Aromaticity is a property of cyclic, planar molecules with a ring of resonance bonds that gives them increased stability compared to other geometric or connective arrangements with the same set of atoms.
Hückel's Rule for Aromaticity:
A molecule is aromatic only if it is:
Cyclic
Planar (all atoms in the ring are sp² hybridized)
Fully Conjugated (a continuous system of p-orbitals)
Contains (4n + 2) π electrons, where 'n' is an integer (n = 0, 1, 2, 3...).
This means aromatic compounds have 2, 6, 10, 14, ... π electrons.
Anti-aromaticity: A molecule that meets the first 3 criteria but has (4n) π electrons (4, 8, 12...). These molecules are *highly unstable*.
Non-aromatic: A molecule that fails any of the first 3 criteria (e.g., it is non-planar or not fully conjugated).
Examples:
Arenes (e.g., Benzene): Cyclic, planar, fully conjugated, and has 6 π electrons (n=1). It is aromatic.
Heterocyclic Compounds: Lone pairs can participate in the π system if they are in a p-orbital.
Pyridine: 6 π electrons. The N lone pair is in an sp² orbital (in the plane of the ring) and does *not* count. Aromatic.
Pyrrole: 6 π electrons. The N lone pair is in a p-orbital and *does* count towards the 6 π electrons. Aromatic.
Furan: 6 π electrons. One O lone pair is in a p-orbital (counts), the other is in an sp² orbital (does not count). Aromatic.
Electrophilic Aromatic Substitution (EAS) - General Mechanism
This is the characteristic reaction of aromatic compounds. An electrophile (E⁺) attacks the electron-rich benzene ring, and a proton (H⁺) is substituted.
General Mechanism (Two Steps):
Step 1: Attack by the electrophile (Slow, Rate-Determining Step) The π-electron system of the ring attacks the electrophile (E⁺) to form a resonance-stabilized carbocation intermediate known as an arenium ion or sigma complex. This step destroys aromaticity and has a high activation energy.
Step 2: Deprotonation (Fast) A base (often the conjugate base of the acid used to generate E⁺) removes the proton from the sp³-hybridized carbon, restoring the aromaticity of the ring.
Specific EAS Reactions
Each reaction follows the general mechanism, differing only in how the electrophile (E⁺) is generated.
Reaction
Reagents
Electrophile (E⁺)
Mechanism of E⁺ Generation
Halogenation
Cl₂ + FeCl₃ or Br₂ + FeBr₃
Cl⁺ or Br⁺ (Halonium ion)
Cl-Cl + FeCl₃ → Cl⁺ + [FeCl₄]⁻
Nitration
Conc. HNO₃ + Conc. H₂SO₄ (Nitrating mixture)
NO₂⁺ (Nitronium ion)
HNO₃ + 2 H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2 HSO₄⁻
Sulphonation
Fuming H₂SO₄ (H₂SO₄ + SO₃)
SO₃ (neutral, but highly electrophilic)
2 H₂SO₄ ⇌ SO₃ + H₃O⁺ + HSO₄⁻
Friedel-Crafts Alkylation
R-Cl + AlCl₃ (Lewis acid catalyst)
R⁺ (A carbocation)
R-Cl + AlCl₃ → R⁺ + [AlCl₄]⁻
Friedel-Crafts Acylation
R-CO-Cl + AlCl₃ (Lewis acid catalyst)
R-C≡O⁺ (Acylium ion)
R-CO-Cl + AlCl₃ → [R-C≡O⁺ ↔ R-C⁺=O] + [AlCl₄]⁻
Limitations of Friedel-Crafts Reactions
Carbocation Rearrangement (Alkylation only): If the initial carbocation (R⁺) is 1°, it will rearrange to a more stable 2° or 3° carbocation if possible. This leads to unexpected products.
Example: Benzene + 1-chloropropane → Isopropylbenzene (Cumene), not n-propylbenzene.
Solution: Use Friedel-Crafts Acylation followed by reduction (e.g., Clemmensen or Wolff-Kishner) to get the straight-chain product. Acylium ions do *not* rearrange.
Deactivated Rings: Friedel-Crafts reactions do not work on strongly deactivated rings (e.g., nitrobenzene, benzoic acid) or on aniline (-NH₂).
Polyalkylation (Alkylation only): The alkyl group (R-) added to the ring is *activating*, making the product (e.g., toluene) *more reactive* than benzene. This leads to multiple alkyl groups being added.
Effects of Substituents on EAS
Substituents already on the ring affect both the rate of the reaction and the position (orientation) of the incoming electrophile.
Activating and Deactivating Groups
Activating Groups: These groups donate electron density to the ring, making it more nucleophilic and *faster* to react. They stabilize the arenium ion intermediate.
Examples: -NH₂, -OH, -OR, -R (alkyl), -O-CO-R.
Deactivating Groups: These groups withdraw electron density from the ring, making it less nucleophilic and *slower* to react. They destabilize the arenium ion.
Examples: -NO₂, -CN, -SO₃H, -COOH, -CHO, -COR, -NR₃⁺.
Ortho/Para vs. Meta Directors
Ortho, Para-Directors: These groups direct the new substituent to the positions *ortho* (1,2) or *para* (1,4) to themselves.
Reason: They can stabilize the intermediate arenium ion via resonance (if they have a lone pair) or hyperconjugation (if they are alkyl groups).
Groups: All activating groups are o,p-directors.
Exception (Halogens): -F, -Cl, -Br, -I are deactivating (due to strong -I effect) but are ortho, para-directing (due to +M effect from lone pairs).
Meta-Directors: These groups direct the new substituent to the *meta* (1,3) position.
Reason: They strongly destabilize the arenium ions formed by ortho or para attack. The meta-attack intermediate is "less bad" by comparison.
Groups: All deactivating groups (except halogens) are m-directors.
Summary Table for Substituents:
Group Type
Effect on Rate
Directing Effect
Examples
Strong Activators
Strongly Activate
Ortho, Para
-NH₂, -OH, -O⁻
Moderate Activators
Activate
Ortho, Para
-OR, -NHCOR
Weak Activators
Weakly Activate
Ortho, Para
-R (alkyl), -Ph
Deactivators
Weakly Deactivate
Ortho, Para
-F, -Cl, -Br, -I
Moderate Deactivators
Deactivate
Meta
-CHO, -COR, -COOH
Strong Deactivators
Strongly Deactivate
Meta
-NO₂, -CN, -SO₃H, -NR₃⁺
Polycyclic Aromatic Hydrocarbons (PAHs) and Annulenes
Naphthalene, Phenanthrene, and Anthracene
These are PAHs with fused benzene rings.
Naphthalene (C₁₀H₈): Two fused rings.
Structure: All 10 carbons are sp². It has 10 π electrons (n=2), so it is aromatic.
Reactions: Undergoes EAS, similar to benzene but is *more reactive*.
Orientation: Attack at C1 (α-position) is kinetically favored (more stable intermediate). Attack at C2 (β-position) is thermodynamically favored (more stable product) but usually occurs only at high temperatures.
Anthracene (C₁₄H₁₀): Three rings fused linearly.
Reactions: The central ring (C9, C10) is the most reactive. It undergoes addition reactions (like Diels-Alder) more readily than substitution, as this retains two stable benzene rings.
Phenanthrene (C₁₄H₁₀): Three rings fused in an angular fashion.
Structure: More stable than its isomer, anthracene.
Reactions: Reacts at C9 and C10 (the "bay" region).
Structure Clarification: Their structures were historically elucidated by oxidative cleavage (e.g., with O₃ or KMnO₄) and analyzing the resulting products (e.g., phthalic acid from naphthalene proves it has one benzene ring fused to another fragment).
Annulenes
Annulenes are monocyclic hydrocarbons with alternating single and double bonds. They are named as [N]annulene, where N is the number of carbons in the ring.
[4]annulene (Cyclobutadiene): 4 π electrons. Anti-aromatic and highly unstable.
[6]annulene (Benzene): 6 π electrons. Aromatic.
[8]annulene (Cyclooctatetraene): 8 π electrons. It *should* be anti-aromatic, but to avoid this instability, it adopts a non-planar "tub" shape. This breaks conjugation, making it non-aromatic.
[10]annulene: 10 π electrons (4n+2). It *should* be aromatic, but the planar isomer has severe angle strain. The molecule is not perfectly planar, so its aromaticity is weak.
[18]annulene: 18 π electrons (n=4). It *is* large enough to be planar and is aromatic.