Introduction 

Terpenes are a huge and different class of organic compounds, generated via a variety of plants, particularly conifers though as well through several insects these as termites or swallowtail butterflies, that release terpenes from their osmeterium. They are frequently strong smelling and therefore might have had a protective function. They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine". When terpenes are modified chemically (via oxidation or rearrangement of the carbon skeleton), the consequential compounds are usually termed to as terpenoids. Terpenoids are as well recognized as isoprenoids.

Several authors utilize the term 'terpenes' more broadly, to include the terpenoids. These terpenes are frequently found in plant essential oils which contain the "Quinta essentia", the plant fragrance. They are universally present in small amounts in living organisms, where they play numerous vital roles in plant physiology in addition to significant functions in all cellular membranes.  Terpenes and terpenoids are the primary constituents of the necessary oils of many kinds of plants and flowers. Essential oils are utilized extensively as natural flavor additives for food, as fragrances in perfumery, and in traditional and alternative medicines these as aromatherapy. Synthetic differences and derivatives of natural terpenes and terpenoids as well greatly enlarge the variety of aromas utilized in perfumery and flavors employed in food additives. Vitamin A is an instance of a terpene. Terpenes are liberated via trees more actively in warmer weather, acting as a natural form of cloud seeding. The clouds reflect sunlight, permitting the forest to control its temperature. The aroma and flavor of hops, extremely desirable in several beers, comes from terpenes. 

Terpenes might be described as a group of molecules whose structure is depend on diverse but definite number of isoprene units (methylbuta-1,3-diene, named hemiterpene, with 5 carbon atoms).  The terpenes have been prized for their necessary oils and utilize as fragrances for over two thousand years. An archaeological investigation in Egypt in the year 1997 unearthed boswellic acids from the resin of frankincense (Boswellia spp.) dating from 400 to 700 AD. Records from the middle Ages of terpene-based necessary oils were protected, and chemical analysis of the oils began early in the nineteenth century. Commerce in necessary oils and aromatherapy continues today. For instance, rose (Rosa spp.) fragrance has enchanted many. Bulgarian rose oil needs over 4000 kg of petals to generate 1 kg of steam-distilled oil. Over 260 constituents have been identified, many of that are olfactory applicable. It should be apparent that even the easy terpenes originate in fragrances have a considerable amount of structural diversity. 

Providentially, although their diversity, the terpenes have an easy unifying characteristic via that they are described and through that they might be simply classified. This generality, termed to as the isoprene rule, was postulated via Otto Wallach in 1887 and it states that Terpene skeleton are shaped via bonding mutually isoprene units through carbon atoms 1 and 4 (head-to-tail), carbon atoms 1 and 1 (head-to-head), carbon atoms 4 and 4 (tail-to-tail) or combination thereof. 

This rule describes all terpenes as having fundamental repeating five-carbon isoprene units. The head-to-tail arrangement is most ordinary. This isoprene rule has showed to be of huge value in deriving the structure of terpenes. Therefore, terpenes are described as a single group of hydrocarbon-based natural products that possess a structure that might be hypothetically derived from isoprene, giving increase to structures, which might be separated into isopentane (2-methylbutane) units. The actual biosynthetic route to terpenes isn't fairly so easy. Two different biosynthetic pathways create the major terpene building block, isopentenyl diphosphate (IPP). The 1^st is termed to as either the MEP (methylerythritolphosphate) or DOX (1- deoxy-D-xylulose) pathway. Here, IPP is shaped in the chloroplast, mostly for the more volatile mono and diterpenes. The 2^nd biosynthetic route is recognized as the MVA (mevalonic acid) pathway. This occurs in the cytosol, producing sesquiterpenes.

Terpenoids are extraordinarily different but they all originate through the condensation of the common phosphorylated derivative of hemiterpene, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) giving geranyl pyrophosphate (GPP).

Fig: geranyl pyrophosphate

Isolation of Terpenes

Terpenes are isolated from essential oils present in plants. The essential oil is removed from the plant tissues via 4 techniques: (1) steam distillation; (2) digestion through solvents; (3) expression; and (4) adsorption in purified fats. 

Method 1 (steam distillation) is the one most extensively utilized. The plant tissue is macerated and then steam distilled. If a particular terpene is decomposed under these conditions, it might be eliminated via extracting through light petrol at 50^oC and the solvent distilled under decreased pressure. On the other hand, the technique of adsorption in fats might be employed. For instance the flower petals are extended over molten fat until the latter is saturated through necessary oil. The fat is then digested through ethanol to eliminate the necessary oil from it.

Essential oils attained from plants as above generally enclose a number of terpenes that are divided via fractional distillation or chromatography. Gas chromatography has been chiefly helpful.

Properties of Terpenes

Physical

Most of the terpenes are colourless fragrant liquids having a boiling point between 150^o and 200^oC. They are lighter than water and are willingly volatile in steam. They melt in organic solvents, but usually not in water. Most of them are optically active although there is no prevailing direction of rotation; several are dextro, others are laevo.

Chemical

Most of the terpenes being unsaturated hydrocarbons are extremely reactive. They undergo addition reactions through hydrogen bromide, bromine, hydrogen, nitrosyl chloride and ozone. They as well shape trait addition compounds through NO₂ and NOBr that are utilized in their identification. Most terpenes are oxidised easily and tend to resinify upon exposure to air.

Classification of Terpenes

A rational classification of the terpenes has been established based upon the number of isoprene (or isopentane) units incorporated in the basic molecular skeleton; a prefix in the name indicates the number of terpene units needed to assemble the molecule. Terpenes are thus classified by the number of five-carbon units they contain:

• Hemiterpenes (C₅) contain a single isoprene unit. Isoprene itself is believed the only hemiterpene, but oxygen-containing derivatives these as prenol and isovaleric acids are hemiterpenoids.
• Monoterpenes (C₁₀) consist of 2 isoprene units and contain the molecular formula C₁₀H₁₆. Examples of monoterpenes are: geraniol, limonene and terpineol.
• Sesquiterpenes (C₁₅) consist of three isoprene units and have the molecular formula C₁₅H₂₄. Instances of sesquiterpenes are: farnesenes, farnesol. (The sesqui prefix means one and a half).
• Diterpenes (C₂₀) are composed for 4 isoprene units and have the molecular formula C₂₀H₃₂. They are obtained from geranylgeranyl pyrophosphate. Instances of diterpenes are cafestol, kahweol, cembrene and taxation (precursor of taxol).
• Diterpenes as well form the basis for biologically important compounds these as retinol, retinal, and phytol. They are recognized to be antimicrobial and anti-inflammatory.
• Sesterterpenes (C₂₅), terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. (The sester prefix means half to three, for instance two and a half). An instance of a sesterterpene is geranylfarnesol.
• Triterpenes (C₃₀) consist of six isoprene units and have the molecular formula C₃₀H₄₈. The linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of 2 molecules of farnesyl pyrophosphate. Squalene is then processed biosynthetically to produce either lanosterol or cycloartenol, the structural precursor to all the steroids.
• Tetraterpenes (C₄₀) contain eight isoprene units and have the molecular formula C₄₀H₆₄. Biologically significant tetraterpenes comprise the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha-carotene and beta-carotene.
• Polyterpenes contain long chains of many isoprene units. Natural rubber consists of polyisoprene in that the double bonds are cis. Several plants produce a polyisoprene through trans double bonds, recognized as gutta-percha.

Table: Classification of Terpenes

Mono-, sesqui-, di-, and sesterpenes contain the isoprene units linked in a head to tail fashion. The triterpenes and carotenoids (tetraterpenes) contain two C₁₅ and C₂₀ units respectively bonded head to head. Many terpenes are hydrocarbons, but oxygen-containing compounds these as alcohols, aldehydes or ketones are as well originate. Such derivatives are frequently named terpenoids. 

Hemiterpenes: C₅

Hemiterpenes are made of one five-carbon unit and are the simplest of all terpenes. Isoprene is emitted from the leaves of many plants and donates to the natural haze (phytochemical smog) in several regions, these as the Smoky Mountains. Abundant five-carbon compounds are recognized that enclose the isopentane skeleton, including isoamyl alcohol, senecioic acid, tiglic acid, angelic acid, α- and β- furoric acid and isovaleraldehyde. There is evidence that such compounds might help in plant protection via repelling herbivores or via attracting predators and parasites of herbivores.

Monoterpenes: C₁₀

A bewildering variety of isoprene-based decane arrangements exist in nature. This provides the term 'terpenoid' a mainly elastic meaning and is reminiscent of a little of the current combinatorial endeavors utilized in the pharmaceutical industry. The monoterpenoids are the main component of many necessary oils and, as these, have economic significance as flavors and perfumes. They are the terpenes that have been known for several centuries as components of the fragrant oils obtained from leaves, flowers and fruits. Monoterpenes, through sesquiterpenes, are the main constituents of essential oils. While a few, these as camphor, take place in a near pure form, most happen as compound mixtures, often of isomers hard to divide. Such necessary oils have numerous actions, such as allelochemical functions between plants as well as between plants and predators, a role in wound healing and many monoterpenes possess antitumor activity in animals. Common acyclic examples include myrcene, geraniol, and linalool. Cyclic structures include many well-known compounds, including menthol, camphor, pinene, and limonene. Most of the monoterpenes come from common sources with which most of us are familiar. 

The thujone diastereomers are rapidly metabolised convulsants. They act as noncompetitive blockers of the γ-aminobutyric acid (GABA) gated chloride channel. Myrcene is found in the essential oil of bay leaves (Laurus nobilis) as well as hops (Humulus lupulus). It is used as an intermediate in the manufacture of perfumes. Geraniol, which is isomeric with linalool, constitutes the major part of the oil of geraniums (Pelargonium graveolens) and is also found in essential oils of citronella (Cymbopogon nardus), lemongrass (Cymbopogon citratus or C. flexuosus), and others. Lavandulol is one of the principal ingredients of oil of lavender (Lavandula augustifolia), commonly used in male perfumes.  Perillene can be found in the perilla (Perilla frutescens), native to South and East Asia. Menthol is a well-known monoterpene that is found in the essential oil of peppermint (Mentha piperita) and other members of the mint family (Lamiaceae). Carvone is a common monoterpene. It is one of the main olfactory components of caraway seed (Carum carvi), and it shows antifungal activity. 3-Carene is a cyclopropane containing monoterpene, derivatives of which have shown anesthetic activity. α-Pinene, the major ingredient in turpentine, may play a significant role in the activity of hydrocarbon-degrading bacteria in nature. Linalool is one of the principle constituents of coriander (Coriandrum sativum), a general spice. It is as well one of the most ordinary floral scent compounds originate in flowering plants, and it is a common flavor compound in a choice of teas.  Safranal is primarily responsible for the trait odor of saffron (Crocus sativus).

Eucalyptol (1, 8-cineole) is the major component of the essential oil of eucalyptus leaf (Eucalyptus globulus). Eucalyptol along with camphor, form the major constituents of rosemary oil. Recent research showed that eucalyptol is effectual in reducing inflammation and pain and in promoting leukemia cell death.

(a)   Acyclic monoterpenes

b)   Monocyclic monoterpenes

They are derived from cyclohexane with an isopropyl substituent. The most typical are:

(c)  Bicyclic monoterpenes

Sesquiterpenes: C₁₅ 

Sesquiterpenoids are described as the group of 15 carbon compounds obtained from three isoprene units, the C₁₅ sesquiterpenes exist in aliphatic, bicyclic, and tricyclic frameworks. Like the monoterpenes, most of the sesquiterpenes are components of the needed oil of the plant from that they are derived. A significant member of this series is farnesol, through pyrophosphate that provides as a key in-between in terpenoid biosynthesis. They are originated mostly in higher plants but as well in invertebrates. Sesquiterpenes, by monoterpenes, are an important constituent of essential oils in plants. They are the most diverse group of isoprenoids. In plants, they function as pheromones and juvenile hormones. 

(a)   Acyclic compounds 

 The acyclic representative are as well said farnesans, term obtained from the basic structure, farnesol.  Farnesol and nerolidol are extremely ordinary and are isolated from necessary oils of diverse sources. 

Fig: Nerolidol 

Farnesol is extensively distributed in many crucial oils these as citronella, neroli, cyclamen, lemon grass, tuberose, rose, musk, and balsam. It is utilized in perfumery to emphasise the odors of perfumes. Furthermore, it is a natural pesticide for mites and is as well a pheromone for numerous insects and mammals, including elephants (territorial marking, individual recognition, mate attraction).

(b)   Cyclic compounds 

Abscisic acid  plays a key role in plants in the regulation of stomatal closure via regulating ion channel activities and water exchanges across the plasma membrane of guard cells.

Fig: Abscisic acid

Abscisic acid has as well a variety of roles in plant expansion, bud and seed dormancy, germination, cell division and movement. It induces as well storage protein synthesis in seeds and might be comprised in defense against insect attack.

Diterpenes: C₂₀ 

They have 20 carbon atoms and are attained from geranylgeraniol pyrophosphate. They are of fungal or plant origin and are originate in resins, gummy exudates, and in the resinous high-boiling fractions residual after distillation of necessary oils. Diterpenoid groups that are physiologically active comprise: vitamin A (retinol), phytohormones that regulate plant growth and germination, for example gibberellin, fungal hormones that arouse the switch from asexual to sexual reproduction, for instance trisporic acid; disease resistance agents (phytoalexins), for illustration casbene and podocarpic acid, the anticancer drug, taxol, from the bark of the yew tree, the cancer promoter, phorbol, and natural cannabinoids. The diterpenes have exceptionally open chain, as originate in geranylgeraniol or phytol that shapes a part of chlorophyll and the side chain of vitamin E and K, and crocetin that is a diacid diterpenoid and the lipid part of the crocins, glycosylated derivatives present in saffron.

Instances of diterpene substances are given below:

Fig: Cembrene

The diterpenes are a extensively varied group of compounds depend on four isoprene groups. Because of their higher boiling points, they aren't considered to be necessary oils. In its place, they are classically considered to be resins, the material that remains after steam distillation of a plant extract. Many interesting instances might be mentioned here. The cyclic ether zoapatanol is derived from the Mexican zoapatle plant (Montanoa tomentosa). It has been utilized as an abortifacient. A number of clerodanes were isolated from Ajuga, Salvia, and Teucrium species.

They have been establishing to possess insect antifeedant activity. A variety of cytotoxic lactones were isolated from Podocarpus species. Such podolactones and nigilactones have plant regulatory properties in addition to antileukemic activity. The gibberellins include a vital group of extensively allocated plant hormones. Such falls into 2 series, including a C₂₀ family symbolized via gibberellin and a C₁₉ series for that gibberellic acid (GA₃) is distinctive. Marrubiin is a diterpene lactone from white horehound (Marrubium vulgare). It has been employed as a vasorelaxant. Taxol or paclitaxol (derived from needles and bark of Taxus spp., yews) is a completely exclusive antimitotic agent utilized to treat breast cancer. Chemically, it is made up of a diterpenoid core through an alkaloid side group. It binds to microtubules and stabilises them, as opposed to all other antimitotics of the tubulin-binding kind, such as vincristine, the podophyllotoxins, and colchicines. 

Sesterpenes

They are derived from geranylfarnesol pyrophosphate and have 25 carbon atoms. They were isolated from insect protective waxes and from fungal sources.

Fig: Geranylfarnesol 

Triterpenes: C₃₀ 

They shape a huge group of natural substances that comprises steroids and therefore sterols. Squalene is the instant biological precursor of all triterpenoids. 

Fig: Squalene

Squalene epoxide (2,3-oxidosqualene) is generated via the enzyme squalene epoxidase that utilize NADPH and oxygen to oxidise squalene. This metabolic step is the 1^st in sterol biosynthesis leading to the configuration of lanosterol or cycloartenol.

Fig: Squalene epoxide

Squalane is a wholly saturated derivative of squalene. Present in sebum, it is mainly utilized as a component in many cosmetic products. It is attained through hydrogenation of squalene extracted from olive oil. Among the huge number of triterpenoid structures, several of them are revealed below.

Fig: Triterpenoid structures

Steroids

The important class of lipids said steroids is in fact metabolic derivatives of terpenes that derived as well from squalene via cyclisation, unsaturation and substitution, but they are customarily treated as a divide group. Steroids might be recognized via their tetracyclic skeleton, consisting of 3 fused six-membered and one five-membered ring, as revealed in the diagram to the right. The 4 rings are delegated A, B, C & D as noted, and the peculiar numbering of the ring carbon atoms (shown in red) is the result of an earlier misassignment of the structure. The substituents designated via R are frequently alkyl groups, but might as well have functionality. The R group at the A:B ring fusion is most usually methyl or hydrogen, that at the C:D fusion is usually methyl. The substituent at C-17 varies considerably, and is generally larger than methyl if it isn't a functional group. The most general locations of functional groups are C-3, C-4, C-7, C-11, C-12 & C-17. Ring A is sometimes aromatic. Because a number of tetracyclic triterpenes as well have this tetracyclic structure, it cannot be considered a unique identifier.

Steroids are extensively distributed in animals, where they are connected through a number of physiological procedures. Instances of several significant steroids are revealed in the subsequent diagram.  Norethindrone is a synthetic steroid; all the other illustrations take place naturally. A common strategy in pharmaceutical chemistry is to take a natural compound, having indeed desired biological properties mutually through undesired side results, and to modify its structure to develop the desired traits and reduce the undesired. The generic steroid structure drawn above has seven chiral stereocenters (carbons 5, 8, 9, 10, 13, 14 & 17), which means that it may have as many as 128 stereoisomers. With the exception of C-5, natural steroids generally have a single common configuration. This is revealed in the last of the toggled displays, along with the termed conformations of the rings.

Fig: Typical animal steroids

Practically all plant steroids are hydroxylated at C-3 and are, actually, sterols. In the animal kingdom, the steroids have profound significance as hormones, coenzymes, and provitamins. Though, the role of the phytosterols is less well understood. There is evidence that several of the phytosterols are effective against cardiovascular disease.

Chemical studies of the steroids were extremely significant to our present understanding of the configurations and conformations of six-membered rings. Substituent groups at dissimilar sites on the tetracyclic skeleton will have axial or equatorial orientations that are fixed because of the rigid structure of the trans-fused rings. This fixed orientation influences chemical reactivity, mainly due to the greater steric obstruction of axial groups versus their equatorial isomers. Therefore an equatorial hydroxyl group is esterified more quickly than its axial isomer.

Tetraterpenes: C₄₀

The most ordinary tetraterpenoids are the carotenoids a extensively allocated group of C₄₀ compounds. Whereas the structures of the di- and triterpenes can have a broad variety of fascinating structures, the carotenoids are usually derived from lycopene. Cyclisation at one end gives γ-carotene and at both ends gives β-carotene. This pigment was 1^st isolated in the year 1831. The nature of such compounds was determined during the 19^th century. In the year 1831, Wachenroder H. proposed the term 'carotene' for the hydrocarbon pigment he had crystallized from carrot roots. Berzelius J. said the more polar yellow pigments extracted from autumn leaves "xanthophylls" and Tswett M., who separated many pigments via column chromatography, said the whole group "carotenoids". It is almost worldwide in the leaves of higher plants. As is evident from this polyene structure, abundant double-bond isomers are probable for such essential structures, all of which can give brightly coloured pigments. In plants, carotenoids serve as essential pigments in photosynthesis, where they are believed to defend plants from over oxidation catalyzed via other light-absorbing pigments, these as the chlorophylls. They are as well dependable for colours varying from yellow to red in both flowers and fruits. This colouration attracts pollinators (flowers) and serves as a source of food for animal herbivores (fruits), thus aiding in seed dispersal. Among this important group, the numerous compounds consist of C₄₀ chains (tetraterpenes) with conjugated double bonds, they illustrate strong light absorption and frequently are brightly coloured (red, orange). They take place as pigments in bacteria, algae and higher plants. 

The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls. Carotenoids are important components of the light harvesting in plants, expanding the absorption spectra of photosynthesis. The major carotenoids in this context are lutein, violaxanthin and neoxanthin. Additionally, there is considerable evidence which indicates a photoprotective role of xanthophylls preventing damage by dissipating excess light. In mammals, carotenoids exhibit immunomodulatory actions, likely related to their anticarcinogenic effects. β-Carotene was thus shown to enhance cell-mediated immune responses. The decrease in prostate cancer risk has been linked to the consumption of tomatoes, vegetable rich in lycopene, as prostatic tissues. While there is yet limited direct evidence linking lycopene and prostate cancer, several observations, including the ability of the prostate to concentrate lycopene, suggest a special protection of lycopene against that pathology. 

Carotenoids consist of eight isoprenoid units joined in such a manner that the arrangement of isoprenoid units is reversed at the centre of the molecule so that the two central methyl groups are in a 1,6-position association and the continuing non-terminal methyl groups are in a 1,5-position relationship. They are, by far the predominant class of tetraterpenes. They may be also classified in the terpenoids. Carotenoids can be considered derivatives of lycopene, originate in tomatoes, fruits and flowers. Its long straight chain is extremely unsaturated and composed of two identical units connected via a double link between carbon 15 and 15'. Each of such 20 carbon units might be considered to be obtained from 4 isoprene units. Lycopene is a bioactive red coloured pigment naturally occurring in plants. Interest in lycopene is rising due to rising evidence proving its antioxidant activities and its preventive properties toward abundant diseases. In vitro, in vivo and ex vivo studies have revealed that lycopene-rich foods are inversely connected to diseases these as cancers, cardiovascular diseases, diabetes, and others.

Carotenoids might be acyclic (seco-carotenoids) or cyclic (mono- or bi-, alicyclic or aryl). Oxyfunctionalisation of diverse carotenoids leads to a huge number of xanthophylls in that the function might be a hydroxyl, methoxyl, carbonyl, oxo, and formyl or epoxy group. Only several of the most ordinary carotenes and xanthophylls are specified below:

In human serum, several carotenes and xanthophylls have been detected. If α- , β-carotene and lycopene are frequently quoted in specialized papers, some others are now determined with precise HPLC techniques (lutein, zeaxanthin, cantaxanthin and β-cryptoxanthin). These compounds create from ingested fruit, green leaves, berries and yellow corn.

Xanthophylls 

Polyterpenes: (C_n)

The 2 most significant polyterpenes are natural rubber and gutta-percha. In natural rubber, the double bonds of the repeating isoprene units are cis; the molecules are able to bend back and forth a little so that the rubber can stretch. Gutta-percha has the trans structure. It is rigid and non-rubbery. Gutta-percha is extensively utilized in electrical insulation and dentistry. Faraday Michael determined in the year 1829 showed that rubber generated through Hevea brasiliensis was made up solely of carbon and hydrogen and had the empirical formula C₅H₈. In the year 1860, the English chemist Greville Williams C. Attained a liquid through the similar formula via distilling rubber, he said it isoprene. In the year 1879, Bouchardat G. obtained isoprene from natural rubber and found that heating isoprene through HCl generated a rubber-like polymer after distillation. He said that this new product had "the elasticity and other properties of rubber itself." This was the 1^st production of artificial rubber. After the early expression that each isoprene unit has one double bond and that rubber has a high molecular weight, the idea that the rubber molecule consisted of long chains shaped via the regular linking of isoprene units was only gradually established after the works of Harries C.D. (between 1902 and 1905) and mainly of Staudinger H. (in the year 1920) who coined the term "macromolecule". It was determined that the hydrocarbon chains were composed of an initial group ω' formed by two or three  trans-isomer units, a long chain formed via a huge number of cis-isomer units, and a not yet wholly decided terminal group α. 

The ω-terminal bonding to proteins was suggested to form physical cross-links, whereas the α-terminal linking to phospholipids to form chemical cross-links through long chain fatty acid ester groups. It was revealed that the bonded fatty acids were composed of saturated and unsaturated C₁₀ to C₂₂ fatty acids, the composition of that was similar to that of mixed fatty acids. It seems that the mechanical properties of natural rubber could be dependent on the composition of these fatty acids.

If natural rubber is shaped via cis-isomer units, the substance recognized as gutta-percha produced via Palaquium gutta (Sapotaceae), and balata, formed by Mimusops globosa, are polyisoprenes having all trans structure. The polymer chains of rubber are extremely extended and have an average molecular weight more than a million. As such long chains aren't naturally cross-linked; rubber is soluble in non-polar solvents and therefore might be considered as lipids.

Goodyear C.  originate in the year 1830 the way to harden the natural rubber in heating the raw product through elementary sulfur, process which  creates chain cross-links and is now known as vulcanisation. It must be noticed that while natural rubber is mostly generated today from Hevea tree, it might as well be attained from guayule (Parthenium argentatum), a xerophytic shrub, which has been exploited as commercial source of rubber because the pre-Columbian times when Indians of Mexico utilized it to form balls for their games.

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