Terpenes are a broad and diverse class of organic compounds produced by a variety of plants (1), particularly conifers, and some insects. They often have a strong odour and can protect the plants that produce them by deterring herbivores and attracting predators and parasites of herbivores. Terpenes are the main components of rosin and turpentine produced by conifers. 

They are the main constituents of the essential oils of many types of plants and flowers. Essential oils are widely used as fragrances in perfumery, medical and alternative therapies such as aromatherapy. Synthetic variations and derivatives of natural terpenes and terpenoids greatly expand the variety of aromas used in perfumery and food additives.

Some terpenes: Linalool, Limonene, Myrcene, Pinene, etc. 

How they are obtained

Terpenes can be obtained biosynthetically from isoprene units, whose molecular formula is C₅H₈. The basic molecular formulas of terpenes are multiples of this, (C₅H₈)n where n is the number of related isoprene units. This is called the isoprene rule or the C₅ rule. Isoprene units can be linked together 'head to tail' to form linear chains or they can be arranged to form rings.

There are different methods for extracting terpenes from plants and the method used may depend on the type of plant and the specific terpenes to be obtained.

Extraction with CO₂. This is a modern method that uses supercritical carbon dioxide to extract the oil from the plant. This method is very efficient and can be developed to extract specific compounds, but requires specialised equipment and is more expensive than other methods.

Solvent extraction. In this method, a solvent (such as ethanol, butane or CO2) is used to extract the oil from the plant. The solvent and the oil are then separated by evaporation or filtration. This method is often used when the oil is too delicate to be extracted by steam distillation.

Steam distillation. This is the most common and one of the oldest methods of extracting terpenes. Plant material is placed in a distillation vessel and steam is used to break down the oily membranes of the plant, releasing the essential oil. The steam and oil are then condensed into a liquid and the oil is separated.

Cold press extraction. This method is often used for citrus peels that contain a large amount of oil. The peels are mechanically pressed, breaking up the oily pockets and releasing the essential oil. 

What they are used for and where

Medical

Terpenes and terpenoids have demonstrated antimicrobial activity with a mechanism that causes cell death in bacteria based on the loss of cell membrane function integrity (2).

Cosmetics

The INCI designation is Terpenes and terpenoids

It is a restricted ingredient III/130 as a Relevant Item in the Annexes of the European Cosmetics Regulation No (EC) 2013/344 because the peroxide value must be less than 10 mmoles/L 

Perfuming. Unlike fragrance, which can also contain slightly less pleasant or characteristic odours, the term perfume indicates only very pleasant fragrances. Used for perfumes and aromatic raw materials.

References_____________________________________________________________________

(1) Boncan, D. A. T., Tsang, S. S., Li, C., Lee, I. H., Lam, H. M., Chan, T. F., & Hui, J. H. (2020). Terpenes and terpenoids in plants: Interactions with environment and insects. International Journal of Molecular Sciences, 21(19), 7382.

Abstract. The interactions of plants with environment and insects are bi-directional and dynamic. Consequently, a myriad of mechanisms has evolved to engage organisms in different types of interactions. These interactions can be mediated by allelochemicals known as volatile organic compounds (VOCs) which include volatile terpenes (VTs). The emission of VTs provides a way for plants to communicate with the environment, including neighboring plants, beneficiaries (e.g., pollinators, seed dispersers), predators, parasitoids, and herbivores, by sending enticing or deterring signals. Understanding terpenoid distribution, biogenesis, and function provides an opportunity for the design and implementation of effective and efficient environmental calamity and pest management strategies. This review provides an overview of plant–environment and plant–insect interactions in the context of terpenes and terpenoids as important chemical mediators of these abiotic and biotic interactions.

(2) Guimarães, Aline Cristina, Leandra Martins Meireles, Mayara Fumiere Lemos, Marco Cesar Cunegundes Guimarães, Denise Coutinho Endringer, Marcio Fronza, and Rodrigo Scherer. 2019. "Antibacterial Activity of Terpenes and Terpenoids Present in Essential Oils" Molecules 24, no. 13: 2471. https://doi.org/10.3390/molecules24132471

Abstract. Background: The antimicrobial activity of essential oils has been reported in hundreds of studies, however, the great majority of these studies attribute the activity to the most prevalent compounds without analyzing them independently. Therefore, the aim was to investigate the antibacterial activity of 33 free terpenes commonly found in essential oils and evaluate the cellular ultrastructure to verify possible damage to the cellular membrane. Methods: Screening was performed to select substances with possible antimicrobial activity, then the minimal inhibitory concentrations, bactericidal activity and 24-h time-kill curve studies were evaluated by standard protocols. In addition, the ultrastructure of control and death bacteria were evaluated by scanning electron microscopy. Results: Only 16 of the 33 compounds had antimicrobial activity at the initial screening. Eugenol exhibited rapid bactericidal action against Salmonella enterica serovar Typhimurium (2 h). Terpineol showed excellent bactericidal activity against S. aureus strains. Carveol, citronellol and geraniol presented a rapid bactericidal effect against E. coli. Conclusions: The higher antimicrobial activity was related to the presence of hydroxyl groups (phenolic and alcohol compounds), whereas hydrocarbons resulted in less activity. The first group, such as carvacrol, l-carveol, eugenol, trans-geraniol, and thymol, showed higher activity when compared to sulfanilamide. Images obtained by scanning electron microscopy indicate that the mechanism causing the cell death of the evaluated bacteria is based on the loss of cellular membrane integrity of function. The present study brings detailed knowledge about the antimicrobial activity of the individual compounds present in essential oils, that can provide a greater understanding for the future researches.