Word of the Day: Aromatic

Here’s an experiment for you: ask a stranger to tell you about something organic and aromatic. Most people will probably look at you strangely and tell you about some beautiful pesticide-free roses, or an expensive boutique wine, If, however, they just say “benzene?”, you’ve probably found yourself a chemist.

Aromatic molecules are a sub-group of organic molecules with at least part of their structure made up of atoms (usually most or all of them being carbon) arranged in a ring with some particular features.

A few different ways of representing the common aromatic molecule, benzene.

A few different ways of representing the common aromatic molecule, benzene.

The hourglass shapes on the representation second along on the bottom row above are p-orbitals, the areas where the electrons that make double bonds are. Note that every atom in the ring has one, and the lobes of the hourglass shapes are perpendicular to the plane of the ring. The number of electrons in p-orbitals around the ring must satisfy Hückel’s Rule; that is, there must be 4n + 2 of them, where n is some positive integer, or zero.

Aromaticity matters because aromatic units are flat, rigid, and extremely stable, which gives them particular, useful chemical properties. Three of the twenty amino acids that form the basic chemistry of living things are aromatic, and so are all the nucleotides in DNA and RNA.

Their flatness, rigidity, and common features with DNA does make some aromatic compounds particularly harmful to living things. You may have heard of polycyclic aromatic hydrocarbons, or PAHs, a byproduct of burning anything from fossil fuels to firewood. Some of these are harmless, but the particular shape of others makes them especially dangerous. Benzo[a]pyrene is one example; it gets inserted into DNA causing mutations which may in turn cause cancer. It is one of the many compounds in cigarette smoke that appears to be involved in the link between smoking and cancers of the respiratory system. (Science 18 October 1996: Vol. 274 no. 5286 pp. 430-432)

Benzo[a]pyrene: it'll really ruin your whole week.

Benzo[a]pyrene: it’ll really ruin your whole week.

Aromatic molecules were probably originally so named because some of the first ones discovered did have strong, distinctive aromas, but most don’t have this feature, and most strongly odorous molecules aren’t organic — they’re far more likely to be terpines or esters… or something else entirely.

One of the handful of aromatics which are aromatic in the other sense: Naphtalene, the smelly component of mothballs.

Thank you to Chris P. for the suggestion, and to the UC Davis ChemWiki and Chris P. for the refresher on the four conditions for aromaticity and Hückel’s Rule.