Isoprene, Butyl and Nitrile are all types of which substance?
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Isoprene.txt Butyl_rubber.txt Nitrile.txt Terpene.txt
Isoprene Butyl rubber Nitrile Terpene
Isoprene, or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2C(CH3)-CH
CH2. In its pure form, it is a colorless volatile liquid. Isoprene is produced by many plants.

Natural occurrences

Isoprene is produced and emitted by many species of trees (major producers are oaks, poplars, eucalyptus and some legumes). Yearly production of isoprene emissions by vegetation is around 600 million tonnes, half from tropical broadleaf trees and the remainder primarily from shrubs.  This is about equivalent to methane emissions and accounts for ~1/3 of all hydrocarbons released into the atmosphere. 

Plants

Isoprene is made through the methyl-erythritol 4-phosphate pathway (MEP pathway, also called the non-mevalonate pathway) in the chloroplasts of plants. One of the two end products of MEP pathway, dimethylallyl pyrophosphate (DMAPP), is catalyzed by the enzyme isoprene synthase to form isoprene. Therefore, inhibitors that block the MEP pathway, such as fosmidomycin, also blocks isoprene formation. Isoprene emission increases dramatically with temperature and maximizes at around 40 °C. This has led to the hypothesis that isoprene may protect plants against heat stress (thermotolerance hypothesis, see below). Emission of isoprene is also observed in some bacteria and this is thought to come from non-enzymatic degradations from DMAPP.

Regulation

Isoprene emission in plants is controlled both by the availability of substrate (DMAPP) and by enzyme (isoprene synthase) activity. In particular, light, CO2 and O2 dependencies of isoprene emission are controlled by substrate availability, whereas temperature dependency of isoprene emission is regulated both by substrate level and enzyme activity.

Other organisms 

Isoprene is the most abundant hydrocarbon measurable in the breath of humans.   The estimated production rate of isoprene in the human body is 0.15 µmol/(kg·h), equivalent to approximately 17 mg/day for a person weighing 70 kg. Isoprene is common in low concentrations in many foods.

Biological roles

Isoprene emission appears to be a mechanism that trees use to combat abiotic stresses.  In particular, isoprene has been shown to protect against moderate heat stress (~40 °C). It may also protect plants against large fluctuations in leaf temperature. Isoprene is incorporated into and helps stabilize cell membranes in response to heat stress. 

Isoprene also confers resistance to reactive oxygen species.  The amount of isoprene released from isoprene-emitting vegetation depends on leaf mass, leaf area, light (particularly photosynthetic photon flux density, or PPFD) and leaf temperature. Thus, during the night, little isoprene is emitted from tree leaves, whereas daytime emissions are expected to be substantial during hot and sunny days, up to 25 μg/(g dry-leaf-weight)/hour in many oak species  

Isoprenes are used in the cell membrane monolayer of many Archaea, filling the space between the diglycerol tetraether head groups. This is thought to add structural resistance to harsh environments in which many Archaea are found.

Isoprenoids

The isoprene skeleton can be found in naturally occurring compounds called terpenes (also known as isoprenoids), but these compounds do not arise from isoprene itself. Terpenes can be viewed as multiples of isoprene subunits, and this perspective is the cornerstone of the "isoprene rule". The precursor to isoprene units in biological systems is dimethylallyl pyrophosphate (DMAPP) and its isomer isopentenyl pyrophosphate (IPP). The plural “isoprenes” is sometimes used to refer to terpenes in general. Isoprene chains are commonly found in numerous biologically active oligomers such as Vitamin A. Similarly, natural rubber is  composed of linear polyisoprene chains of very high molecular weight and other natural molecules.Heinz-Hermann Greve "Rubber, 2. Natural" in Ullmann's Encyclopedia of Industrial Chemistry, 2000, Wiley-VCH, Weinheim. 

Impact on aerosols

After release, isoprene is converted by free radicals (like the hydroxyl radical) and to a lesser extent by ozone   into various species, such as aldehydes, hydroperoxides, organic nitrates, and epoxides, which mix into water droplets and help create aerosols and haze.   

While most experts acknowledge that isoprene emission affects aerosol formation, whether isoprene increases or decreases aerosol formation is debated. A second major effect of isoprene on the atmosphere is that in the presence of nitric oxides (NOx) it contributes to the formation of tropospheric (lower atmosphere) ozone, which is one of the leading air pollutants in many countries. Isoprene itself is not normally regarded as a pollutant, as it is a natural plant product. Formation of tropospheric ozone is only possible in presence of high levels of NOx, which comes almost exclusively from industrial activities. Isoprene can have the opposite effect and quench ozone formation under low levels of NOx.

Industrial production

Isoprene was first isolated by thermal decomposition of natural rubber.  It is most readily available industrially as a byproduct of the thermal cracking of naphtha or oil, as a side product in the production of ethylene. About 800,000 tonnes are produced annually. About 95% of isoprene production is used to produce cis-1,4-polyisoprene—a synthetic version of natural rubber.

Natural rubber consists mainly of poly-cis-isoprene with a molecular weight of 100,000 to 1,000,000. Typically natural rubber contains a few percent of other materials, such as proteins, fatty acids, resins, and inorganic materials. Some natural rubber sources, called gutta percha, are composed of trans-1,4-polyisoprene, a structural isomer that has similar, but not identical, properties.

Structural motif

Isoprene is a common structural motif in biological systems. The isoprenoids (for example, the carotenes are tetraterpenes) are derived from isoprene. Also derived from isoprene are phytol, retinol (vitamin A), tocopherol (vitamin E), dolichols, and squalene. Heme A has an isoprenoid tail, and lanosterol, the sterol precursor in animals, is derived from squalene and hence from isoprene. The functional isoprene units in biological systems are dimethylallyl pyrophosphate (DMAPP) and its isomer isopentenyl pyrophosphate (IPP), which are used in the biosynthesis of naturally occurring isoprenoids such as carotenoids, quinones, lanosterol derivatives (e.g. steroids) and the prenyl chains of certain compounds (e.g. phytol chain of chlorophyll). Butyl rubber is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for isobutylene isoprene rubber. Polyisobutylene, also known as "PIB" or polyisobutene, (C4H8)n, is the homopolymer of isobutylene, or 2-methyl-1-propene, on which butyl rubber is based. Butyl rubber is produced by polymerization of about 98% of isobutylene with about 2% of isoprene. Structurally, polyisobutylene resembles polypropylene, having two methyl groups substituted on every other carbon atom. Polyisobutylene is a colorless to light yellow viscoelastic material. It is generally odorless and tasteless, though it may exhibit a slight characteristic odor.

Butyl rubber has excellent impermeability, and the long polyisobutylene segments of its polymer chains give it good flex properties.

The formula for PIB is:
–(–CH2–C(CH3)2–)n–

The formula for IIR is:

It can be made from the monomer isobutylene or CH2=C(CH3)2 only via cationic addition polymerization.

A synthetic rubber, or elastomer, butyl rubber is impermeable to air and used in many applications requiring an airtight rubber. Polyisobutylene and butyl rubber are used in the manufacture of adhesives, agricultural chemicals, fiber optic compounds, ball bladders, caulks and sealants, cling film, electrical fluids, lubricants (2 cycle engine oil), paper and pulp, personal care products, pigment concentrates, for rubber and polymer modification, for protecting and sealing certain equipment for use in areas where chemical weapons are present, as a gasoline/diesel fuel additive, and even in chewing gum. The first major application of butyl rubber was tire inner tubes. This remains an important segment of its market even today.

History

Isobutylene was discovered by Michael Faraday in 1825. Polyisobutylene (PIB) was first developed by the BASF unit of IG Farben in 1931 using a boron trifluoride catalyst at low temperatures and sold under the trade name Oppanol B. PIB remains a core business for BASF to this day.

It was later developed into butyl rubber in 1937, by researchers William J. Sparks and Robert M. Thomas, at Standard Oil of New Jersey's Linden, N.J., laboratory. Today, the majority of the global supply of butyl rubber is produced by just two companies, ExxonMobil, the successor to Standard Oil, and Polymer Corporation, a Canadian federal crown corporation established in 1942 to produce artificial rubber to substitute for overseas supply cut off by World War II. It was renamed Polysar in 1976 and the rubber component became a subsidiary, Polysar Rubber Corp. The company was privatized in 1988 with its sale to NOVA Corp which, in turn, sold Polysar Rubber in 1990 to Bayer AG of Germany. In 2005 Bayer AG spun off chemical divisions, including most of the Sarnia site, creating LANXESS AG, also of Germany. 

PIB homopolymers of high molecular weight (100 000 - 400 000 or more) are polyolefin elastomers: tough extensible rubber-like materials over a wide temperature range; with low density: 0.913-0.920, low permeability and excellent electrical properties.

In the 1950s and 1960s, halogenated butyl rubber (halobutyl) was developed, in its chlorinated (chlorobutyl) and brominated (bromobutyl) variants, providing significantly higher curing rates and allowing covulcanization with other rubbers such as natural rubber and styrene-butadiene rubber. Halobutyl is today the most important material for the inner linings of tubeless tires.

In the spring of 2013 two incidents of PIB contamination in the English Channel, believed to be connected, were described as the worst UK marine pollution 'for decades'. The RSPB estimated over 2,600 seabirds were killed by the chemical and hundreds more were rescued and decontaminated. 

Uses

Fuel and lubricant additive

Polyisobutylene can be reacted with maleic anhydride to make polyisobutenylsuccinic anhydride (PIBSA), which can then be converted into polyisobutenylsuccinimides (PIBSI's)by reacting it with various amines. These derivatives have interesting properties when used as an additive in lubricating oils and motor fuels. Polyisobutylene added in small amounts to the lubricating oils used in machining results in a significant reduction in the generation of oil mist and thus reduces the operator's inhalation of oil mist.  It is also used to clean up waterborne oil spills as part of the commercial product Elastol. When added to crude oil it increases the oil's viscoelasticity when pulled, causing the oil to resist breakup when it is vacuumed from the surface of the water.

As a fuel additive, polyisobutylene has detergent properties. When added to diesel fuel, it resists fouling of fuel injectors, leading to reduced hydrocarbon and particulate emissions.  It is blended with other detergents and additives to make a "detergent package" that is added to gasoline and diesel fuel to resist buildup of deposits and engine knock. 

Polyisobutylene is used in some formulations as a thickening agent.

Explosives 

Polyisobutylene is often used by the explosives industry as a binding agent in plastic explosives such as C-4.  Polyisobutylene binder is used because it makes the explosive more insensitive to premature detonation as well as making it easier to handle and mold.

Sporting equipment 

Butyl rubber is used for the bladders in sporting balls (e.g. Rugby balls, footballs, basketballs, netballs) to provide a tough, airtight inner compartment.

Damp proofing and roof repair 

Butyl rubber sealant is used for damp proofing, rubber roof repair and for maintenance of roof membranes (especially around the edges). It is important to have the roof membrane fixed, as a lot of fixtures (i.e., air conditioner vents, plumbing and other pipes, etc.) can considerably loosen it.

Rubber roofing typically refers to a specific type of roofing materials that are made of ethylene propylene diene monomers (EPDM rubber). It is crucial to the integrity of such roofs to avoid using harsh abrasive materials and petroleum-based solvents for their maintenance.

Polyester fabric laminated to butyl rubber binder provides a single-sided waterproof tape that can be used on metal, PVC, and cement joints. It is used for repairing and waterproofing metal roofs.

Gas masks and chemical agent protection 

Butyl rubber is one of the most robust elastomers when subjected to chemical warfare agents and decontamination materials. It is a harder and less porous material than other elastomers, such as natural rubber or silicone, but still has enough elasticity to form an airtight seal. While butyl rubber will break down when exposed to agents such as NH3 (ammonia) or certain solvents, it breaks down more slowly than comparable elastomers. It is therefore used to create seals in gas masks and other protective clothing.

Pharmaceutical Stoppers 

Butyl and Bromobutyl rubber are commonly used for manufacturing rubber stoppers used for sealing medicine vials and bottles. 

Chewing gum 

Most modern chewing gum uses food-grade butyl rubber as the central gum base, which contributes not only the gum's elasticity but an obstinate, sticky quality which has led some municipalities to propose taxation to cover costs of its removal. 

Tires 

Butyl rubber and halogenated rubber are used for the innerliner that holds the air in the tire.

Notes A nitrile is any organic compound that has a −C≡N functional group.  The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, and nitrile rubber, a nitrile-containing polymer used in the latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons.

Inorganic compounds containing the −C≡N group are not called nitriles, but cyanides instead.  Though both nitriles and cyanides can be derived from cyanide salts, most nitriles are not nearly as toxic.

Structure and basic properties

The N−C−C geometry is linear in nitriles, reflecting the sp hybridization of the triply bonded carbon. The C−N distance is short at 1.16 Å, consistent with a triple bond.   Nitriles are polar, as indicated by high dipole moments.  As liquids, they have high dielectric constants, often in the 30s.

History

The first compound of the homolog row of nitriles, the nitrile of formic acid, hydrogen cyanide was first synthesized by C. W. Scheele in 1782.   In 1811 J. L. Gay-Lussac was able to prepare the very toxic and volatile pure acid.  
The nitrile of benzoic acids was first prepared by Friedrich Wöhler and Justus von Liebig, but due to minimal yield of the synthesis neither physical nor chemical properties were determined nor a structure suggested. Théophile-Jules Pelouze synthesized propionitrile in 1834 suggesting it to be an ether of propionic alcohol and hydrocyanic acid. 
The synthesis of benzonitrile by Hermann Fehling in 1844, by heating ammonium benzoate, was the first method yielding enough of the substance for chemical research. 
He determined the structure by comparing it to the already known synthesis of hydrogen cyanide by heating ammonium formate to his results. He coined the name "nitrile" for the newfound substance, which became the name for this group of compounds. 

Synthesis 

Industrially, the main methods for producing nitriles are ammoxidation and hydrocyanation. Both routes are green in the sense that they do not generate stoichiometric amounts of salts.

Ammoxidation

In ammoxidation, a hydrocarbon is partially oxidized in the presence of ammonia. This conversion is practiced on a large scale for acrylonitrile: 
CH3CHCH2  +  O2  +  NH3   →   NCCH
CH2  +  3 H2O
In the production of acrylonitrile, a side product is acetonitrile. Most derivatives of benzonitrile, phthalonitrile, as well as Isobutyronitrile are prepared by ammoxidation.  The process is catalysed by metal oxides and is assumed to proceed via the aldehyde.

Hydrocyanation

Hydrocyanation is an industrial method for producing nitriles from hydrogen cyanide and alkenes.  The process requires homogeneous catalysts.  An example of hydrocyanation is the production of adiponitrile, a precursor to nylon-6,6 from 1,3-butadiene:
CH2CH−CH
CH2  +  2 HCN   →   NC(CH2)4CN

From organic halides and cyanide salts

Often for more specialty applications, nitriles can be prepared by a wide variety of other methods. For example, alkyl halides undergo nucleophilic aliphatic substitution with alkali metal cyanides in the Kolbe nitrile synthesis. Aryl nitriles are prepared in the Rosenmund-von Braun synthesis.

Cyanohydrins

The cyanohydrins are a special class of nitriles that result from the addition of metal cyanides to aldehydes in the cyanohydrin reaction. Because of the polarity of the organic carbonyl, this reaction requires no catalyst, unlike the hydrocyanation of alkenes.

Dehydration of amides and oximes

Nitriles can be prepared by the dehydration of primary amides. Many reagents are available, the combination of ethyl dichlorophosphate and DBU just one of them in this conversion of benzamide to benzonitrile: 
:Two intermediates in this reaction are amide tautomer A and its phosphate adduct B.
In a related dehydration, secondary amides give nitriles by the von Braun amide degradation. In this case, one C-N bond is cleaved.
The dehydration of aldoximes (RCH=NOH) also affords nitriles. Typical reagents for this transformation are triethylamine/sulfur dioxide, zeolites, or sulfuryl chloride. Exploiting this approach is the One-pot synthesis of nitriles from aldehyde with hydroxylamine in the presence of sodium sulfate.  
* from aryl carboxylic acids (Letts nitrile synthesis)

Sandmeyer reaction

Aromatic nitriles are often prepared in the laboratory from the aniline via diazonium compounds. This is the Sandmeyer reaction. It requires transition metal cyanides. 
+ CuCN → ArCN + N2 + Cu+

Other methods

*A commercial source for the cyanide group is diethylaluminum cyanide Et2AlCN which can be prepared from triethylaluminium and HCN.  It has been used in nucleophilic addition to ketones.  For an example of its use see: Kuwajima Taxol total synthesis
* cyanide ions facilitate the coupling of dibromides. Reaction of α,α′-dibromoadipic acid with sodium cyanide in ethanol yields the cyano cyclobutane: 
: In the so-called Franchimont Reaction (A. P. N. Franchimont, 1872) an α-bromocarboxylic acid is dimerized after hydrolysis of the cyanogroup and decarboxylation 
* Aromatic nitriles can be prepared from base hydrolysis of trichloromethyl aryl ketimines (RC(CCl3)=NH) in the Houben-Fischer synthesis 
* Nitriles can be obtained from primary amines via oxidation. Common methods include the use of potassium persulfate,  Trichloroisocyanuric acid,  or anodic electrosynthesis. 
* α-Amino acids form nitriles and carbon dioxide via various means of oxidative decarboxylation.   Henry Drysdale Dakin discovered this oxidation in 1916. 

Reactions 

Nitrile groups in organic compounds can undergo a variety of reactions depending on the reactants or conditions.  A nitrile group can be hydrolyzed, reduced, or ejected from a molecule as a cyanide ion.

Hydrolysis 

The hydrolysis of nitriles RCN proceeds in the distinct steps under acid or base treatment to achieve carboxamides RC(=O)NH2 and then carboxylic acids RCOOH. The hydrolysis of nitriles to carboxylic acids is efficient. When conducted with base or acids, the reactions cogenerate salts, which can be problematic.

The kinetic studies show that the second-order rate constant for hydroxide-ion catalyzed hydrolysis of acetonitrile to acetamide is 1.6 × 10−6, which is slower than the hydrolysis of the amide to the carboxylate (7.4 × 10−5 M−1 s−1). Thus, the base hydrolysis route will afford amides contaminated with the carboxylate. The acid catalyzed reactions requires a careful control of the temperature and of the ratio of reagents in order to avoid the formation of polymers, which is promoted by the exothermic character of the hydrolysis.  The [http://www.wikipremed.com/03_organicmechanisms.php?mch_code=030209_100 mechanism] is depicted in the enlisted link.

Reduction 

Nitriles are susceptible to hydrogenation over diverse metal catalysts.  The reaction can afford either the primary amine (RCH2NH2) or the tertiary amine ((RCH2)3N), depending on conditions.   In conventional organic reductions,  nitrile is reduced by treatment with lithium aluminium hydride to the amine.  Reduction to the imine followed by hydrolysis to the aldehyde takes place in the Stephen aldehyde synthesis, which uses stannous chloride in acid.

Alkylation 

Alkyl nitriles are sufficiently acidic to form nitrile anions, which alkylate a wide variety of electrophiles.  Key to the exceptional nucleophilicity is the small steric demand of the CN unit combined with its inductive stabilization. These features make nitriles ideal for creating new carbon-carbon bonds in sterically demanding environments for use in syntheses of medicinal chemistry. Recent developments have shown that the nature of the metal counter-ion causes different coordination to either the nitrile nitrogen or the adjacent nucleophilic carbon, often with profound differences in reactivity and stereochemistry. 

Nucleophiles 

The carbon center of a nitrile is electrophilic, hence it is susceptible to nucleophilic addition reactions:
* with an organozinc compound in the Blaise reaction
* with alcohols in the Pinner reaction.
* with amines, e.g. the reaction of the amine sarcosine with cyanamide yields creatine 
* Nitriles react in Friedel–Crafts acylation in the Houben–Hoesch reaction to ketones

Miscellaneous methods and compounds 

* In reductive decyanation the nitrile group is replaced by a proton.The reductive decyanation reaction: chemical methods and synthetic applications Jean-Marc Mattalia, Caroline Marchi-Delapierre, Hassan Hazimeh, and Michel Chanon Arkivoc (AL-1755FR) pp 90–118 2006 [http://www.arkat-usa.org/ark/journal/2006/I04_Lattes/1755/AL-1755FR%20as%20published%20mainmanuscript.asp Article] Decyanations can be accomplished by dissolving metal reduction (e.g. HMPA and potassium metal in tert-butanol) or by fusion of a nitrile in KOH.  Similarly, α-aminonitriles can be decyanated with other reducing agents such as lithium aluminium hydride.
* Nitriles self-react in presence of base in the Thorpe reaction in a nucleophilic addition
* In organometallic chemistry nitriles are known to add to alkynes in carbocyanation: 
Nitrile derivatives

Organic cyanamides

Cyanamides are N-cyano compounds with general structure R1R2N−CN and related to the inorganic parent cyanamide. For an example see: von Braun reaction.

Nitrile oxides

Nitrile oxides have the general structure R−CNO.
R-C{\equiv}\overset{\oplus}N-O^\ominus

Occurrence and applications

Nitriles occur naturally in a diverse set of plant and animal sources. Over 120 naturally occurring nitriles have been isolated from terrestrial and marine sources. Nitriles are commonly encountered in fruit pits, especially almonds, and during cooking of Brassica crops (such as cabbage, brussel sprouts, and cauliflower), which release nitriles through hydrolysis. Mandelonitrile, a cyanohydrin produced by ingesting almonds or some fruit pits, releases hydrogen cyanide and is responsible for the toxicity of cyanogenic glycosides. 

Over 30 nitrile-containing pharmaceuticals are currently marketed for a diverse variety of medicinal indications with more than 20 additional nitrile-containing leads in clinical development. The nitrile group is quite robust and, in most cases, is not readily metabolized but passes through the body unchanged. The types of pharmaceuticals containing nitriles are diverse, from vildagliptin, an antidiabetic drug, to anastrozole, which is the gold standard in treating breast cancer. In many instances the nitrile mimics functionality present in substrates for enzymes, whereas in other cases the nitrile increases water solubility or decreases susceptibility to oxidative metabolism in the liver.  The nitrile functional group is found in several drugs.

File:Periciazine.svg|Structure of periciazine, an antipsychotic studied in the treatment of opiate dependence.
File:Citalopram structure.svg|Structure of citalopram, an antidepressant drug of the selective serotonin reuptake inhibitor (SSRI) class.
File:Cyamemazine.svg|Structure of cyamemazine, an antipsychotic drug.
File:Fadrozole.png|Structure of fadrozole, an aromatase inhibitor for the treatment of breast cancer.
File:Letrozole.svg|Structure of letrozole, an oral non-steroidal aromatase inhibitor for the treatment of certain breast cancers. Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers,  though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeteria. They are often strong-smelling.  They may protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores.  Many terpenes are aromatic hydrocarbons and thus may have had a protective function.  The difference between terpenes and terpenoids is that terpenes are hydrocarbons, whereas terpenoids contain additional functional groups.

They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine".  In addition to their roles as end-products in many organisms, terpenes are major biosynthetic building blocks within nearly every living creature.  Steroids, for example, are derivatives of the triterpene squalene.

When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids.  Some authors will use the term terpene to include all terpenoids. Terpenoids are also known as isoprenoids.

Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers.  Essential oils are used widely as fragrances in perfumery, and in medicine and alternative medicines such as aromatherapy.  Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives.  Vitamin A is a terpene.

Terpenes are released by trees more actively in warmer weather, acting as a natural form of cloud seeding.  The clouds reflect sunlight, allowing the forest to regulate its temperature. 

The aroma and flavor of hops, highly desirable in some beers, comes from terpenes. Of the terpenes in hops myrcene, β-pinene, β-caryophyllene, and α-humulene are found in the largest quantities.Glenn Tinseth, "Hop Aroma and Flavor", January/February 1993, Brewing Techniques.  Accessed July 21, 2010.

Terpenes are also major constituents of Cannabis sativa plants, which contain at least 120 identified compounds.  In addition to being responsible for the plant’s aroma, they can act synergistically with cannabinoids. In fact, there are several promising applications based on the combined use of cannabinoids and terpenes, such as new acne therapies utilizing CBD with the monoterpenes limonene, linalool, and pinene; new antiseptic agents with CBG and pinene; treatment of social anxiety disorder using CBD with limonene and linalool; and treatment of sleeping disorders by adding caryophyllene, linalool, and myrcene to 1:1 CBD/THC extracts.

Structure and biosynthesis
 

Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C5H8.  The basic molecular formulae of terpenes are multiples of that, (C5H8)n where n is the number of linked isoprene units. This is called the isoprene rule or the C5 rule. The isoprene units may be linked together "head to tail" to form linear chains or they may be arranged to form rings. One can consider the isoprene unit as one of nature's common building blocks.

Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway.  IPP is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. An alternative, totally unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP(2-Methyl-D-erythritol-4-phosphate)-pathway, which is initiated from C5-sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.

As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes,  diterpenes, sesterterpenes, triterpenes, and tetraterpenes. Essentially, they are all synthesised by terpene synthase.

Types

Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule.

* Hemiterpenes consist of a single isoprene unit.  Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids.
* Monoterpenes consist of two isoprene units and have the molecular formula C10H16. Examples of monoterpenes and monoterpenoids include geraniol, terpineol (present in lilacs), limonene (present in citrus fruits), myrcene (present in hops), linalool (present in lavender) or pinene (present in pine trees).   Iridoids derive from monoterpenes. 
* Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24.  Examples of sesquiterpenes and sesquiterpenoids include humulene, farnesenes, farnesol. (The sesqui- prefix means one and a half.)
* Diterpenes are composed of four isoprene units and have the molecular formula C20H32.  They derive from geranylgeranyl pyrophosphate.  Examples of diterpenes and diterpenoids are cafestol, kahweol, cembrene and taxadiene (precursor of taxol). Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol.
* Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. (The sester- prefix means half to three, i.e. two and a half.) An example of a sesterterpenoid is geranylfarnesol.
* Triterpenes 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 two molecules of farnesyl pyrophosphate. Squalene is then processed biosynthetically to generate either lanosterol or cycloartenol, the structural precursors to all the steroids.
* Sesquarterpenes are composed of seven isoprene units and have the molecular formula C35H56.    Sesquarterpenes are typically microbial in their origin. Examples of sesquarterpenoids are ferrugicadiol and tetraprenylcurcumene.
* Tetraterpenes contain eight isoprene units and have the molecular formula C40H64.  Biologically important tetraterpenoids include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes.
* Polyterpenes consist of long chains of many isoprene units.  Natural rubber consists of polyisoprene in which the double bonds are cis. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha.
* Norisoprenoids, such as the C13-norisoprenoids 3-oxo-α-ionol present in Muscat of Alexandria leaves and 7,8-dihydroionone derivatives, such as megastigmane-3,9-diol and 3-oxo-7,8-dihydro-α-ionol found in Shiraz leaves (both grapes in the species Vitis vinifera)  or wine   (responsible for some of the spice notes in Chardonnay), can be produced by fungal peroxidases  or glycosidases. 

Research

A range of terpenes have been identified as high-value chemicals in food, cosmetic, pharmaceutical and biotechnology industries.   Chemical synthesis of terpenes can be problematic because of their complex structure, and plants produce very small amounts of these valuable chemicals, making it difficult, time consuming and expensive to extract them directly from plants. Scientists are working to identify the key enzymes and pathways that produce terpenes in plants. In 2014, researchers at the John Innes Centre  searched the genomes of 17 plant species to find the genes that encode terpenoid synthase enzymes that give terpenes their basic structure, and cytochrome P450s that modify this basic structure.  Once genes have been identified for the synthesis pathway of a specific terpene, synthetic biology techniques could be used to make high levels of this terpene, using plants or microorganisms (e.g. yeast) as “factories”. This could mean cheaper and more sustainable production of economically and medicinally important terpenes. For example, a precursor to the effective anti-malarial terpene artemesinin, from the plant Artemisia annua, can now be produced in yeast. 

Other uses

Research into terpenes has found that many of them possess qualities that make them useful active ingredients as part of natural agricultural pesticides. 

Terpin hydrate is a derivative of turpentine. An expectorant and humectant, it is commonly used in the treatment of acute or chronic bronchitis and related conditions.

Terpenes are used by termites of the Nasutitermitinae family to attack enemy insects, through the use of a specialized mechanism called a fontanellar gun.
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