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Anyone who has walked through a forest or who enjoys the variety of fragrances that flowers have to offer knows that many plants and trees have their own distinctively pleasant odors. These odors are due to the volatile essential oils, many of which have been prized commodities since ancient times. If a list of commercially important essential oils was compiled the list would exceed 200 essential oils. Thyme, garlic, peppermint, and spearmint are but a few examples of the many essential oils used in commercial goods.
Essential oils have many applications in the world that can range from flavor enhancement to perfumes. Some oils have also been found to have medicinal properties such as antibacterial and antifungal abilities. Chaulmoogra oil is one of the few know curative agents for leprosy. The oils can also be used in the paint industry, where turpentine is used as a solvent for many paint products. These oils are often found in the intracellular spaces and glands of plant tissue.
The oils may be located throughout the entire plant, but are often concentrated in the seeds or flowers of the plant. Many of the essential oils are steam-volatile and can undergo steam distillation to isolate the oils. Another method of oil extraction from these plants is through both solvent extraction and pressing methods. The functional group that is mostly responsible for the characteristic odors and flavors of fruit and flowers is the ester.
Although the ester composes most of the scents it is not totally responsible for all scents. Beside the ester functional group an essential oil can also possess complex mixtures of hydrocarbons, alcohols, and carbonyl groups. These compounds usually belong to one of two groups of natural products called terpenes or phenylpropanoids. Investigation of the essential oils in the nineteenth century found that many of the pleasant odors we obtain from nature contained exactly 10 carbon atoms in the compound.
These 10-carbon compounds later became known as terpenes if they were hydrocarbons. If the compound contained oxygen in an alcohol, ketone, or aldehyde functional group it was called a terpenoid. It was eventually found that minor and less volatile compounds containing 15, 20, 30, and 40 carbon atoms also exist that give an odor. Since the 10-carbon compounds were at the time called terpenes they changed the name to monoterpenes. The other terpenes were classified in the following way. Class