All essential oils consist of multiple components, which means they are made up of a wide range of organic compounds. To give you a glimpse into the intriguing effects of these oils, the primary categories of their ingredients are outlined below. These substances can be categorized together due to their similar biochemical structures and effects. The names of the ingredients and their groups are based on their chemical makeup.
The descriptions of their effects are informed by scientific studies on the individual components as well as practical observations.
It's important to remember that the characteristics of a pure, natural essential oil cannot be understood solely by examining the individual components; rather, the unique synergy among them contributes to the distinctiveness of each oil.
The ingredients can be roughly divided into two main groups:
- The terpenes and their derivatives, which make up the largest proportion at around 90%.
- Aromatic compounds are characterized by the presence of a benzene ring. Plants produce these compounds through various synthesis methods. Notable examples include phenylpropane compounds.
Generally, different groups of ingredients vary based on their functional groups. In these compounds, one or more hydrogen atoms on the terpene structure or the benzene ring can be substituted with other atoms or groups, such as alcohol, aldehyde, ketone, acid, or oxide residues. These functional groups, along with the core structure, play a crucial role in defining the properties, color, fragrance, and effects of each ingredient.
Terpenes
Terpenes are commonly found in both the plant and animal kingdoms. In plants, they serve as a key ingredient in essential oils, particularly in flowers, leaves, fruits, bark, and roots. Additionally, terpenes encompass substances like balms, resins, steroids, pheromones, plant hormones, and dyes.
The amount of terpenes released by coniferous forests around the globe is substantial, estimated to exceed 1 billion tons annually! This means we are constantly surrounded by terpenes and their oxidation byproducts. Therefore, any concerns regarding the oxidation products of essential oils should be approached with caution; otherwise, we might as well place a stop sign at the entrance of every forest.
The chemical structure
Terpenes are commonly found in both the plant and animal kingdoms. In plants, they serve as a key ingredient in essential oils, particularly in flowers, leaves, fruits, bark, and roots. Additionally, terpenes encompass substances like balms, resins, steroids, pheromones, plant hormones, and dyes.
Terpenes are composed of isoprene units, which are naturally occurring hydrocarbons that have two double bonds and follow a specific biochemical structure. The fundamental unit is isoprene, made up of five carbon atoms and eight hydrogen atoms, but it lacks functional properties.
A terpene is formed by linking two or more isoprene units, adhering to what is known as the isoprene rule. When two isoprene units combine, they create monoterpenes, which contain ten carbon atoms. Three isoprene units yield sesquiterpenes, consisting of fifteen carbon atoms, while four isoprene units result in diterpenes, which have twenty carbon atoms. Diterpenes are large, low-volatile compounds that are typically found only in trace amounts in essential oils.
Terpenes with more than twenty carbon atoms are non-volatile and, as a result, are absent from the distillates of essential oils. Additionally, there are naturally occurring terpenes that do not follow the isoprene rule, known as irregular terpenes. However, their biosynthesis also begins with two isoprene units.
| The most important terpenes and their occurrence | ||
|---|---|---|
| Monoterpenes | 2 x 5 C-atoms | Most essential oils, pheromones |
| Sesquiterpenes | 3 x 5 C-atoms | Essential oils, bitter substances, balsams |
| Diterpenes | 4 x 5 C-atoms | Essential oils, balsams, resins, phytol, vitamin A, plant growth hormones, pheromones |
| Triterpenes | 6 x 5 C-atoms | Resins, sterols, steroids, plant hormones |
| Tetraterpenes | 8 x 5 C-atoms | Plant pigments, e.g., carotenoids |
| Polyterpene | more than 8 x 5 C-atoms | Plant milks, rubber |
Key Elements of Essential Oils
Essential oils are primarily composed of monoterpenes and sesquiterpenes, along with their derivatives. During the biosynthesis of isoprenoids, various functional groups—such as oxygen, nitrogen, or sulfur—are added to different points in the carbon framework of the terpenes. The most significant derivatives are those containing oxygen, which are products of oxidation. Depending on their chemical structure, these compounds can be classified as terpene alcohols, aldehydes, ketones, oxides, phenols, and esters. Each of these has unique properties and effects that set them apart from pure hydrocarbon compounds (terpenes).
Terpenes can be categorized as acyclic (linear), monocyclic (single ring), dicyclic (two rings), or tricyclic (three rings). This diversity in structure accounts for the vast range of fragrances found in essential oils. The table below outlines the main groups of terpenes and their derivatives, typically referring to them by their functional group names. The terms "monoterpenes" and "sesquiterpenes" are often used to encompass the entire group, including their derivatives.
| Terpenes and their derivatives (derivatives/oxidation products) | |||
|---|---|---|---|
| Functional group | Terpene groups then C atoms | 15 C-atoms | |
| Pure hydrocarbon compound | Monoterpenes | Sesquiterpenes | |
| Derivats (Oxidation products) | Alcohols | Monoterpenols | Sesquiterpenols |
| Ester | Monoterpene esters | Sesquiterpene esters | |
| Aldehydes | Monoterpene aldehydes | Sesquiterpene aldehydes | |
| Ketones | Monoterpene ketones | Sesquiterpene ketones | |
| Oxides | Monoterpene oxides | Sesquiterpene oxides | |
| Phenols | Monoterpene phenols | ||
Aromatic Compounds - Benzene Compounds
The benzene ring serves as the foundation for many aromatic compounds that have a strong scent. This structural feature is a defining characteristic across all compounds in this category. The terminology for these compounds evolved over time alongside the discovery of the benzene ring system, which has led to some confusion. Terms like benzene, benzyl, phenyl compounds, and aromatic compounds are often used interchangeably to refer to the same substances.
The benzene ring
In a benzene ring, represented by the chemical formula C6-H6, six carbon atoms are linked together in a circular formation, with six hydrogen atoms evenly spread around the ring. This structure is often referred to as an "aromatic ring."
Traditional structural formulas fail to capture the unique characteristics of the aromatic ring. To maintain the stability of this ring system, some electrons are delocalized, allowing them to be evenly distributed throughout the structure. In structural representations, this delocalization is typically indicated by an inner ring.
When one or more hydrogen atoms are substituted with different atoms or molecules, the resulting structures are known as benzyl compounds, phenyl compounds, or aromatic compounds.
Classification of ingredient groups
This group of substances is created through various synthetic processes within the plant. In the following sections, the classification of aromatic ingredient groups will be based not only on their origin or synthesis methods but also on their physical and psychological effects. As a result, multiple groups can be grouped together and discussed collectively.
| Classification of ingredient groups | |
|---|---|
| Monoterpene phenols | |
| Phenylpropane derivatives | Eugenol |
| Cinnamaldehyde | |
| Ether | |
| Coumarins | |
| Aromatic Esters and Alcohols | Aromatic esters |
| Aromatic alcohols | |
| Aromatic Aldehydes, Ketones and Acids | Aromatic aldehydes |
| Aromatic acids | |
| Aromatic ketones | |
Chemical Constituents Distribution of Essential Oils (360 profiles was used to generate this graph)
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