15.6 Polycyclic Aromatic Compounds

Polycyclic Aromatic Compounds

Polycyclic aromatic compounds contain two or more fused benzene rings sharing adjacent carbon atoms. Their extended π-electron systems give them distinct stability, reactivity, and physical properties compared to single-ring aromatics. These compounds show up in coal tar, combustion products, and biological molecules.

Structure of Polycyclic Aromatics

The simplest polycyclic aromatic hydrocarbon (PAH) is naphthalene, which has two fused benzene rings. Anthracene has three fused benzene rings arranged in a linear chain. Phenanthrene is an isomer of anthracene where the three rings are fused in an angular arrangement.

All carbons in these systems are $sp^2$-hybridized, which keeps the molecules planar. That planarity allows continuous overlap of p-orbitals across the entire fused ring system, creating a delocalized π-electron cloud that extends over multiple rings.

Key physical properties:

• Stability: The delocalized π-electron system provides significant resonance stabilization, well beyond what isolated double bonds would give
• High melting/boiling points: The flat, rigid structures pack efficiently in the solid state, increasing London dispersion forces between molecules
• Low water solubility: These are nonpolar, hydrophobic molecules
• UV-Vis absorption: The extended conjugation shifts absorption to longer wavelengths compared to benzene. Naphthalene absorbs in the UV; larger PAHs can absorb visible light (anthracene fluoresces blue)

Aromaticity and Electronic Properties

For a polycyclic compound to be aromatic, it must satisfy the same criteria as benzene: the molecule must be cyclic, planar, fully conjugated, and contain $4n + 2$ π-electrons (Hückel's rule). Naphthalene, for example, has 10 π-electrons ($n = 2$), and anthracene has 14 ($n = 3$).

Resonance stabilization in PAHs comes from delocalization of π-electrons across the entire fused ring system. You can draw multiple resonance structures for naphthalene (three major ones), which illustrates that not all C–C bonds are equivalent. Some bonds have more double-bond character than others, unlike in benzene where all bonds are identical. This uneven electron distribution directly affects where reactions occur on the ring.

Reactivity of Polycyclics vs. Benzene

PAHs are more reactive than benzene in electrophilic aromatic substitution (EAS). The reason: when an electrophile attacks a PAH, the intermediate carbocation (arenium ion) retains aromaticity in the ring(s) that aren't directly involved in the reaction. Benzene, with only one ring, completely loses its aromaticity in the intermediate. This lowers the activation energy for PAHs.

The reactivity trend is: Anthracene > Naphthalene > Benzene.

Naphthalene regioselectivity:

1. Electrophilic substitution occurs preferentially at the 1-position (α-position).
2. Attack at C-1 gives an intermediate where the untouched ring remains fully aromatic, and you can draw more resonance structures stabilizing the positive charge.
3. Attack at the 2-position (β-position) gives a less stable intermediate with fewer resonance contributors, so it's the minor product.

Anthracene regioselectivity:

1. Electrophilic substitution occurs preferentially at the 9-position (the center ring).
2. Attack at C-9 allows both outer rings to retain full aromaticity in the intermediate, providing maximum stabilization.
3. Substitution at C-1 or C-2 is less favorable because fewer intact aromatic rings remain in the intermediate.

Carcinogenicity: Some PAHs (notably benzo[a]pyrene, found in cigarette smoke and charred food) are metabolized in the body to reactive epoxide intermediates. These epoxides can form covalent adducts with DNA bases, leading to mutations and potentially cancer.

Heterocyclic Analogs in Biology

When you replace a CH group in a PAH with a heteroatom like nitrogen, you get a heterocyclic aromatic compound. Several of these are biologically critical.

• Quinoline: A benzene ring fused to a pyridine ring (nitrogen replaces a CH in the ring equivalent to naphthalene's 1-position). Quinine, the classic antimalarial drug, contains a quinoline core.
• Isoquinoline: An isomer of quinoline with the nitrogen in the other ring. Morphine and other opiate alkaloids are built on an isoquinoline framework.
• Indole: A benzene ring fused to a pyrrole ring. This is the ring system in the amino acid tryptophan and the neurotransmitter serotonin. Many drugs contain the indole scaffold, including sumatriptan (migraine treatment) and LSD.
• Purine: A pyrimidine ring fused to an imidazole ring. Purines form the backbone of adenine and guanine, two of the four nucleic acid bases in DNA and RNA. Caffeine and uric acid are also purine derivatives.

These heterocyclic systems maintain aromaticity because the heteroatom contributes to the π-electron count while preserving planarity and conjugation across the fused rings.