Important terpenoids derived from monocyclic terpenes are menthol, thymol, carvacrol and many others. Condensation with acetylene gives 2-methylbut-3-yn-2-ol, precursor to synthetic terpenes and terpenoids. Major classes of phytochemical include phenols, polyphenols, tannins, terpenes, and alkaloids. Taste and aroma compounds include sucrose, glucose, organic acids, terpenes, aldehydes and lactones. While terpenes and terpenoids occur widely, their extraction from natural sources is often problematic. Lemons contain numerous phytochemicals, including polyphenols, terpenes, and tannins. Lemon balm contains eugenol, tannins, and terpenes. Although sometimes used interchangeably with "terpenes", terpenoids contain additional functional groups, usually O-containing. The wood extractives are fatty acids, resin acids, waxes and terpenes. A major class of VOCs is terpenes, such as myrcene. One of the terpenes in the secretion, pinene, functions as an alarm pheromone. Hundreds of peroxides and hydroperoxides are known, being derived from fatty acids, steroids, and terpenes. Examples include antibiotics and pigments such as resins and terpenes etc. Most plant resins are composed of terpenes. Hundreds of hydroperoxides are known, being derived from fatty acids, steroids, and terpenes.