Aromatic Compounds

Benzene (C₆H₆) is the parent compound of aromatic compounds. Its structure was proposed by August Kekulé (1865): a hexagonal ring of 6 carbon atoms with alternating single and double bonds (three conjugated double bonds).

However, benzene does not behave like a normal conjugated triene:

• It does not undergo electrophilic addition (like alkenes), but electrophilic aromatic substitution
• All C—C bonds in benzene are identical (length 139.7 pm, intermediate between C—C single 154 pm and C=C double 134 pm)
• The resonance energy of benzene is about 152 kJ/mol: it is 152 kJ/mol more stable than the hypothetical cyclohexatriene with localized double bonds

The modern description: benzene is a resonance hybrid of two equivalent Kekulé structures. The six $pi$ electrons are delocalized over the entire ring, forming a continuous $pi$ system above and below the molecular plane.

Aromaticity confers thermodynamic and chemical stability: aromatic compounds tend to keep the $pi$ system intact, preferring substitution (which preserves aromaticity) over addition (which destroys it).

Real-world applications: benzene is a crucial industrial intermediate (production of styrene, phenol, cyclohexane for nylon, aniline). However, it is carcinogenic (leukemia), so its use is strictly regulated.

Hückel's rule (Erich Hückel, 1931) establishes the criteria for aromaticity: a planar cyclic compound is aromatic if it possesses $(4n+2)$ delocalized $pi$ electrons, where $n$ is an integer (0, 1, 2, 3...).

Necessary conditions for aromaticity:

1. Cyclic structure (closed ring of atoms)
2. Planarity (all ring atoms must allow continuous $pi$ overlap)
3. Every atom of the ring must have a p orbital perpendicular to the ring plane (sp² or sp hybridization)
4. $(4n+2)$ delocalized $pi$ electrons (Hückel's rule)

Examples:

• Benzene (C₆H₆): 6 $pi$ electrons (n=1: 4·1+2=6) → aromatic
• Naphthalene (C₁₀H₈): 10 $pi$ electrons (n=2: 4·2+2=10) → aromatic (bicyclic)
• Anthracene (C₁₄H₁₀): 14 $pi$ electrons (n=3) → aromatic (tricyclic)
• Cyclopentadienyl (C₅H₅⁻): 6 $pi$ electrons (anion) → aromatic
• Cycloheptatrienyl (C₇H₇⁺): 6 $pi$ electrons (tropylium cation) → aromatic
• Cyclobutadiene (C₄H₄): 4 $pi$ electrons (n=0.5, not integer) → antiaromatic (unstable, non-planar)

Compounds with 4n $pi$ electrons (4, 8, 12...) are antiaromatic: very unstable, tend to distort planarity to avoid delocalization.

Aromaticity is not limited to hydrocarbons: heterocyclic compounds such as pyridine (C₅H₅N, 6 $pi$ electrons), pyrrole (C₄H₅N, 6 $pi$ electrons), thiophene (C₄H₄S) are aromatic (the heteroatom contributes a lone pair to the $pi$ system).

Real-world applications: heterocyclic aromatic compounds are ubiquitous in biochemistry: the nitrogenous bases of DNA/RNA (purines, pyrimidines), chlorophyll (porphyrin ring with 22 $pi$ electrons, n=5), the heme of hemoglobin.

Electrophilic aromatic substitution (SEAr) is the characteristic reaction of aromatic compounds. An electrophile replaces a hydrogen atom on the ring, preserving aromaticity. The general two-step mechanism:

1. Electrophilic attack (slow, rate-determining step): the electrophile E⁺ attacks the $pi$ cloud of the ring, forming a Wheland intermediate ($sigma$ complex, or arenium ion). This intermediate is a delocalized carbocation (non-aromatic, less stable) stabilized by resonance over three ring positions.
2. Proton loss (fast step): a base (typically the counterion of the electrophile, e.g., AlCl₄⁻) removes the hydrogen from the attacked carbon, restoring the aromatic $pi$ system.

Main SEAr reactions:

• Nitration: $mathrm{C_6H_6 + HNO_3/H_2SO_4 o C_6H_5NO_2 + H_2O}$. The electrophile is the nitronium ion NO₂⁺, generated from the sulfuric-nitric acid mixture. Product: nitrobenzene (intermediate for aniline and azo dyes).
• Halogenation: $mathrm{C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl}$. The electrophile is Cl⁺ (polarized by FeCl₃ or AlCl₃). Bromination uses Br₂/FeBr₃, iodination is more difficult (I₂/HNO₃).
• Sulfonation: $mathrm{C_6H_6 + H_2SO_4 o C_6H_5SO_3H + H_2O}$. The electrophile is SO₃ (or HSO₃⁺). It is reversible: desulfonation is possible with hot dilute acid.
• Friedel-Crafts alkylation: $mathrm{C_6H_6 + RCl \xrightarrow{AlCl_3} C_6H_5R + HCl}$. The electrophile is the carbocation R⁺ (generated from RCl + AlCl₃). Limitations: possible carbocation rearrangements and polyalkylation.
• Friedel-Crafts acylation: $mathrm{C_6H_6 + RCOCl \xrightarrow{AlCl_3} C_6H_5COR + HCl}$. The electrophile is the acylium ion RCO⁺. No rearrangements occur and it produces aromatic ketones (e.g., acetophenone).

Real-world applications: nitration of toluene produces TNT (trinitrotoluene, explosive). Friedel-Crafts alkylation is used to produce ethylbenzene (precursor of styrene for polystyrene) and cumene (for phenol and acetone).

When a monosubstituted benzene (C₆H₅—G) undergoes a second electrophilic substitution, the attack position (ortho, meta, or para) is determined by the nature of the existing substituent G. Substituents fall into three classes:

1. Ortho-para directing activators: accelerate the reaction (compared to benzene) and direct the electrophile to ortho and para positions.

• Groups with lone pairs donating by resonance: —NH₂, —OH, —OR (strongly activating). The lone pair delocalizes into the ring, increasing electron density especially at ortho and para positions.
• Alkyl groups (—CH₃, —C₂H₅): weakly activating by inductive effect (+I) and hyperconjugation. They are ortho/para directors. Toluene nitrates faster than benzene, forming predominantly o- and p-nitrotoluene.

2. Meta-directing deactivators: slow down the reaction and direct to the meta position.

• Electron-withdrawing groups by resonance and inductive effect (−M, −I): —NO₂, —CN, —COR, —COOH, —SO₃H. They withdraw electron density from the ring, deactivating it.

3. Ortho-para directing deactivators: slow down the reaction but direct to ortho and para.

• Halogens (—F, —Cl, —Br, —I): they are deactivating (strong −I inductive effect) but ortho/para directing (by resonance +M effect: the lone pairs delocalize into the ring). Unique case: they deactivate but direct ortho/para.

Theoretical explanation: the Wheland intermediate (carbocation) has the positive charge delocalized over three positions. Donor substituents stabilize the positive charge at ortho and para (where resonance allows direct delocalization). Withdrawing substituents destabilize the charge at ortho/para (unfavorable resonance), so the charge is preferentially located at meta.

Real-world applications: knowledge of orientation is fundamental in the synthesis of polysubstituted aromatic compounds. For example, to obtain p-nitrotoluene, toluene is nitrated (methyl activating ortho/para) and the ortho isomer is separated. To introduce a group at the meta position, a nitro group (meta-directing deactivator) is used first, then reduced to an amine.