Identity and Names
Eugenol's preferred IUPAC name is 2-Methoxy-4-(prop-2-en-1-yl)phenol. The name is derived from Eugenia caryophyllata, the former Linnaean nomenclature term for cloves. The currently accepted name is Syzygium aromaticum.
Occurrence in Cloves and Other Plants
Eugenol comprises 72-90% of the essential oil extracted from cloves, and is the compound most responsible for clove aroma. Clove bud oil contains eugenol at 70-90%, eugenyl acetate at up to 17% and beta-caryophyllene at 5-12%. Clove stem oil has a eugenol content of 90-95%, but a low eugenyl acetate content. Clove leaf oil usually has a eugenol content of 80-88%, a low eugenyl acetate content and a high caryophyllene content.
The clove tree yields three different essential oils: from the flower buds, which hold 15-17% oil, from the flower stems, which hold 6%, and from the leaves, which hold 2-3%. In general, the best oil comes from the flower bud, the second best from the flower stem and the third from the leaf. Clove bud oil is a clear, colourless to yellow liquid that turns browner with age. Clove stem oil is a colourless to light yellow liquid. Crude clove leaf oil is dark brown, often with a purple or violet tinge, and somewhat cloudy; once rectified it is a clear pale yellow.
The clove itself is the flower bud of an evergreen tree that grows up to 8-12 metres tall; the bud is 1-2 cm long and is picked just before it opens.
Chemistry and Physical Behavior
The biosynthesis of eugenol begins with the amino acid tyrosine. Eugenol and isoeugenol are both floral volatile scent compounds. Isoeugenol, a propenyl-substituted guaiacol, can be synthesized from eugenol. Eugenol is slightly soluble in water and soluble in ether and most fixed oils. It darkens and thickens on exposure to air. Eugenol is denser than water and sinks in it. Its pKa is 10.19 at 25 °C.
In the biosynthesis, L-tyrosine is first converted to p-coumaric acid by the enzyme tyrosine ammonia lyase. In the genus Gymnadenia, a single type of enzyme catalyzes the formation of both eugenol and isoeugenol. Eugenol dissolves in water at 2460 mg/L at 25 °C, and it is miscible with alcohol, chloroform, ether and oils. One mL of it dissolves in 2 mL of 70% alcohol. It also dissolves in glacial acetic acid and in aqueous solutions of fixed alkali hydroxides. Its viscosity is 7.817 centipoise at 20 °C. When heated to decomposition it gives off acrid smoke and irritating fumes. Eugenol benzoate forms crystals that melt at 69-70 °C.
Isoeugenol occurs in the essential oil of ylang-ylang and is a component of wood smoke and liquid smoke. Its trans isomer is crystalline, while its cis isomer is a liquid. Eugenol has not been classified for its potential toxicity, but it was toxic to test organisms at concentrations of 50, 75 and 100 mg per liter.
Dental and Other Uses
Eugenol can be combined with zinc oxide to form zinc oxide eugenol, which has restorative and prosthodontic applications in dentistry. Eugenol-zinc oxide paste is also used for root canal sealing. In zinc oxide eugenol pastes, clove oil, which contains 70% to 85% eugenol, is sometimes used instead of eugenol because it causes less burning sensation in patients when it comes into contact with soft tissues. Eugenol is attractive to males of various species of orchid bees, which apparently gather the chemical to synthesize pheromones. It is commonly used as bait to attract and collect these bees for study. Eugenol is an ingredient in some insecticides.
Zinc oxide eugenol forms in an acid-base reaction that produces zinc eugenolate chelate; water catalyzes the reaction and metal salts speed it up. The material serves as a dental filling or dental cement, often when decay is very deep or very close to the nerve or pulp chamber. It is classified as an intermediate restorative material and has anaesthetic and antibacterial properties. A typical formulation contains about 69% zinc oxide, 29.3% white rosin, 1% zinc acetate and 0.7% zinc stearate, with a liquid of about 85% eugenol and 15% olive oil. A dental composite resin should not be placed on it, because it inhibits resin polymerization through a radical-scavenging effect.
For a dry socket after tooth extraction, packing the socket with a eugenol-zinc oxide paste on iodoform gauze reduces acute pain. The paste is also used as an impression material in making complete dentures, and as an antimicrobial additive in paint. Eugenol also attracts female cucumber beetles. In the United States, cloves and clove bud, stem and leaf oils have the regulatory status of generally recognized as safe.
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