Guaiacol
Guaiacol () is a naturally-occurring organic compound with the formula CH(OH)(OCH). Although it is biosynthesized by a variety of organisms, this aromatic oil is usually derived from guaiacum or wood creosote. It is also found in essential oils from celery seeds, tobacco leaves, orange leaves, and lemon peels. The pure substance is colorless, but samples become yellow upon exposure to air and light. Guaiacol is present in wood smoke, resulting from the pyrolysis of lignin. The compound contributes to the flavor of many substances such as whisky and roasted coffee. Preparation. The compound was first isolated by Otto Unverdorben  in 1826. Guaiacol is produced by methylation of "o"-catechol, for example using potash and dimethyl sulfate: Laboratory methods. Guaiacol can be prepared by diverse routes in the laboratory. "o"-Anisidine, derived in two steps from anisole, can be hydrolyzed via its diazonium derivative. Guaiacol can be synthesized by the dimethylation of catechol followed by selective mono-demethylation. Uses and chemical reactions. Guaiacol is a useful precursor for the synthesis of other compounds. Being derived from biomass, it is a potential component or precursor to "green fuels". Guaiacol is also a useful reagent for the quantification of peroxidases, as in the presence  of hydrogen peroxide these enzymes will catalyse with it the formation of tetraguaiacol, a coloured compound that can be quantified by its absorbance at 420–470 nm, following the equation: 4 + 2 → + 8 . Medicinal and food. Guaiacol is a precursor to various flavorants, such as eugenol. An estimated 85% of the world's supply of vanillin comes from guaiacol. The route entails condensation of glyoxylic acid with guaiacol to give mandelic acid, which is oxidized to produce a phenylglyoxylic acid. This acid undergoes a decarboxylation to afford vanillin. Guaiacol is also used medicinally as an expectorant, antiseptic, and  local anesthetic. Guaiacol is produced in the gut of desert locusts, "Schistocerca gregaria", by the breakdown of plant material. This process is undertaken by the gut bacterium "Pantoea agglomerans" ("Enterobacter"). It is one of the main components of the pheromones that cause locust swarming. Safety. Methoxyphenols are potential biomarkers of biomass smoke exposure, such as from inhalation of woodsmoke. Dietary sources of methoxyphenols overwhelm the contribution from inhalational exposures to woodsmoke.