Experiment 4: Isolation of Essential Oils from Eugenol
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
No. of Carbons
No. of Carbons
As the knowledge of terpenes expanded it was found that they had a repeating structural unit that consisted of five carbons. The structure of this repeating unit was found to correspond to that of isoprene. This compound was first observed when chemists performed thermal “cracking” on natural rubber.
As a result of this similarity, a diagnostic rule for terpenes became known as the isoprene rule. This rule states that a terpene should be divisible into separate isoprene units. The following shows some examples of how a terpene follows the isoprene rule.
Many of these compounds represent odors that you should be familiar with. Although the isoprene rule was developed to classify these compounds it is now known that the production of terpenes proceeds via a biological route and not naturally through the combination of isoprenes. Aromatic compounds also contribute greatly to the amount of essentials oils in the natural world. These aromatic compounds are classified as phenylpropaniods which consist of a phenylpropane skeleton. Some of these compounds are constructed of cylic terpenes that have been aromatized, but have a different origin. A phenylpropanoid is closely related in structure to the amino acids phenylalanine and tyrosine. Many of these compounds are derived from a biological pathway...
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