Polycyclic aromatic hydrocarbon

A polycyclic aromatic hydrocarbon (PAH) is a type of chemical made of rings of carbon and hydrogen atoms connected together. These chemicals form when organic materials do not burn completely. This can happen in car exhaust, when tobacco is burned, in forest fires, or when meat cooks over an open flame. Some simple PAHs are found in everyday things like mothballs, which contain a PAH called naphthalene.

PAHs are also in coal and petroleum. While they are usually colorless and flat, being around them can be unhealthy. Scientists have found that some PAHs may increase the risk of cancer and heart issues.

Besides pollution, PAHs are also interesting to scientists studying how life began. These chemicals might have been important for the very first living things on Earth. Their ability to form easily makes them a key topic in research about life’s beginnings.

Nomenclature and structure

Polycyclic aromatic hydrocarbons, or PAHs, are groups of connected rings of atoms. The simplest PAH is naphthalene, which has two rings. Other common PAHs are anthracene and phenanthrene. You can find these compounds in engine exhaust, smoke, and grilled meats.

Most PAHs are flat, with all their atoms in the same plane. This flat shape helps them fit together easily. Some PAHs can be bent or twisted, especially with many rings. For example, corannulene looks like a small bowl. These shapes affect how the molecules act and work with other substances.

Bonding and aromaticity

See also: Clar's rule

Polycyclic aromatic hydrocarbons (PAHs) have different levels of aromaticity. This means how much they follow special patterns in their structure. According to Clar's rule, the structure with the most benzene-like parts, called pi sextets, is the most important for understanding a PAH.

For example, phenanthrene has two main structures. One has one aromatic sextet in the middle ring. The other has two sextets in the first and third rings. The structure with two sextets shows that the outer rings are more aromatic and less reactive. The middle ring is less aromatic and more reactive.

In contrast, anthracene has structures where the aromatic sextet can be in any of its three rings. This spreads the aromaticity more evenly. This difference affects how these molecules absorb light. Phenanthrene absorbs at 293 nm and anthracene at 374 nm. In chrysene, a four-ring PAH, there are three structures with two sextets each. This shows that the outer rings are more aromatic than the inner rings.

Properties

PAHs do not mix well with water. They are called nonpolar and lipophilic. Bigger PAHs, like perylene, usually do not dissolve in water or even in organic solvents and lipids. They often have strong colors.

When PAHs are treated with alkali metals, they can form radicals and anions. Larger PAHs can even form dianions. Their redox potential depends on their size.