ANETHOLE: STRUCTURE, PROPERTIES, USES, TOXICITY - CHEMISTRY - 2025

The anethole is an organic compound of molecular formula C ₁₀ H ₂₂ O, derived from the phenylpropene. It has a characteristic smell of anise oil and a sweet taste. It is naturally found in some essential oils.

Essential oils are liquid at room temperature, being responsible for the smell of plants. They are mainly found in plants of the labiate family (mint, lavender, thyme and rosemary) and the umbelliferous (anise and fennel); from the latter, anethole is extracted by steam stripping.

Star anise. Source: Pixabay

Anise and fennel are plants that contain anethole, a phenolic ether, which in the case of anise is found in its fruit. This compound is used as a diuretic, carminative, and expectorant. It is also added to food to give them flavor.

It is synthesized by esterification of p-cresol with methyl alcohol, and subsequent condensation with aldehyde. Anethole can be extracted from plants that contain it by steam distillation.

Anethole has toxic effects, and can cause irritation to the skin, eyes, respiratory tract or digestive tract, depending on the site of contact.

Structure of anethole

Anethole molecule. Source:..TTT.., from Wikimedia Commons

The structure of an anethole molecule is shown in the upper image in a sphere and rod model.

Here you can see why it is a phenolic ether: on the right is the methoxy group, –OCH ₃, and if you ignore CH ₃ for a moment, you will have the phenolic ring (with a propene substituent) without hydrogen, Ring-. Therefore, in summary form its structural formula could be visualized as ArOCH ₃.

It is a molecule whose carbon skeleton can be located in the same plane, as almost all of its atoms have sp ² hybridization.

Its intermolecular force is of the dipole-dipole type, with the highest electron density located towards the ring region and the methoxy group. Note also the relatively amphiphilic character of anethole: –OCH ₃ is polar, and the rest of its structure is apolar and hydrophobic.

This fact explains its low solubility in water, behaving like any fat or oil. It also explains its affinity for other fats present in natural sources.

Geometric isomers

Isomers cis (Z), top, and trans (E), bottom. Source: Jü, from Wikimedia Commons.

Anethole can be present in two isomeric forms. The first image of the structure showed the trans (E) form, the most stable and abundant. Again, this structure is shown in the upper image, but accompanied by its cis (Z) isomer, at the top.

Note the difference between the two isomers: the relative position of –OCH ₃ with respect to the aromatic ring. In the cis isomer of anethole, –OCH ₃ is closer to the ring, resulting in steric hindrance, which destabilizes the molecule.

In fact, such is destabilization that properties such as the melting point are altered. As a general rule, cis fats have lower melting points, and their intermolecular interactions are less efficient compared to trans fats.

Properties

Names

annexol and 1-methoxy-4-propenyl benzene

Molecular formula

C ₁₀ H ₂₂ O

Physical description

White crystals or colorless liquid, sometimes pale yellow.

Boiling point

454.1 ° F to 760 mmHg (234 ° C).

Melting point

704 ° F (21.3 ° C).

Ignition point

195ºF.

Water solubility

It is practically insoluble in water (1.0 g / l) at 25 ºC.

Solubility in organic solvents

In a 1: 8 ratio in 80% ethanol; 1: 1 in 90% ethanol.

Miscible with chloroform and ether. It reaches a concentration of 10 mM in dimethyl sulfoxide. Soluble in benzene, ethyl acetate, carbon disulfide, and petroleum ether.

Density

0.9882 g / ml at 20ºC.

Vapor pressure

5.45 Pa at 294ºK.

Viscosity

2.45 x 10 ^-3 Poise.

Refractive index

1,561

Stability

Stable, but it is a combustible compound. Incompatible with strong oxidizing agents.

Storage temperature

Between 2 and 8 ºC.

pH

7.0.

Applications

Pharmacological and therapeutic

Anethole trithione (ATT) is attributed numerous functions, including increased salivary secretion, which helps in the treatment of xerostomia.

In anethole, and in the plants that contain it, an activity related to the respiratory system and the digestive system has been demonstrated, in addition to having an anti-inflammatory, anticholinesterase and chemopreventive action.

There is a relationship between the anethole content of a plant and its therapeutic action. Therefore, the therapeutic action is attributed to atenol.

The therapeutic activities of plants that contain anethole make all of them antispasmodic, carminative, antiseptic and expectorant. They also have eupeptic, secretolytic, galactogogic properties and, at very high doses, emenogogic activity.

Anethole shows a structural similarity to dopamine, which is why it is pointed out that it can interact with neurotransmitter receptors, inducing the secretion of the hormone prolactin; responsible for the galactogogic action attributed to atenol.

Star anise

Star anise, a culinary flavoring, is used to treat stomach aches. In addition, it is attributed analgesic, neurotropic and fever-reducing properties. It is used as a carminative and in the relief of colic in children.

Insecticidal, antimicrobial and antiparasitic action

Anise is used against aphid insects (aphids), which suck the leaves and shoots causing them to curl.

Anethole acts as an insecticide on the larvae of the mosquito species Ochlerotatus caspices and Aedes egypti. It also acts as a pesticide on the mite (arachnid). It has an insecticidal action on the cockroach species Blastella germanica.

Likewise, it acts on several adult species of weevils. Finally, anethole is a repellent agent for insects, especially mosquitoes.

Anethole acts on the enteric Salmonella bacteria, acting in a bactericidal and bacteriostatic way. It has antifungal activity, especially on Saccharomyces cerevisiae and Candida albicans species, the latter being an opportunistic species.

Anethole exerts an anthelmintic action in vitro on the eggs and larvae of the nematode species Haemonchus contortus, located in the digestive tract of sheep.

In food and beverages

Anethole, as well as plants that have a high content of the compound, is used as a flavoring in many foods, beverages and confectioneries, due to its pleasant sweet taste. It is used in alcoholic beverages such as ouzo, raki, and Pernoud.

Due to its low solubility in water, anethole is responsible for the ouzo effect. When water is added to the ouzo liquor, tiny drops of anethole are formed that cloud the liquor. This is proof of its authenticity.

Toxicity

It can cause eye and skin irritation by contact, manifesting on the skin in erythema and edema causing flaking. By ingestion it can produce stomatitis, a sign that accompanies the toxicity of anethole. While by inhalation there is irritation of the respiratory tract.

Star anise (high in anethole) can cause allergies, especially in babies. Likewise, excessive consumption of anethole can induce the appearance of symptoms, such as muscle spasms, mental confusion and drowsiness due to its narcotic action.

Star anise poisoning increases when used intensively in the form of pure essential oils.

References

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3. Be Kind Botanicals. (2017). Fennel & other essential oils with (E) -anethole. Recovered from: bkbotanicals.com
4. Chemical Book. (2017). trans-Anethole. Recovered from: chemicalbook.com
5. PubChem. (2019). Anethole. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Wikipedia. (2019). Anethole. Recovered from: en.wikipedia.org
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8. Admin. (October 21, 2018). What is anethole? Trusted Health Products. Recovered from: trustedhealthproducts.com