Terpenes are the largest and most diverse class of natural products. They are found in all parts of plants, including the leaves, flowers, stems, roots, bark, and resin. More than 20,000 terpenes have been characterized from a variety of plant species. Terpenes are also present in many other organisms such as bacteria, fungi, and animals.
The term “terpene” was coined in 1866 by German chemist August Kekulé von Stradonitz from the word turpentine. Terpenes are hydrocarbons; that is, they are made up of only carbon and hydrogen atoms. The name “terpene” comes from the fact that many of these molecules were first isolated from pine trees (Pinus spp.). In addition to being found in plants, some terpenes occur naturally in other organisms such as insects (e.g., beeswax) and animals (e.g., musk).
Most terpenes are bicyclic or tricyclic compounds that contain 10 or 15 carbon atoms arranged in a ring structure (Figure 1). The simplest terpene is geraniol, which has a 10-carbon skeleton with an alcohol group attached to one end (Figure 2). Other common examples include limonene (a monocyclic compound with a 11-carbon skeleton), myrcene (a bicylic compound with a 14-carbon skeleton), and pinene (a tricyclic compound with two fused rings containing 15 carbons each). These three compounds plus another important terpene called linalool make up the majority of the volatile oil content in citrus peels.