sulfonation | SO₃/H₂SO₄ (fuming) | SO₃ (or HSO₃⁺) | Reversible! Can remove SO₃H with dilute H₂SO₄/heat |
| Friedel-Crafts Alkylation | RCl/AlCl₃ | R⁺ (carbocation) | Rearrangements possible. Polyalkylation is a problem. |
| Friedel-Crafts Acylation | RCOCl/AlCl₃ | RCO⁺ (acylium ion) | NO rearrangements. Product is a ketone. |
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.
| Strength | Substituents |
|---|---|
| Strongly activating | -NH₂, -NHR, -NR₂, -OH, -O⁻ |
| Moderately activating | -OR, -NHCOR |
| Weakly activating | -R (alkyl), -Ar (aryl) |
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.
| Strength | Substituents |
|---|---|
| Strongly deactivating | -NO₂, -NR₃⁺, -CF₃, -CCl₃ |
| Moderately deactivating | -CN, -SO₃H, -CHO, -COR, -COOH, -COOR |
| Special | Halogens (-F, -Cl, -Br, -I): deactivating but o/p |
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
- Alkylation: Carbocation rearrangements are possible (primary RCl may rearrange via hydride shift).
- Alkylation: Polyalkylation is hard to avoid (product is more reactive than starting material).
- Both: No reaction on strongly deactivated rings (if the ring has a meta director, especially NO₂, no FC reaction).
- Both: No reaction with aromatic amines (the NH₂ group complexes with AlCl₃, deactivating the ring).
- 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).
Eli explains
The same idea, in plain words
Explain it like I’m 10
Aromatic molecules are like a perfectly balanced circle of kids holding hands — super stable and happy. Hückel's rule is the "magic number" rule: 2, 6, 10, 14 π electrons = aromatic. EAS is like a bully (electrophile) trying to join the circle. Groups already on the ring tell the bully where to go: helpful (electron-donating) groups say "stand next to me" (ortho/para); greedy (electron-withdrawing) groups say "go away to the opposite side" (meta). Halogens are the confusing ones — they're greedy but still bossy about where you stand!
Key takeaways
- 4n+2 π electrons = aromatic. 4n π electrons with planarity = antiaromatic. Nonplanar with 4n = nonaromatic.
- Activating groups → ortho/para. Deactivating groups → meta. Halogens: deactivating BUT ortho/para.
- Friedel-Crafts acylation is cleaner than alkylation (no rearrangements, no polyacylation).
- Sulfonation is reversible — a useful blocking/directing strategy in synthesis.
- The sigma complex in EAS has the positive charge on the carbons ortho and para to the site of attack (never meta).
- Is the cyclopentadienyl anion aromatic? Answer: Yes. It is cyclic, planar, fully conjugated (all sp² carbons with p-orbitals), and has 6 π electrons (5 from the double bonds + 2 from the lone pair on the anionic carbon) → 4n+2 with n=1.
- Predict the major product: toluene + HNO₃/H₂SO₄ → ? Answer: o-nitrotoluene and p-nitrotoluene (mixture). The methyl group is an activating ortho/para director. The ortho and para positions are both reactive; para is slightly favored due to sterics but both form.
- Why does nitrobenzene + CH₃Cl/AlCl₃ give no reaction? Answer: The nitro group is strongly deactivating (electron-withdrawing by both resonance and induction). The ring is too electron-poor for the electrophile to attack. Friedel-Crafts reactions fail on rings with strong meta directors.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Determine if a compound is aromatic, antiaromatic, or nonaromatic
- Predict EAS products based on substituent directing effects
- Draw mechanisms for nitration, halogenation, sulfonation, and Friedel-Crafts reactions
- Rank substituents by activating/deactivating strength
- Recognize limitations of Friedel-Crafts reactions
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