Chemistry and properties
Isoeugenol is a propenyl-substituted guaiacol. Its formula is C₁₀H₁₂O₂ and its molecular weight is 164.20. Its IUPAC name is 2-methoxy-4-[(E)-prop-1-enyl]phenol. It may occur as either the cis (Z) or the trans (E) isomer. The trans isomer is crystalline, while the cis isomer is a liquid. Eugenol, from which isoeugenol can be synthesized, is an allyl chain-substituted guaiacol, whereas isoeugenol carries a propenyl chain.
Isoeugenol is an isomer of eugenol in which the allyl substituent is replaced by a prop-1-enyl group. Commercial isoeugenol is a mixture of the (E) and (Z) isomers, and the (E) isomer dominates because it is thermodynamically more stable. Heating the sodium or potassium salt of eugenol isomerizes it to isoeugenol, and ruthenium or rhodium compounds can bring about the same change catalytically. Catalytic hydrogenation converts it to dihydroeugenol, and esterification or etherification of its hydroxy group yields further fragrance materials.
As a liquid it is pale yellow and viscous, and it easily becomes somewhat yellow. It melts at -10 °C and boils at 266 °C. Its density is 1.080 g/cm³ at 25 °C, so it is denser than water and sinks. Its vapour pressure is 0.0135 mmHg at 25 °C (extrapolated) and its log Kow is 3.04. Its refractive index is 1.5739 at 19 °C, its viscosity is 7.476 centipoise at 20 °C and its surface tension is 30.813 dynes/cm. When heated to decomposition it emits acrid smoke and fumes. Its solubility in water is 810 mg/L at 25 °C; it is miscible with alcohol and ether and dissolves in most fixed oils. Its pKa is 9.88 at 25 °C.
Occurrence and uses
Isoeugenol is one of several structurally similar phenylpropenoid compounds produced by plants. It occurs in the essential oils of plants such as ylang-ylang (Cananga odorata). It is also a component of wood smoke and liquid smoke. Extractions have been reported from calamus, savory, basil, clove, tuberose, jonquil, nutmeg, tobacco, sandalwood, dill seed, mace, gardenia and petunia. It has also been reported in nutmeg oil, in the oil from the flowers of Michelia champaca and in the oil from the seeds of Nectandra puchury. In some chemotypes of the betel plant (Piper betle), isoeugenol is among the dominant compounds. Plants produce it as a defence against animals and microorganisms and as a floral attractant for pollinators. It can also act as an inhibitor of lipid peroxidation and as a free-radical scavenger. It has been used in the manufacture of vanillin, and vanillin was formerly prepared by oxidation of isoeugenol. Isoeugenol is one of several phenolic compounds responsible for the mold-inhibiting effect of smoke on meats and cheeses.
It is also used in food, household products, animal feed, and veterinary medicines. Its veterinary ATC code is QN01AX94. Firefighters and workers involved in isoeugenol synthesis or handling isoeugenol-containing products may be exposed. Some individuals experience a hives-like reaction to long-term exposure to isoeugenol. Sensitivity to isoeugenol may be identified with a clinical patch test.
Toxicology
In the National Toxicology Program studies, male and female F344/N rats and B6C3F1 mice were given isoeugenol of 99% or greater purity in corn oil by gavage for 3 months or 2 years. The two-year dose groups received 0, 75, 150 or 300 mg/kg. There was no evidence of carcinogenic activity in female rats, and the evidence in female mice was equivocal, based on increased histiocytic sarcoma. Exposure caused nonneoplastic lesions of the nose in male and female rats, of the nose, forestomach and glandular stomach in male and female mice, and of the kidney in female mice.
Gene mutation tests in Salmonella typhimurium and Escherichia coli were negative, as were chromosomal aberration tests in cultured Chinese hamster ovary cells. Micronucleated erythrocytes in mouse peripheral blood were negative in males and positive in females.
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