Are all terpenes aromatic?

Aromatic is a common description of terpenes. However, aromaticity only occurs at a specific chemical structure. And of two different terpene categories, only one has aromatic molecules.

Terpenes often emit pungent or strong pleasant smells in essential oils, cannabis, and various plants. Our nose receives while nasal receptors respond to the molecules. Aromatic compounds, especially terpenes, are responsible for many smells in the world. However, 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 these rules are far more stable. Myrcene is acyclic and non-aromatic. It is an example of a pure, aliphatic terpene.

Linalool has a strong odor, but like myrcene, it is an aliphatic terpene. Aromatic molecules must have at least one unique ring. Especially molecules without aromatic rings, like limonene, are almost always aliphatic rather than aromatic.

The aromatic ring of a terpene

The most common aromatic ring is the six-sided hexagon of carbon and hydrogen atoms known as benzene. Even more important, however, are the double bonds within benzene. They are fully conjugated and unsaturated.

Aromatic rings must be flat (planar) and their electrons (double bonds) must align in a specific way. Furan, pyridine, pyrole and thiophene also possess aromaticity.

Pinene consists of two rings (bicyclic) but the molecule is not flat. Also, the electrons of pinene are not conjugated. It is therefore not an aromatic terpene.

before they became known

Scientists discovered many terpenes in the decades following Michael Faraday’s discovery of benzene in 1825. In the mid to late 19th century, scientists characterized certain terpenes as having aromatic or aliphatic properties.

Consumers need not worry about Hückel’s rule of aromaticity, which was developed by Erich Hückel in 1931. But not all terpenes are aromatic. And chemists relied on aromaticity to identify specific terpenes nearly two hundred years ago.

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