Polycyclic aromatic hydrocarbon

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

PAHs are also present in coal and petroleum. While they are usually colorless and flat in shape, exposure to them can be harmful. Scientists have found that some PAHs may increase the risk of cancer, problems during pregnancy, and heart issues.

Besides their role in pollution, PAHs are also interesting to scientists studying the origins of life. These chemicals might have been important building blocks for the very first living things on Earth. Their ability to form under simple conditions makes them a key topic in research about how life could begin.

Nomenclature and structure

Polycyclic aromatic hydrocarbons, often called PAHs, are compounds made of multiple connected rings of atoms. The simplest PAH is naphthalene, which has two rings. Other common PAHs include anthracene and phenanthrene. These compounds are found in things like engine exhaust, smoke, and even grilled meats.

Most PAHs are flat, meaning all their atoms lie in the same plane. This flat shape helps them fit together easily. However, some PAHs can be bent or twisted, especially when they have many rings. For example, corannulene looks like a small bowl. These shapes change how the molecules behave and interact with other substances.

Bonding and aromaticity

See also: Clar's rule

Polycyclic aromatic hydrocarbons (PAHs) have different levels of aromaticity, which 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 the properties of a PAH.

For example, phenanthrene has two main structures: one with one aromatic sextet in the middle ring and another with two sextets in the first and third rings. The structure with two sextets shows that the outer rings are more aromatic and less reactive, while 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, spreading the aromaticity more evenly. This difference affects how these molecules absorb light, with phenanthrene absorbing at 293 nm and anthracene at 374 nm. In chrysene, a four-ring PAH, there are three structures with two sextets each, showing that the outer rings are more aromatic than the inner rings.

Properties

PAHs are nonpolar and lipophilic, meaning they do not mix well with water. Larger PAHs, such as perylene, are usually not soluble in water or even in organic solvents and lipids, and they often have strong colors.

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