Gvajakol

Guaiacol has the formula C₆H₄(OH)(OCH₃), and its IUPAC name is 2-methoxyphenol. It occurs widely in nature, and wood pyrolysis commonly yields it. The pure substance is colorless, but samples turn yellow when exposed to air and light. It solidifies at 28 °C, yet it may stay liquid for a long time well below that temperature. It mixes in all proportions with alcohol, chloroform, ether, oils and glacial acetic acid. In the chemical literature it is described as colorless to amber crystals or liquid. It is slightly soluble in water and dissolves in sodium hydroxide solution. Heated to decomposition, it gives off acrid smoke and irritating fumes.

Occurrence

Guaiacol is usually derived from guaiacum or from wood creosote. A variety of plants produce it. Essential oils from celery seeds, tobacco leaves, orange leaves and lemon peels also contain it. Wood smoke contains the compound, formed by the pyrolysis of lignin. Pyrolysis of lignin from gymnosperms gives more guaiacol, which arises when the propenyl group of coniferyl alcohol is removed. Such lignins are said to have a high guaiacyl content. Lignins derived from sinapyl alcohol give syringol instead, and a high syringyl content indicates lignin from angiosperms. Sugarcane bagasse is one useful source: pyrolysing its lignins yields guaiacol, 4-methylguaiacol and 4-vinylguaiacol.

Wood-Tar Creosote

Chemically, guaiacol is a methyl ether of pyrocatechin, and creosol is the methyl ether of the next homolog, methyl-pyrocatechin. Methyl ethers differ from simple phenols in being less hydrophilic, caustic and poisonous. Wood-tar creosote is used for its guaiacol and creosol content. It is therefore generally derived from beechwood, which distills with a higher proportion of those chemicals relative to other phenolics than other woods do. In a typical beech-tar creosote, guaiacol makes up 25.0% and creosol with its homologs 35.0%.

Industrial Uses

Guaiacol is a useful precursor for the synthesis of other compounds. Because it comes from biomass, it is a potential component or precursor of green fuels. An estimated 85% of the world's vanillin and ethyl vanillin comes from guaiacol. Of the several routes from guaiacol to vanillin, the most significant at present is a two-step process that Rhodia has practiced since the 1970s. In it, guaiacol reacts with glyoxylic acid by electrophilic aromatic substitution. The vanillylmandelic acid that forms is then converted to vanillin by oxidative decarboxylation. The crude vanillin can then be purified by vacuum distillation and recrystallization. Because consumers tend to prefer natural vanillin, methods such as microbial fermentation have also been adopted. Guaifenesin, also called glyceryl guaiacolate, is an expectorant medication, and chemically it is an ether of guaiacol and glycerine. Guaiacol is also used medicinally as an expectorant, an antiseptic and a local anesthetic. It is a useful reagent for quantifying peroxidases, which in the presence of hydrogen peroxide form tetraguaiacol, a coloured compound that can be quantified by its absorbance at 420-470 nm.

Safety

Guaiacol carries the hazard statements H302, H315 and H319. Its oral LD50 in rats is 520 mg/kg. Methoxyphenols are potential biomarkers of exposure to biomass smoke, such as from inhaling woodsmoke. Dietary sources of methoxyphenols far outweigh the contribution from inhaled woodsmoke.

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