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Alkenes

Alkenes are hydrocarbons which contain carbon-carbon double bonds.  Their general formula is CnH2n for molecules with one double bond (and no rings).  Alkenes are also known as olefins, after the original name for ethene, olefiant gas.

Alkenes take part in a wide variety of chemical reactions, and are found as parts of many highly colored systems (see below for examples).

Straight-chain and branched alkenes are named by using the stem that indicates the number of carbon atoms to which is added the suffix -ene.  A locator number is placed immediately in front of the prefix to indicate which carbon number the double bond starts on.  Thus, the word "1-butene" indicates a chain of four carbons, with a double bond between carbons 1 and 2; the word "2-butene" indicates a chain of four carbons, with a double bond between carbons 2 and 3.

Double-bonded carbons are sp2-hybridized, and have trigonal planar shapes, with the bonded atoms at angles of 120° to each other.  Free rotation is not possible around carbon-carbon double bonds in alkenes, making the carbon chains less flexible and "floppy" than those of alkanes with the same number of carbons.  This lack of free rotation also gives rise to geometric isomerism in alkenes (see 2-butene below for an example).

Alkenes are nonpolar, since they contain nothing but carbon-carbon and carbon-hydrogen bonds, and are not soluble in water; they are also generally less dense than water.

Many alkenes are isolated from petroleum, and may serve as the starting points for the synthesis of more complex molecules.

 

Ethene (Ethylene) 3D

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Ethene is more commonly known under the trivial name ethylene.  It is the simplest of the alkenes, consisting of two carbon atoms connected by a double bond.  This leaves each carbon free to bond to two hydrogen atoms.  Since both carbon atoms are trigonal planar in shape, all six atoms lie in the same plane, and ethylene is a flat molecule.  It is a colorless gas (bp -103.9°C), having a sweet odor and taste.  It is synthesized during the cracking of hydrocarbons at high temperatures, and can also be produced from the dehydration of ethanol.  It is used in the manufacture of the plastic polyethylene (see section on Addition Polymers).  The release of ethylene stimulates the beginning of the ripening process in many plants; some fruits and vegetables are picked while unripe, when they are less fragile, and ripened when they reach their destination by exposure to ethylene gas.

 
   
Propene (Propylene) 3D

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Propene, more commonly known as propylene, is a colorless gas (bp -47.7°C) produced in the cracking of hydrocarbons.  It is used in the manufacture of the polymer polypropylene.

 
   
1-Butene 3D

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1-Butene consists of a chain of four carbons, having a double bond between carbons 1 and 2.  Because carbon 1 has two of the same substituent (in this case, H), 1-butene does not exhibit geometric isomerism, unlike its structural isomer, 2-butene (see below).

 
   
2-Butene 3D

2-Butene is a structural isomer of 1-butene, having the double bond located between carbons 2 and 3.  Because free rotation is not possible around carbon-carbon double bonds, there are two isomers of 2-butene:  one in which the two methyl groups at the end are pointing to the same side of the molecule, known as cis-2-butene:

 

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and one in which the two methyl groups are pointing to opposite sides of the molecule, known as trans-2-butene:

 

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Molecules which exhibit this kind of isomerism are known as geometric isomers (or cis-trans isomers).  Despite the close similarity between these two structures, they are distinctly different compounds:  the cis isomer boils at 3.7°C and melts at -139°C, while the trans isomer boils at 0.88°C and melts at -105.8°C.

 
   
Chloroethene (Vinyl chloride) 3D

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Chloroethene, or vinyl chloride, is a carcinogenic gas manufactured from ethylene.  It is used in the manufacture of the plastic polyvinyl chloride (PVC).  The —CH=CH2 group occurs in many organic molecules, and is commonly known as the vinyl group (or more formally as the ethenyl group).

 
   
Tetrachloroethylene (Perchloroethylene, "Perc") 3D

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Tetrachloroethylene, better known as perchloroethylene ("Perc"), and more officially as tetrachloroethene, is a non-flammable organic solvent, widely used in dry cleaning; it is also used as an industrial solvent, degreaser, and paint remover.

 
   
2-Methyl-1-propene; Isobutylene 3D

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Isobutylene is copolymerized with isoprene to form butyl rubber (see entry on polyisoprene).

 
   
Muscalure 3D

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Muscalure, or cis-9-tricosene, is the sex pheromone of the female common housefly (Musca domestica).

 
   
1,3-Butadiene 3D

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Molecules which have two carbon-carbon double bonds are named with the suffix -diene; hence a four carbon chain with double bonds starting at carbon numbers 1 and 3 is named as 1,3-butadiene.  Molecules with carbon-carbon double bonds adjacent to each other are referred to as conjugated alkenes; molecules with large numbers of conjugated double bonds are responsible for many of the colors that we see in the world around us (see the section on Terpenes for examples).

 
   
1,2-Propadiene 3D

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1,2-Propadiene is an example of an allene, in which there are two double bonds to the same carbon atom.  (In fact, the common name of 1,2-propadiene is allene.)  An interesting feature in the shape of these molecules is that the interior "double-double-bonded" carbon is sp-hybridized, and the two pi-systems of the double bonds are perpendicular to each other; therefore the substituents on the exterior double bonded carbon atoms are at right angles to each other.

 
   
Cyclopropene 3D

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Cyclopropene is an example of a cycloalkene, a cyclic structure containing a carbon-carbon double bond.  In cycloalkenes smaller than cyclohexene, the double bond can only exist in the cis-configuration, since the ring strain would be too great to accommodate a trans double bond.  Cyclopropene itself is a highly strained molecule, since the bond angles are 60°, which is very far from the ideal sp2-hybridized geometry of 120°.

 
   
Cyclobutene 3D

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Cyclobutene is another highly strained molecule, with bond angles of approximately 90°, which is again very far from the ideal 120° geometry of alkenes.

 
   
Cyclopentene 3D

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Cyclopentene is not as highly strained as cyclobutene, since the bond angles of 108° are a little closer to the ideal 120° geometry of alkenes.

 
   
Cyclohexene 3D

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Cyclohexene rings are considerably less strained than the smaller cycloalkenes, and occur commonly in nature.

 
   
Cyclooctene 3D

cis-cyclooctene


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Cyclooctene has a large enough ring that it can exist in both the cis and trans stereoisomers.  trans-Cyclooctene is a chiral compound, and can be found in two enantiomeric forms:

 

trans-cyclooctene


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Cyclobutadiene 3D

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Rings of four or more carbons can accommodate two double bonds; 1,3-cyclobutadiene is one example of this phenomenon.  Since the ring is too small to accommodate two double bonds to the same carbon atom, the position numbers of 1 and 3 are often dropped from the name, since there is no other set of position numbers possible for the ring.

 
   
Cyclopentadiene 3D

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1,3-Cyclopentadiene is a ring of five carbons containing two double bonds.  As with cyclobutadiene, the position numbers can be dropped from the name, since no other positioning of the double bonds is possible.  When left to itself, cyclopentadiene undergoes a Diels-Alder reaction with itself, producing a dimer called dicyclopentadiene; this is the form in which cyclopentadiene is usually sold. 

 
 

By heating the dicyclopentadiene is a distillation apparatus, it is possible to "crack" the dimer back into the cyclopentadiene monomer, which distills out and can be collected in a cooled receiving flask.  Cyclopentadiene is a common reagent in organic labs for use as a reagent in various Diels-Alder reactions.

 
   
Cyclooctatetraene 3D

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1,3,5,7-Cyclooctatetraene was first synthesized by Richard Willstätter in 1905.  Unlike benzene, it is not an aromatic compound, despite having an alternating series of double and single bonds.  (See the section on Annulenes on the Aromatic Rings page.)

 

 

 

 

Terpenes, Terpenoids, and Essential Oils

The terpenes are a diverse group of natural products which can be considered as being composed of isoprene units (although their biosynthesis does not actually involve the isoprene molecule).  The monoterpenes contain 10 carbon atoms, the sesquiterpenes have 15, the diterpenes have 20, the triterpenes have 30, the tetraterpenes have 40, and so on.  A number of related molecules also contain oxygen, and are classified as terpenoids

Terpenes and terpenoids are found in many plants, and often have distinctive flavors and aromas.  They are often components of essential oils, so named because they have a characteristic “essence” or fragrance. Many of these molecules are components of common foods and perfumes.  They can often be obtained from natural sources by steam distillation, or other fairly simple purification techniques.  Many of these compounds have important pharmaceutical and industrial uses, and may also be used as the starting point for the synthesis of other important compounds.

Isoprene (2-methyl-1,3-butadiene) 3D

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Isoprene (more formally 2-methyl-1,3-butadiene) can be considered the building block of the terpene family, although isoprene itself is not actually involved in the biosynthesis of these molecules.  Instead, the natural precursor is isopentenyl pyrophosphate (3-methyl-3-butenyl pyrophosphate).  Two isopentenyl pyrophosphate units combine enzymatically to form geranyl pyrophosphate, which is the precursor for the monoterpenes; reaction with a third isopentenyl pyrophosphate molecule produces farnesyl pyrophosphate, the precursor for the sesquiterpenes.

 

Isopentenyl pyrophosphate
 

Geranyl pyrophosphate
 

Farnesyl pyrophosphate
 

 

Monoterpenes

Myrcene 3D

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Myrcene is found in bay leaves and oil of bay.

 
   
Geraniol 3D

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Geraniol is found in roses (with 2-phenylethanol), citronella oil, pomerosa oil, geraniums, and other flowers.

 
   
Citral (Geranial) 3D

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Citral, or geranial, is found in oil of lemon; it is also secreted by some insects to repel predators.  It is used commercially in lemon-smelling perfumes and in the synthesis of Vitamin A.  The structure shown here is citral-a; there is also a citral-b isomer, in which the aldehyde and methyl group on the bottom double bond are in a trans relationship.

 
   
a-Pinene 3D

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a-Pinene is the major component of oil of turpentine, which is obtained by steam distillation of the resin from several species of pine.

 
   
b-Pinene 3D

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b-Pinene is found in oil of turpentine, but in much smaller amounts than a-pinene.

 
   
a-Terpineol 3D

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a-Terpineol is found in pine oil and juniper; it is used as a perfume and bactericide in many household cleaners.

 
   
(R)-(+)-Limonene 3D

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Limonene is isolated from oil of lemon or orange; it is found in the zest of the fruit.

 
   
(R)-(-)-Carvone 3D

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(R)-Carvone is found in spearmint oil.

 
   
(S)-(+)-Carvone 3D

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(S)-Carvone is found in caraway seed oil and dillseed oil; this molecule is the enantiomer of R-carvone above.  The difference in orientation around the chiral carbon atoms gives these molecules a slightly different overall shape, resulting in slightly different odors when they interact with the olfactory receptors in the nasal passages.

 
   
Thujene 3D

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Thujene is found in oil of thuja, sage, tansy, and wormwood.

 
   
Thujone 3D

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Thujone is found in oil of thuja, sage, tansy, and wormwood.

 
   
a-Phellandrene 3D

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The (-) stereoisomer of a-phellandrene is found in eucalyptus oils; the (+) stereoisomer is found in bitter fennel oil, ginger-grass oil, and cinnamon oil.

 
   
b-Phellandrene 3D

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The (+) and (-) stereoisomers of b-phellandrene is found in a variety of essential oils, including water-fennel oil, lemon oil, and some eucalyptus oils.

 
   
Menthol 3D

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Menthol is found in peppermint and other mint oils.  It has a pleasant cooling taste, since it causes the “cold” receptors on the tongue to activate at higher temperatures than normal (such as body temperature).  It is used in cough drops, shaving lotion, and mentholated tobacco.

 
   
Menthone 3D

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Menthone is the oxidized form of menthol, and has a similar taste and physiological effect.

 
   
Cineole (Eucalyptol) 3D

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Cineole, or eucalyptol, is found in eucalyptus oils.  It is the active component of medicinal eucalyptus oils.

 
   
Camphene 3D

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Camphene is found in turpentine oil, rosemary, cypress oil, and oil of citronella.

 
   
Camphor 3D

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Camphor is obtained from the wood of the camphor tree (found in China and Japan).  It is used medicinally as a counter-irritant (a substance which produces a superficial inflammation to reduce deeper inflammation) and an anti-itching agent.  It produces a cooling sensation, because it stimulates cold receptors (see menthol).  Its strong odor inspires deep breathing, but in large doses can lead to respiratory collapse.

 

 

Sesquiterpenes

a-Farnesene 3D

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a-Farnesene is a waxy substance found on the outer skin of apples.

 
   
b-Selinene 3D

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b-Selinene is found in oil of celery.

 
   
Caryophyllene 3D

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Caryophyllene is found in oil of cloves

 

 

Diterpenes

Vitamin A (Retinol) 3D

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Vitamin A (also known as retinol) is a fat-soluble vitamin, which is produced by the breakdown of the carotenes (especially b-carotene).  It is found in liver, egg yolks, butter, and milk.  It is a precursor to retinal (see below), the primary dye involved in vertebrate vision.  It is also involved in cell growth and maintenance of healthy skin tissue, bones, and teeth.

 

 

Triterpenes

Squalene

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Squalene is found in shark liver oil, and is also a major component of the lipids on the surface of human skin.  Although it is not obvious from the way the structure above is drawn, squalene is a precursor for the biosynthesis of cholesterol.  Through a complex series of enzymatically controlled reactions, squalene is converted into an intermediate called lanosterol, which undergoes a number of subsequent reactions to become cholesterol.

 

Tetraterpenes

In addition to essential oils, terpenes and terpenoids are also found in naturally occurring dyes.  Molecules which contain large numbers of carbon-carbon double bonds adjacent to each other (a conjugated double bond system) absorb light at lower frequencies than molecules with no double bonds or just a few double bonds; some of these compounds absorb light in the visible region of the electromagnetic spectrum, and produce colors which we can see.  (Compare the structures of the molecules below with the ones in the section on the Chemistry of Vision.)

Lycopene

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Lycopene is a red pigment found in tomatoes, watermelon, guava, papaya, pink grapefruit, apricots, and rosehips.  Unripe tomatoes are green in color because of their chlorophyll, but as they ripen, the chlorophyll breaks down, unmasking the red color of the lycopene.  Lycopene is a good antioxidant, and is more readily absorbed from cooked tomatoes and tomato paste, especially if the foods contain fat.  This molecule, and the ones below, are structurally similar to that of carotene, and are referred to as carotenoids.

 
Zeaxanthin

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Zeaxanthin is a yellow pigment found in corn, egg yolk, orange juice, mangoes; also contributes to the yellowish color of animal fats. It is also found in the macula region of the retina (macula lutea, “yellow spot”), where it filters out some blue and UV light, acting like internal sunglasses; macular degeneration is the most common cause of blindness in the elderly.  Carotenoids containing hydroxyl groups are sometimes referred to as xanthophylls.

 
Astaxanthin

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Astaxanthin is a pink pigment found in salmon, trout, red seabream and the carapaces of lobster and shrimp. In live shellfish, the astaxanthin forms a complex with a protein which gives it a blackish color; when the shellfish are boiled, the protein breaks down, unmasking the pink astaxanthin.

 
Canthaxanthin

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Canthaxanthin is a pink pigment found in the feathers of American flamingos. It is obtained from shrimp in their diet; flamingos in captivity turn into plain white birds unless they are fed adequate amounts of shrimp.

 

 

 

The Chemistry of Vision

“I can see clearly now . . .”

Vitamin A (Retinol)

See entry above.

 
b-Carotene

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b- and a-Carotene are tetraterpenes.  b-Carotene absorbs blue and indigo light, giving it an orange color; it is found in carrots, yams, mangoes, and persimmons. The yellow color of butter and animal fats comes from carotene and related molecules. Carotene is found along with chlorophyll in photosynthetic organisms; it protects cells by reacting with O2 molecules. The yellow color of autumn leaves is due to the carotene, which is unmasked as the chlorophyll in the leaves breaks down.  b-Carotene is broken down in the body into two molecules of Vitamin A.

 
a-Carotene

a-Carotene is different from b-carotene in the position of the double bond on the far right.  It is broken down in the body into one molecule of Vitamin A.

 
g-Carotene

In g-carotene, the one of the cyclohexene rings in b-carotene is opened up, changing the location of one double bond.  It is broken down in the body into one molecule of Vitamin A.

 
Retinal and Vision

Retinal is the oxidized form of Vitamin A, having an aldehyde group at the end instead of an alcohol.  It combines with the protein opsin to form rhodopsin, the primary light-gathering pigment in vertebrate retinas.  In the rod and cone cells in the retina, the retinal in rhodopsin is found “at rest” in the cis form. When it absorbs a photon (hn) of light, one of the p-bonds is broken, causing the molecule to rotate and lock into the trans form, which has a completely different shape. This starts a long chain of chemical processes which eventually results in a visual image in the brain. The trans-retinal molecule is then twisted back into the cis form by another enzyme. When you look directly at a very bright light, the “afterimage” that you see in front of your eyes is the result of a large amount of cis-retinal having been transformed into trans-retinal all at once; the enzymes take a little bit of time to go through and “reset” all of these molecules back into the cis form.

 

 

 

References

P. W. Atkins, Molecules, 2nd ed.  Cambridge: Cambridge University Press, 2003, p. 73-74, 77-78, 149-155, 164-173.

Paula Yurkanis Bruice, Organic Chemistry, 4th ed.  Upper Saddle River:  Prentice Hall, 2004, p. 111-115.

Marye Anne Fox and James K. Whitesell, Organic Chemistry, 3rd ed.  Sudbury:  Jones and Bartlett Publishers, p. 49-51, 79.

Maitland Jones, Jr., Organic Chemistry.  New York:  W. W. Norton & Company, 1997, p. 115, 136-138.

Richard J. Lewis, Sr., Hawley's Condensed Chemical Dictionary, 13th ed.  New York:  Van Nostrand Reinhold, 1997.

G. Marc Loudon, Organic Chemistry, 4th ed.  New York:  Oxford University Press, 2002, p. 111.

Robert Thornton Morrison and Robert Neilson Boyd, Organic Chemistry, 6th ed.  Englewood Cliffs:  Prentice Hall, 1992, p. 273-285.

Royston M. Roberts, John C. Gilbert, and Stephen F. Martin.  Experimental Organic Chemistry:  A Miniscale Approach.  Fort Worth:  Saunders College Publishing, 1994, p. 122, 184.

D. W. A. Sharp, The Penguin Dictionary of Chemistry, 2nd ed.  London:  Penguin Books, 1990.

Graham Solomons and Craig Fryhle, Organic Chemistry, 7th ed.  New York:  John Wiley & Sons, Inc., 2000, p. 52, 287, 934-937, 1152-1157.

L. G. Wade, Jr., Organic Chemistry, 5th ed.  Upper Saddle River:  Prentice Hall, 2003, p. 272, 276-278.

Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.