Aromatic is a common description of terpenes. Yet aromaticity only occurs with a specific chemical structure. And out of two different terpene categories, only one features aromatic molecules.
Terps often give off pungent or strongly pleasant odours in essential oils, cannabis, and various plants. Our nose receives while nasal receptors react to the molecules. Aromatic compounds, especially terpenes, are responsible for many smells in the world. Yet, some terps are aliphatic.
Molecular requirements for aromaticity
Aromaticity cannot be quantified, but general rules apply to the principle. And molecules, including terpenes, that follow those rules are far more stable. Myrcene is acyclic and not aromatic. It is an example of a straight, aliphatic terpene.
Linalool gives off a powerful odour, but like myrcene, it is an aliphatic terpene. Aromatic molecules must have at least one unique ring. Straight molecules without aromatic rings, such as limonene, are almost always aliphatic rather than aromatic.
A terpene’s aromatic ring
The most common aromatic ring is the six-sided hexagon built with carbon and hydrogen atoms known as benzene. More importantly, though, are the double bonds within benzene. They are fully conjugated and unsaturated.
Aromatic rings must be flat (planar), and their electrons (double bonds) must orientate themselves in a particular manner. Furan, pyridine, pyrole, and thiophene also possess aromaticty.
Pinene consists of two rings (bicyclic), but the molecule is not flat. Moreover, pinene’s electrons are not conjugated. It is, therefore, not an aromatic terpene.
Before they were known
Scientists discovered many terps in the decades following Michael Faraday’s discovery of benzene in 1825. In the mid to late 1800s, scientists characterized certain terpenes by aromatic or aliphatic properties.
Consumers do not need to worry about Hückel’s Rule of aromaticity, developed by Erich Hückel in 1931. But not all terpenes are aromatic. And chemists depended on aromaticity to identify certain terpenes nearly two hundred years ago.