Science Makes Sense: Week47-Organic Chemistry-Terpenes and Terpenoids

With the recent election in 2016, several ballot measures were passed that supported medical marijuana as well as recreational marijuana in some states in the U.S.  Terpenes are the main class of aromatic compounds found in cannabis or marijuana as it is popularly called.  Yet, this is only a part of the field of terpenes, terpenoids that are organic hydrocarbons produced by a wide variety of plants.(Ref.1)

The term ‘terpene’ is ascribed to compounds isolated from terpentine, the volatile liquid found in pine trees.   Terpenes were originally used to describe a mixture of isomeric hydrocarbons of the molecular formula C10H16 found in the essential oils obtained from the sap and tissues of plants and trees.  The modern definition is more broad-based:Terpenoids are the hydrocarbons of plant origin of the general formula(C5H8)n, as well as their oxygenated,hydrogenated and de-hydrogenated derivatives. (Ref.2)

Here is a simple table to illustrate the classification of terpenes:

#of carbon atoms      Value of n       Class

10                                   2                     Monoterpenoids(C10H16)

15                                    3                  Sesquiterpenoids(C15H24)

20                                   4.                 Diterpenoids(C20H32)

25                                   5                    Sesterpenoids(C25H40)

30                                   6                   Troterpenoids(C30H48)

40                                   8                   Tetraterpenoids (C40H64)

More than 40     More than 8        Polyterpenoids(C5H8)n

Each class can also be subdivided according to the number of rings present in the structure:

1.Acyclic terpenoids-open structure

2.Monocyclic /bicyclic/ tricyclic terpenoids-containing one/ two/three rings in structure. (Ref.2)

Thermal decomposition of a terpene leads to isoprene as one of the products. (Ref.2). What is an isoprene?  This is the C5H8 building block for a terpene, (where (C5H8)2; i.e., n=2 for the first terpene, as shown in the table above) the structure includes two double bonds: CH2=C(CH3)CH=CH2.

Most terpenoids are colorless fragrant liquids, lighter than water and volatile with steam.  A few are solids like camphor.  All are soluble in organic solvents and mostly insoluble in water, and optically active. Since they can be open-chain or cyclic unsaturated compounds, they can undergo addition reactions with halogens, hydrogen, acids. The addition products have antiseptic properties.  Terpenes also undergo oxidation, polymerization.   On thermal decomposition, most of the terpinoids result in isoprene as one of the products. (Ref.2)

Let us look at some common examples of terpenes:

1.Limonene is comprised of two isoprene units and can be expressed as (C5H8)2 which is then C10H16.  From the above table it is a mono terpene.  Limonene is a mono terpene responsible for the fragrance of citrus fruits.  D- Limonene is widely used as a flavor additive in the food industry as well as an aroma in the perfume industry.  It is also used medicinally to reduce heartburn and gastric reflux.  Sometimes it is used in cleaning solvents and for stripping paint. (Ref.1)  It is a Monocyclic monoterpenoid. (Ref.2)

2. Myrcene is another mono terpene and is the most abundant terpene found in cannabis.  But it is also found in bay leaves, wild thyme, hops, verbena.  It produces analgesic effects in laboratory testing on rats. (Ref.1).  Myrcene is an Acyclic mono terpene. (Ref.2)

3.Pinene: as the name suggests, is found in pines and conifers and it exists as two isomers, alpha and beta pinene, mono terpenes. Turpentine is a large part of pinene.  These pinenes are also present in sage,( both isomers).  Olive, rosemary , sassafras and bergamot contain the alpha isomer while hops and cumin have the beta isomer.  It acts as a natural herbicide and also acts as a bronchodilator in humans. In addition, pinene shows antibacterial, antibiotic properties. (Ref.1)

4. Linalool is a mono terpenoid of the chemical formula C10H18O. It is found in mint,laurel, cinnamon, birch , lavender and some citrus. There are two non- superimposable mirror image isomers for linalool.  Linalool is used as an anxiety inducing drug.  Lavender has been used for thousands of years as a calmative.  Recent tests on rats exhibit its sedative and motor relaxant effects. (Ref.1)( More information about the isomers and examples in Nuggets)

There are several more examples that have not been mentioned here, but suffice to see that terpenes and terpenoids are a wide and extensive resource for us.  Their medicinal uses go back centuries; lavender is just one example. Many herbs and spices like ginger , cloves and cinnamon have been used for centuries in countries like India for home- made health remedies and in the field of Ayurveda medicine.  Today, especially with the possibility of the usage of medical marijuana, the additional terpenes and terpenoids can be combined with cannabis in research and enhancement of therapeutic effects.

Activities for Middle School Teachers:

Students can create the basic isoprene unit and look at common terpene and terpenoid structures.  What limits the structural possibilities? What is the gem dialkyl rule? (Ref.2)

Students  can get a spice or herb used in their foods and find out if it is a terpene or terpenoid.  Let them then build the structure and do a presentation on their terpene/ terpenoid.

Students should have an open discussion about cannabis and talk about the pros and cons of using cannabis.  What kind of societies are created in countries where it is legalized ? Is it better or worse than in the U.S.?  Should there be incarceration for using /possessing small amounts of cannabis? How about focusing on the medicinal uses of cannabis?

Nuggets of information:

Beta- carophyllene, a sesquiterpene is found in cloves, rosemary and hops and exhibits anti- inflammatory effects.  Carophyllene oxide is the substance in cannabis that is identifiable by drug-sniffing dogs. (Ref.1)

Both alpha and beta pinene are present in cannabis; in fact cannabis may contain over 120 terpenes and terpenoids, albeit many of them may be present in trace amounts.  Today the medicinal uses of cannabis, besides the popular assumption that it is mainly used for recreational purposes, is being seriously studied. (Ref.1)

Citral, very similar to Limonene, but has an aldehyde and is an Acyclic mono terpenoid.  It is found in lemon grass oil. (Ref.2)

Other common terpenoids include eugenol (cloves), anethol (fennel, anise), menthol (common cold products),thymol (thyme,oregano), and geraniol (roses).   Camphor is also a terpene. (Ref.3)

Notice that most of the above terpenoids end in ‘ol’ indicating an alcohol or presence of an -OH group.

As terpenoids were found in essential oils, they were used for religious reasons in Ancient Egypt.  Camphor was introduced to Europe from the East by the Arabs in the 11th century.  The process of obtaining  some 60 plant essential oils was described in a book written in 1592. (Ref.3)

 

References:

1.sensiseeds.com/en/blog/medicinal-properties-terpenes-terpenoids/

2.nsdl.niscair.res.in/jspui/bitstream/123456789/700/1/revi

3.food.info.net/uk/qa/qa-fi69.htm

 

 

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