Terpenes encompass a class of naturally occurring hydrocarbons in plants and animals and are arguably the largest and most diverse class of natural products. Although most terpenes are found in plants, some of the larger and more complex forms of terpenes such as lanosterol and squalene occur in animals. Most of the common functional groups of terpenes are known, and this doesn’t provide a useful means of classification. A definitive characteristic among them is the number of carbon atoms and their structural organization.
Terpenes are empirically regarded as built up from isoprene units (precisely isopentane units), which consists of five carbon atoms attached to eight hydrogen atoms to form C5H8. The term terpene refers to the naturally occurring compounds derived from a single isoprene unit and is often used interchangeably with ‘terpenoids’, which are the oxygenated versions of terpenes. The modern usage of the terms has largely become less precise, and the isoprenoids is also used to refer to the same. The word ‘terpene’ is actually derived from an older spelling of the word “turpentine/terpentine, a cleaner mixture of isoprenoids that was originally a resin of the terebinth plant.
Monoterpenes (C10H16) are the smallest occurring terpene molecules, containing 10 carbon atoms. The subsequent larger molecules, which increase by one isoprene unit at a time, are:
- Sesquiterpenes (C15H24)
- Diterpenes (C20H32)
- Triterpenes (C30H48)
- Tetraterpenes (C40H64)
Monoterpenes, diterpenes, and sesquiterpenes, are plentiful in the essential oils of plants. Turpentine contains a number of monoterpenes, while rosin acids are diterpenes. An important, yet common diterpenes is Vitamin A.
Monoterpenes are generally the most volatile, which gives them their distinct fragrances. As the molecular weight of the terpenes increases, they become less volatile, though sesquiterpenes account for the flavors of some foods.
Terpenes of Plants and Animals
The biological formation of terpenes occurs when two molecules of acetic acid (CH3COOH) are combined to produce mevalonic acid (C6H12O4 ), which is then converted into isopentenyl pyrophosphate. This contains the 5-carbon-atoms isoprene skeleton, whose further transformation yields the true terpenes and terpenoids.
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Plant isoprenoids occur in essential oils, and they are found in oleoresins and latexes (the gummy exudates) of various shrubs and trees. These compounds influence the growth of plants and account for the yellow, red, and orange pigments such as carotenoids. The green pigment, chlorophyll, which is vital in photosynthesis, is partly an isoprenoid, just as certain nitrogen-containing compounds and alkanoids found in plants. In animals, isoprenoids occur as waxy or oily substances such as wool wax, fish liver oils, and the yellow pigment in butterfat, egg yolk, fish scales, and feathers.
Although terpenes are not uniformly distributed among animals and plants, certain classes are typically found in a broad group of tissues. For instance, trees in the pine family produce resins with non-volatile substances containing diterpenes carboxylic acids. These belong to palustric, abeitic, and elliotionic carboxylic acids. A few species of plants have latexes that are comprised of polyterpene hydrocarbons gutta-percha or rubber. Other plant species (some related) are characterized by the presence of terpenes such as citral, menthol, limonene, camphor, and α-pinene.
The purpose of the production of sesquiterpenes and monoterpenes in plants is yet to be established. However, it’s been suggested that these compounds help to attract certain animals (insects) and repel others. Some plants produce isoprene compounds that are quite similar to the kind of hormones involved in the growth of insects that ideally prey on those plant. These compounds stop the maturation of the insect so that the plant can benefit from the protection. The gutta-percha and rubber in latexes of certain plants and the rosin acids produced by the pine trees may also serve as healing agents.
Some terpenes are essential in the metabolic processes of animals. The carotenoid tetraterpene pigments are a source of Vitamin A, which is involved in growth and is crucial for vision, neural development, and the reproductive function in animals. Vitamin K, which is necessary for blood clotting, and Vitamin E, necessary for reproduction, are also partly terpenoids.1 The livers of some animals and fish are rich in acyclic triterpenoid oils, particularly squalene. In insects, some terpenoids substances influence mating behavior and maturation, mark the way between sources of food and the next, or even repel predators.
In studies conducted in the 1980s, scientists found out that many proteins in animals are made up of terpenoid structures that contain 15 or 20 carbon atoms attached to a certain part of the chain of the protein. However, the terpenoid is added when the protein is complete. Steroids are another class of compounds that are essential to both animals and plants that, although not terpenoids, are directly derived from them.
Structural Features of Terpenes
Terpenes present a number of problems in their molecular structure, which have challenged organic chemists since the 19th century. Initial studies focused on the structures of monoterpenes since these only contain 10 carbon atoms and were easier to study. Nonetheless, as investigation techniques became more advanced and the structural patterns became more familiar, attention was gradually focused to the terpenes containing 15 to 40 carbon atoms.
In 1887, Otto Wallach, a German chemist, identified the fact that the fundamental unit made up of 5 carbon atoms could be connected in many different ways to create the many different kinds of carbon atoms arrangements in monoterpenes and sesquiterpenes. His proposal is what’s widely known as the isoprene rule, which has helped organic chemists to understand the structures of even the more complex terpene compounds.
Isoprene itself is a gaseous hydrocarbon (C5H8) emitted by certain plants as a natural byproduct of plant metabolism. Along with methane, it’s the most volatile organic compound present in the atmosphere. 5-carbon-atoms isoprene skeleton describes the pattern in which carbon atoms are bonded and arranged in a molecule, without accounting for the atoms of other elements and the differences between multiple and single bonds. That’s because much of organic chemical reactions typically don’t break the bonds between carbon atoms, meaning the carbon skeleton is left unchanged.
Structural Classification of Terpenoids
Terpenes are broadly categorized based on the number of isoprenes (C5H8) units they’re made up of, and range in size from the highly volatile monoterpene oils (C10H16) to the giant isoprenes like natural rubber that contain over 4,000 isoprene units. Most of the terpenes contain carbon skeletons that might be regarded as containing isoprene units that are linked from head to tail. This means that a carbon atom 1 of one isoprene unit is bonded to a carbon atom 4 of the adjacent isoprene unit.
The formation of subsequent bonds in different ways results into monocyclic, bicyclic, and more subclasses with one, two, or more rings. This resulting classification is demonstrated by:
- Limonene �” monocyclic monoterpene
- Myrcene �” an acyclic monoterpene
- Vitamin A �” oxygenated monocyclic diterpenes
- α-pinene �” a bicyclic monoterpene
Most triterpenes have structures that show they were formed by a tail to tail bond (between carbon 4 atom and carbon 4 of the next) between two smaller isoprene units. For example, the structural formula of squalene exhibits a bond uniting two sesquiterpenes units.
Uses of Terpenes
Most of the uses of terpenes were discovered in antiquity, primarily as ingredients of incense and perfumes, spices and flavorings, medicine, and varnishes. The fossilized terpenoid called amber has been prized as a gemstone since the pre-historic times. Central and South American Indians used rubber widely before the Spanish conquest.
Modern application of terpenes is quite diverse. Turpentine has widely been used as a solvent. Its individual components have for a long time been used as a source of raw materials for certain forms of chemical processing. Some of the products derived from turpentine include ingredients for Vitamin A, perfumes, insecticides, lubricant additives, industrial chemicals, and resins used in making adhesives.
Resin, which is usually modified through chemical treatment, is popularly used in making inexpensive soaps and different kinds of coating materials. Other terpenoids such as menthol, camphor, natural rubber, and citronella oil are very valuable in commerce and industry.
- Alpha-pinene, one of the most known terpenes, is known to act as a natural expectorant and bronchodilator for respiratory health and can help a person focus better. It also has powerful energizing effects on the brain and can act as a topical antiseptic.
- Limonene has antibacterial, antifungal, and antidepressant effects. It has been suggested that it contains anti-carcinogenic properties, though more research is needed to confirm this. Limonene is also known to increase blood pressure.1
- Myrcene has been shown to change the permeability of cell membranes, which allows for more absorption of cannabinoids in the brain. It has widely been used in the perfume industry. It contains antiseptic, anti-microbial, anti-carcinogenic, and anti-inflammatory properties, and acts as a natural anti-depressant.
Isolation of Terpenes
A number of procedures have been developed to perform the isolation processes of terpenoids from their natural sources. The choice of a technique is ideally dependent on a number of factors, including the physical and chemical properties of the compound, its abundance in the source, and its distribution in nature.
For instance, the most abundant and volatile substances like turpentine are extracted through the distillation of rosin acids, oleoresins, and fatty acids in a tall oil, which is a by-product of the paper pulp manufacturing process from pine wood. The compounds are then separated via fractional distillation at a low pressure. On the other hand, the extremely rare, non-volatile compounds are isolated through the chromatography process, which is a method for separating the different components of a mixture or solution based on the adsorption onto a column or layer of a suitable material.
To isolate the heat-sensitive ingredients for making perfume from flower petals, enfleurage is used. This laborious process involves placing the petals in thin layers of carefully purified fat, where the floral oils dissolve. The solution is then washed with alcohol to recover the floral oils.
The chemical isolation of terpenoids follows the classic pattern that has steps with the goal of furnishing a full description of the molecule. The description generally involves the identification of the different kinds and the number of atoms present in the molecule, the arrangement (3-D) of atoms, and the chemical bonds holding the atoms together. The operation necessary to perform the identification includes the determination of atomic composition, purification, and identifying the assignment of structure.
Types of Terpenes
Monoterpenes are isolated from their natural sources through the steam distillation of the plant materials. The resulting oils are highly volatile, with normal boiling points ranging from 300 to 365 degrees Fahrenheit (150 to 185 degrees Celsius), and have densities lesser than that of water. Purification is generally performed through fractional distillation conducted at low pressures or through regeneration from crystalline derivatives.
Although acyclic monoterpene compounds are quite a few in number, their oxygenated variants are much more widespread in nature and are generally of greater importance. Some of the most important oxygenated acyclic monoterpene compounds include the terpene aldehyde citronellal and it precursor alcohol citronellal. Both of these occur in the citral found in lemongrass, the oil of citronella, and geraniol, which is found in the Turkish geranium oil.
Citronellal is converted into isopulegol, a monocyclic monoterpene alcohol from which a mixture of stereoisomeric menthols is produced through a process known as hydrogenation. This process is commercially used to supplement the natural sources of the peppermint oil (menthol), which is widely used for food flavoring and in aromatherapy.
Upon reduction with a sodium amalgam, citral yields geraniol, which is an important component of the various rose scented perfumes. Citral can also be condensed using acetone to create pseudo-ionone, an important intermediate from which β-ionone is extracted by treating it with acid. Although β-ionone is not exactly a terpene, it’s vastly important as a starting substance for the synthesis of vitamin A as well as an ingredient in violet-scented perfumes.
Limonene is an oil with a boiling point of 352 °F (178 °C) and serves as a major component of lemon and orange essential oils. It’s a typical monocyclic monoterpene compound, along with terpiolene, and phellandrene, which ideally share an exact carbon skeleton with limonene. They only differ in terms of where the two carbon 1 to carbon 4 bonds is located. Limonene is also optically active and rotates the plane of polarized light. This is a property that terpenes containing asymmetric carbon atoms and their derivatives exhibit. When limonene comes into contact with a heated metallic filament, it’s converted to isoprene.
There are few commercial uses for monocyclic monoterpene compounds, other than their use in food flavoring. Menthol, as mentioned earlier, and the oxygenated derivatives terpin hydrate and α-terpineol are commercially viable chemicals. When the two compounds are mixed with terpinolene, α-terpineol, and the terpines derived from the treatment of α-pinene with acid, the resulting mixture is pine oil; a deodorant, disinfectant, and a wetting agent.
α-Pinene is a representative of bicyclic monoterpenes and has a boiling point of 313 degrees Fahrenheit (156 °C). it ideally one of the most plentiful and important monoterpene. The compound is a major component of the ordinary turpentine, which is derived from pine trees and stumps either via steam distillation or extraction followed by rectification. α-Pinene is also the main component of sulfate turpentine, which is a by-product of paper manufacture, and makes up a major ingredient of varnishes and paint and as the raw material for the manufacture of a wide range of products in the chemical industry.
Treating α-Pinene with acids under certain conditions results in a number of different products, including terpinenes, terpinolene, α-terpineol, fenchyl alcohol, borneol, terpin, and camphene. The formation of camphene, fenchyl alcohol, and borneol involves the rearrangement of the molecular structure, and this is quite important as it aids in the commercial synthesis of bicyclic terpenes such as ketone camphor.
Sesquiterpenes have a lower volatility than monoterpenes, and they are commonly extracted from their natural sources through steam distillation or by direct extraction. They are then purified via chromatography or by fractional distillation. Sesquiterpenes exhibit an even greater complexity of structure in comparison to monoterpenes, and their oxygenated variants are quite abundant.
Bicyclic sesquiterpenes with two arrangements of the isoprene units are the eudalene and cadalene types. The fundamental carbon skeleton of sesquiterpenes is often determined by heating the substance with selenium or sulfur to allow for dehydrogenation of the corresponding naphthalenic hydrocarbons: eudalene (7-isopropyl-1-methylnaphthalene) and cadalene (4-isopropyl-1,6-dimethylnaphthalene). In case the dehydrogenation by sulfur doesn’t yield substantial information about the carbon structures of the sesquiterpenes, a systematic degradation through oxidation to compounds of a known structure is important.
The principal component of the oils of cade and cubeb, cadinene, is a typical sesquiterpene. It’s optically active and has a boiling point of 525 °F (274 °C). A typical form of eudalne is β-Selinene, which is commonly present in celery oil.
An oxygenated variant of acyclic diterpenes, phytol, is an essential building block of the chlorophyll molecule. Phytol is obtained through the treatment of chlorophyll with an alkali solution. The isoprene units in phytol are arranged in a way identical to that of vitamin A, which is a monocyclic diterpene derivative that features a head to tail arrangement of the isoprene units in most terpenes.
The wide commercial importance of α-pinene is paralleled by abietic acid in diterpenes, which is a tricyclic carboxylic acid that makes up a major component of rosin. Rosin is a member of the non-volatile oleoresin in the pine family and is usually the residue of turpentine isolation. Rosin is mostly used in the manufacture of coating materials and varnish. When made into sodium salt, resin is used to sizing paper and in the production of synthetic rubber. It’s one of the cheapest organic acids available.
Squalene is one of the most popular acyclic triterpene hydrocarbons, and make up over half the liver oil of some species of shark. It’s also widely distributed in nature and has been found in the liver oil of other fish species, in fungi, vegetable oils, and human sebaceous and earwax secretions.
Radioactive carbon labeling revealed the biochemical importance of squalene in the biosynthesis of cholesterol as a metabolic intermediate. Although cholesterol is not actually a terpene, it has a terpene compound as a precursor in metabolism, which brought about a major advance in the understanding of the chemical relationship between these two essential classes of compounds.2
Although there are known tricyclic and tetracyclic triterpenes are known, and the most abundant of them contain 5 carbon rings. These pentacyclic triterpenes are either combined with sugars in glycosides or free and occur in many parts of plants.
The orange, yellow, or red fat-soluble animal and plant pigments called carotenoids are a popular form of tetraterpenes, though they have a general molecular formula of C40H56 instead of C40H64. The justification for them being classified as terpenes lies in the fact that their structures can be built from the basic isoprene units. Carotenoids are usually isolated from their natural sources through solvent extraction and are then purified through chromatography.
The red pigment of ripe tomato, lycopene, exemplifies the acyclic tetraterpene class. The usual head to tail attachment of isoprene units is interrupted at the center of the molecule under this category, with a single tail to tail attachment that creates a symmetric structure. This characteristic is generally found in tetraterpenes, as well as the long series of the alternating single and double carbon atom bonds that form a conjugated system, which is responsible for light absorption and the bright color of these compounds.
β-carotene, the principal yellow pigment of carrots, is the most abundant and important tetraterpene. It has a high nutritional importance because animals can cleave the molecule at its point of symmetry with the production of Vitamin A. Research has exemplified the purpose of the structurally related terpenoid molecules and vitamin A in the synthesis of the pigment in the eye, which is necessary for vision.
A popular polyterpene is a rubber, a substance that occurs in the latice of the rubber tree, with a molecular composition of (C5H8)n where n represents 4,000 to 5,000. The rubber vulcanization process entails establishing cross-links between the isoprene chains with sulfur atoms.
Another popular polyterpene, gutta-percha, differs from rubber in the way the methylene groups are arranged in its molecular structure. In gutta-percha, these methylene groups are arranged on opposite sides of the double bonds of the carbon atoms, while they are in the same side in rubber.
Here’s to Terpenes!
Terpenes are an endlessly fascinating corner of nature that has been drawing the interest of scientists and chemists for hundreds of years. As more and more research is conducted on these compounds, the results are proving not only positive but downright miraculous. It suggests that we should be incorporating the use of them in our daily lives on a regular basis to maintain a healthy lifestyle and stay centered, well rested and protected from harmful environmental pollutants.
People grew up with terpenes throughout evolutionary history, and it wasn’t until the modern era that we breathe air that is essentially devoid of terpenes. At MONQ® we call this Terpene Deficiency Syndrome.
For more detailed information, we have a curated list of aromatherapy references concerning terpenes.