polycyclic aromatic hydrocarbon
/ PAH (pol-ee-SY-klik) /
A polycyclic aromatic hydrocarbon, or PAH, is a molecule made of two or more benzene rings fused together, sharing edges so that neighboring rings have a common side. Imagine benzene rings stuck side by side like floor tiles or honeycomb cells. The simplest is naphthalene, two fused rings (the original mothball compound); anthracene and phenanthrene have three rings, and the family extends to large flat sheets that are essentially small fragments of graphite.
Fused aromatics are still aromatic, but the picture is a little richer than for a single ring. The pi electrons are delocalized across the whole fused framework rather than circling each ring separately, and the rings share that electron density unevenly, so not every bond in naphthalene is identical the way every bond in benzene is. Each added ring brings additional aromatic stabilization, though the bonus per ring is smaller than benzene's because the electrons are spread thinner. Chemically, PAHs behave like sturdier, more elaborate benzenes: they undergo substitution rather than addition, and they tend to react at the positions that best preserve aromaticity in the remaining rings.
PAHs matter well beyond the lab. They form whenever organic material burns incompletely, so they appear in soot, smoke, grilled and charred food, vehicle exhaust, and tobacco smoke. This is a real-world caution: several PAHs, such as benzo[a]pyrene, are potent carcinogens, because their flat shape lets them slip between the bases of DNA and their metabolites damage it. PAHs are also of deep interest in astronomy, where they are thought to be among the most abundant carbon-bearing molecules in interstellar space. The same fused-ring stability that makes them persistent pollutants makes them survivors across the cosmos.
Naphthalene (two fused rings, C10H8) is the classic mothball smell; anthracene (three rings in a row) is used in dyes; benzo[a]pyrene from char and smoke is a well-known carcinogen.
From mothballs to soot to interstellar dust, fused benzene rings are everywhere.
Unlike benzene, not all bonds in a fused PAH are equivalent; the shared edges and the way pi density distributes mean some bonds have more double-bond character than others, so naphthalene is not just two identical benzenes glued together.