TERPENOIDS (Terpenes and Camphors)

Introduction to terpenes, classification (based on number, ring), examples for each class with application.

Introduction:

It has been known for centuries that distillation of many plant materials with steam produces a fragrant mixture of liquids called essential oils. For hundreds of years, such plant extracts have been used as medicines, spices, and perfumes.

Chemically, plant essential oils consist largely of mixtures of compounds known as terpenoids—small organic molecules with an immense diversity of structure. More than 35,000 different terpenoids are known. Some are open-chain molecules, and others contain rings; some are hydrocarbons, and others contain oxygen. Hydrocarbon terpenoids, in particular, are known as terpenes, all of which contain double bonds.

Their oxygen derivatives are known as Camphors.

Terpenes and Camphors are both Terpenoids.

Structural Unit:

With the exception of a few (eg. Tatarol, eremophilone, etc), terpenes in general have the empirical formula C5H5 and are supposed to be derived from isoprene (C5H8)

According to the isoprene rule, Terpenes can be thought of as arising from head-to-tail joining of 5-carbon isoprene units (2-methyl- buta-1,3-diene). Carbon 1, which is the branching end of the terpene, is the head of the isoprene unit, and carbon 4 is the tail.

Therefore, terpenes are also referred to as isoprenoids.

(Image 3:- 1,4 linkage, according to isoprene rule)

However, there are some terpenoids which contain carbon atoms which are not multiples of five, and in some terpenoids, head to tail joining rule is violated.

Classification:

The terpenoids are classified according to the number of isoprene (C5) units present in them.

1. Hemiterpenoids: Only one isoprene unit. C5

2. Monoterpenoids: with two isoprene units. C10

3. Sesquiterpenoids: with three isoprene units. C15

4. Diterpenoids: with four isoprene units. C20

5. Triterpenoids: with six isoprene units. C30

6. Polyterpenoids: with several isoprene units. (C5)n

The only known hemiterpenoid is isoprene itself, and that too does not occur in any botanical species.

These terpenoids are further subdivided as acyclic or cyclic depending on whether they have open chain or cyclic structures. The cyclic terpenoids may be monocyclic, bicyclic, or tricyclic according to the number of rings present in them. Furthurmore, these terpenoids may either be hydrocarbon terpenoids or oxygenated terpenoids.

Some of the important classes are:

1. Acyclic monoterpenoid hydrocarbons. C10H16, eg, Myrcene2. Acyclic Polyterpenoid hydrocarbons. (C5H8)n eg. Natural rubber, and gutta-

percha3. Monocyclic monoterpenoid hydrocarbons C10H16 eg. Limonene.4. Bicyclic monoterpenoid hydrocarbons C10H16 eg α-pinene.5. Oxygenated acyclic terpenoids. eg. Geraniol, citral etc.6. Oxygenated cyclic terpenoids. eg menthol, camphor, vitamin A etc.

General Properties of Terpenoids:

1. They are generally colourless pleasant smelling liquids, lighter than water, and possessing high refractive index. A few solid terpenoids are known, eg. Camphor.

2. They are steam volatile.3. They are generally optically active.4. They are insoluble in water, and soluble in organic solvents.5. They are reactive, and exhibit reactions due to double bond and functional

group present in them.6. A number of them posess antiseptic property.

Acyclic Monoterpenoids:

A. Myrcene (7-methyl-3methyleneocta-1,6-diene) (C10H16)

It is well known acyclic terpenoid hydrocarbon. It occurs in oil of bay, verpena, hops, and turpentine.

It can be isolated from essential oils, by fractional distillation in vacuum (b.p 76-78 ° C/33mm).

Myrcene is optically inactive compound and colourless. (b.p 166-168 ° C).

Uses:

1. In the synthesis of Citral.

B. Geraniol C10H18O

This terpene alcohol occurs in large numbers in essential oils, especially rose, palmarosa, citronella, geranium, coriander, lemon-grass and lavender.

Rose oil, (obtained by steam distillation) of fresh flowers of Rosa damascene contains 40-60% of geraniol. However, due to high cost, rose oil is primarily used for high grade perfumery, instead of as a source of geraniol.

Geraniol exhibits optical isomerism. Geroniol is trans, while the other naturally occurring alcohol nerol is cis isomer.

Both geraniol and nerol are colourless liquids having rose like pleasant odour.

OR

Uses:Both geraniol and nerol are extensively used in perfumery, especially in the preparation of artificial rose scents.

C. Citral C10H16O

It is the most important open chain terpene aldehyde, and it occurs in the oil of lemon grass (70-80%) and is also present in the oil of oranges, oil of lemon, oil of citronella, etc.

It is colourless oily liquid with a strong lemon-like odour (b.p. 224-228 ° C).

It is optically inactive.

It is also referred to as geronial, and it’s cis isomer is called neral.

OR

Uses:

1. It is used as starting material for the manufacture of α and β ionones.2. As a starting material for the synthesis of vitamin A3. In the compounding of synthetic lemon flavours.

D. Linalool (C10H18O)

Linalool is an optically active liquid (b.p 198-199 ° C). The (+) form occurs in orange, and coriander oils, and the (-) form occurs in rose, bergamot, and ylang-ylang oils.

Linalool is an isomer of geraniol and nerol but it differs from them in being tertiary alcohol. Of all the acyclic oxygenated monnoterpenes, it is the most reactive.

Uses:

1. (R)-(-)-Linalool smells more woody, while the lavender fragrance of the (S)-(+)-enantiomer is more sweetish. Thus, it is used in perfumes.

2. In the synthesis of geraniol.3. In the synthesis of farnesol.

Monocyclic Terpenoids:

Monocyclic terpenoids are divided into three categories, according to the nature of carbon skeleton present in them.

1. p-menthane derivatives2. m-menthane derivatives.3. 1,1,3-trimethylcyclohexane derivatives.

1,1,3-trimethylcyclohexane) p-menthane

p-menthane does not occur in nature, but is easily synthesized by the hydrogenation of c-cymene or limonene.

A. Limonene [1,8,(9)-Menthandiene] C10H16

Limonene is an important member of monoterpenoid class. It is most widely distributed, optically active, terpenoid.

d-limonene occurs in lemon, orange, bergamot, caraway, and celery oils and l-limonene occurs in the needle of Douglas fir and peppermint oil. The racemic form (±) called dipentene occurs in lemongrass oil and turpentine oil.

Limonene is a liquid with lemon like odour. The dextro and laevo forms have b.p at 177 °C. It is insoluble in water.

Uses:

1. As a flavour in beverages, pharmaceutical preparations, and food.2. As dipentene in the synthesis of isoprene, p-cymene, and p-menthane.3. As terbene (constituting chiefly of dipentene and terpinenes) in expectorants.4. In the synthesis of carvone.

B. α-Terpineol , C10H18O

α-Terpineol is perhaps the most important monoterpenoid. It is naturally occurring, optically active terpenoid whose (+)(-) and (±) forms occur in nature.

The m.p of racemic form is 35 ° C, and it found in the form of various esters or alone in essential oils.

The (+) form is found in petitgrain and neroli oils, and (-) form in camphor oil and the racemic form in cajuput oil.

Uses:

1. In the synthesis of carvone.2. Having the very pleasant smell of lilac blossoms, it is an important raw

material in perfumery3. In the synthesis of α-Thio- terpineol, (p-menth-1-en-8-thiol), which is used in

flavouring grape juice.

C. Carvone, C10H14O

Carvone occurs in various essential oils, both in optically active form and racemic form. Dill, spearmint and caraway oils are the most common sources.

It has two isomers. (S)-(+)-Carvone produces the typical odor and taste of caraway, whereas its (R)-(-)- enantiomer in the oil of spearmint, smells like peppermint.

Uses:

1. Used for flavoring liqueurs2. Fragrance in soaps3. As a carminative.

D. Menthol, Hexahydrothymol (p-menthane-3-ol), C10H19OH

In it’s laevo form, menthol occurs in peppermint oil, obtained from Mentha piperita, and Japanese mint oil, obtained from Mentha arvensis.

Menthol is obtained from chilling essential oils.

Menthol forms colourless, volatile prismatic crystals (m.p. 45 ° C and b.p 212 ° C) possessing strong peppermint odour and cooling taste. It is optically active. It exists in 8 sterioisomeric forms.

Naturally occurring isomer is l-menthol, while the synthetic one is dl-isomer.

Uses:

1. It is used as an antiseptic in ointments, nasal sprays, and gargles.2. It is used in medicine and pharmaceuticals.3. Because of it’s cooling effect, it is used both in toothpaste, and tooth powder,

and also in shaving creams.

Bicyclic Terpenoids (Individual members)

A. α-Pinene

α and β pinenes are the chief constituents of oil of turpentine, and the α-form greatly predominates. α-pinene is also found in eucalyptus oil. It contains two rings, one being six membered, and the other four-membered, both belonging to different planes.

In nature, α-pinene occurs in dextro-form [α]D = 48.8 °

It boils at 156 °C

Uses:

Terpentine oil, which is largely pinene, is used

1. As solvent for resins.2. Thinner for paints and varnishes.3. Preparing synthetic camphor.

B. Camphor

Camphor is a bicyclic monoterpenoid ketone, and the main constituent of camphor oil, obtained from the leaves of camphor tree Cinnamomum camophora.

Camphor is obtained by chilling the essential oil and can be purified by recrystallization.

It is a soft crystalline substance (m.p 178-179 °C, b.p 209 °C). It possesses a familiar camphor like odour, and is slightly soluble in water, to which it imparts the characteristic taste and smell. It dissolves readily in alcohol and other organic solvents. Natural camphor is dextro rotatory. [α]D = +44 °

Uses:

1. As a moth-repellent.2. As a preservative in the manufacture of celluloid and photographic films.3. In medicines, ointments, tinctures, and liniments due to it’s analgesic,

expectorant, and antiseptic properties.4. As a component of ‘dhoop’ which is a repellent of ants, flies, and other

insects.5. As a solvent in Rast’s method of determination of molecular weight.6. As a preservative in cosmetics.

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