Essay Terpenes and Phenylpropanoids

Essential oil components are often found in the glands or intercellular spaces in plant tissue. They may exist in all parts of the plant but are often concentrated in the seeds or flowers. Many components of essential oils are steam-volatile and can be isolated by steam distillation. Other methods of isolating essential oils include solvent extraction and pressing (expression) methods. Esters (see the essay Esters-Flavors and Fragrances ) are frequently responsible for the characteristic odors and flavors of fruits and flowers, but other types of substances may also be important components of odor or flavor principles. Besides the esters, the ingredients of essential oils may be complex mixtures of hydrocarbons, alcohols, and carbonyl compounds. These other components usually belong to one of the two groups of natural products called terpenes or phenylpropanoids. 

Chemical investigations of essential oils in the 19th century found that many of the compounds responsible for the pleasant odors contained exactly 10 carbon atoms. These 10-carbon compounds came to be known as terpenes if they were hydrocarbons and as terpenoids if they contained oxygen and were alcohols, ketones, or aldehydes. 

Eventually, it was found that there are also minor and less volatile plant constituents containing 15, 20, 30, and 40 carbon atoms. Because compounds of 10 carbons were originally called terpenes, they came to be called monoterpenes. The other terpenes were classified in the following way. 

Further chemical investigations of the terpenes, all of which contain multiples of five carbons, showed them to have a repeating structural unit based on a five-carbon pattern. This structural pattern corresponds to the arrangement of atoms in the simple five-carbon compound isoprene. Isoprene was first obtained by the thermal cracking of natural rubber. 

As a result of this structural similarity, a diagnostic rule for terpenes, called the isoprene rule, was formulated. This rule states that a terpene should be divisible, at least formally, into isoprene units. The structures of a number of terpenes, along with a diagrammatic division of their structures into isoprene units, is shown in the figure on the next page that accompanies this essay. Many of these compounds represent odors or flavors that should be familiar to you. 

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Essential oils are volatile compounds responsible for the aromas commonly associated with many plants (see essay "Terpenes and Phenylpropanoids")- The chief constituent of the essential oil from cloves is aromatic and volatile with steam. In this experiment, you will isolate the main component derived from this spice by steam distillation. Steam distillation provides a means of isolating natural products, such as essential oils, without the risk of decomposing them thermally. Identification and characterization of this essential oil will be accomplished by infrared spectroscopy. 

Explain how caryophyllene fits the isoprene rule (see essay, "Terpenes and Phenylpropanoids"). 

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