The repeating monomer of Ultradur is shown in B. Ultradur can be found in showerheads, toothbrush bristles, plastic housing for fiber-optics cables, and in automobile exterior and interior components. Biologically important molecules, such as deoxyribonucleic acid, DNA C also contain an aromatic ring structures.
Thus, they have formulas that can be drawn as cyclic alkenes, making them unsaturated. However, due to the cyclic structure, the properties of aromatic rings are generally quite different, and they do not behave as typical alkenes.
Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics e. The two simplest unsaturated compounds—ethylene ethene and acetylene ethyne —were once used as anesthetics and were introduced to the medical field in However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in Ethylene was thought to be safer, but it too was implicated in numerous lethal fires and explosions during anesthesia.
Even so, it remained an important anesthetic into the s, when it was replaced by nonflammable anesthetics such as halothane CHBrClCF 3. Collectively, they are called unsaturated hydrocarbons , which are defined as hydrocarbons having one or more multiple double or triple bonds between carbon atoms. As a result of the double or triple bond nature, alkenes and alkynes have fewer hydrogen atoms than comparable alkanes with the same number of carbon atoms.
Mathematically, this can be indicated by the following general formulas:. In an alkene, the double bond is shared by the two carbon atoms and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious, ie the condensed formula for ethene is CH 2 CH 2.
The double or triple bond nature of a molecule is even more difficult to discern from the molecular formulas. Note that the molecular formula for ethene is C 2 H 4 , whereas that for ethyne is C 2 H 2. Thus, until you become more familiar the language of organic chemistry, it is often most useful to draw out line or partially-condensed structures, as shown below:. The physical properties of alkenes are similar to those of the alkanes.
Table 8. For molecules with the same number of carbon atoms and the same general shape, the boiling points usually differ only slightly, just as we would expect for substances whose molar mass differs by only 2 u equivalent to two hydrogen atoms.
Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents. Some representative alkenes—their names, structures, and physical properties—are given in Table 8. The first two alkenes in Table 8.
Ethylene is a major commercial chemical. The US chemical industry produces about 25 billion kilograms of ethylene annually, more than any other synthetic organic chemical. More than half of this ethylene goes into the manufacture of polyethylene, one of the most familiar plastics.
Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products.
Ethene and Propene. Alkenes occur widely in nature. Ripening fruits and vegetables give off ethylene, which triggers further ripening.
Fruit processors artificially introduce ethylene to hasten the ripening process; exposure to as little as 0. Other alkenes that occur in nature include 1-octene, a constituent of lemon oil, and octadecene C 18 H 36 found in fish liver. Dienes two double bonds and polyenes three or more double bonds are also common. Lycopene and the carotenes are isomeric polyenes C 40 H 56 that give the attractive red, orange, and yellow colors to watermelons, tomatoes, carrots, and other fruits and vegetables.
Vitamin A, essential to good vision, is derived from a carotene. The world would be a much less colorful place without alkenes. Briefly describe the physical properties of alkenes. How do these properties compare to those of the alkanes? Without consulting tables, arrange the following alkenes in order of increasing boiling point:.
Alkenes have physical properties low boiling points, insoluble in water quite similar to those of their corresponding alkanes. Without referring to a table or other reference, predict which member of each pair has the higher boiling point. Which is a good solvent for cyclohexene? Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon. Briefly identify the important distinctions between an alkene and an alkane.
Classify each compound as saturated or unsaturated. Identify each as an alkane, an alkene, or an alkyne. Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather unreactive. An alkene has a double bond; an alkane has single bonds only.
The simplest alkyne—a hydrocarbon with carbon-to-carbon triple bond—has the molecular formula C 2 H 2 and is known by its common name—acetylene Fig 8. Acetylene ethyne is the simplest member of the alkyne family. Acetylene is used in oxyacetylene torches for cutting and welding metals.
The flame from such a torch can be very hot. Most acetylene, however, is converted to chemical intermediates that are used to make vinyl and acrylic plastics, fibers, resins, and a variety of other products.
Alkynes are similar to alkenes in both physical and chemical properties. For example, alkynes undergo many of the typical addition reactions of alkenes. The names of other alkynes are illustrated in the following exercises. Briefly identify the important differences between an alkene and an alkyne. How are they similar? Do alkynes show cis-trans isomerism?
Alkenes have double bonds; alkynes have triple bonds. Both undergo addition reactions. No; a triply bonded carbon atom can form only one other bond. It would have to have two groups attached to show cis-trans isomerism. Draw the structure for each compound. Next we consider a class of hydrocarbons with molecular formulas like those of unsaturated hydrocarbons, but which, unlike the alkenes, do not readily undergo addition reactions. These compounds comprise a distinct class, called aromatic hydrocarbons.
Aromatic hydrocarbons are compounds that contain a benzene ring structure. The formula C 6 H 6 seems to indicate that benzene has a high degree of unsaturation. Hexane, the saturated hydrocarbon with six carbon atoms has the formula C 6 H 14 —eight more hydrogen atoms than benzene. However, despite the seeming low level of saturation, benzene is rather unreactive. This is due to the resonance structure formed from the alternating double bond structure of the aromatic ring.
It is the aromatic hydrocarbon produced in the largest volume. It was formerly used to decaffeinate coffee and was a significant component of many consumer products, such as paint strippers, rubber cements, and home dry-cleaning spot removers.
It was removed from many product formulations in the s, but others continued to use benzene in products until the s when it was associated with leukemia deaths.
Benzene is still important in industry as a precursor in the production of plastics such as Styrofoam and nylon , drugs, detergents, synthetic rubber, pesticides, and dyes. It is used as a solvent for such things as cleaning and maintaining printing equipment and for adhesives such as those used to attach soles to shoes.
Benzene is a natural constituent of petroleum products, but because it is a known carcinogen, its use as an additive in gasoline is now limited. Most of the benzene used commercially comes from petroleum.
It is employed as a starting material for the production of detergents, drugs, dyes, insecticides, and plastics. Once widely used as an organic solvent, benzene is now known to have both short- and long-term toxic effects.
The inhalation of large concentrations can cause nausea and even death due to respiratory or heart failure, while repeated exposure leads to a progressive disease in which the ability of the bone marrow to make new blood cells is eventually destroyed. This results in a condition called aplastic anemia , in which there is a decrease in the numbers of both the red and white blood cells.
How do the typical reactions of benzene differ from those of the alkenes? Briefly describe the bonding in benzene. Benzene is rather unreactive toward addition reactions compared to an alkene. Valence electrons are shared equally by all six carbon atoms that is, the electrons are delocalized.
The six electrons are shared equally by all six carbon atoms. Which compounds are aromatic? Five examples are shown below. In these structures, it is immaterial whether the single substituent is written at the top, side, or bottom of the ring: a hexagon is symmetrical, and therefore all positions are equivalent. These compounds are named in the usual way with the group that replaces a hydrogen atom named as a substituent group: Cl as chloro, Br as bromo, I as iodo, NO 2 as nitro, and CH 3 CH 2 as ethyl.
Although some compounds are referred to exclusively by IUPAC names, some are more frequently denoted by common names, as is indicated below. Some common aromatic hydrocarbons consist of fused benzene rings—rings that share a common side. These compounds are called polycyclic aromatic hydrocarbons PAHs An aromatic hydrocarbon consisting of fused benzene rings sharing a common side.
The three examples shown here are colorless, crystalline solids generally obtained from coal tar. Naphthalene has a pungent odor and is used in mothballs. Anthracene is used in the manufacture of certain dyes. Steroids, including cholesterol and the hormones, estrogen and testosterone, contain the phenanthrene structure. The intense heating required for distilling coal tar results in the formation of PAHs. For many years, it has been known that workers in coal-tar refineries are susceptible to a type of skin cancer known as tar cancer.
Investigations have shown that a number of PAHs are carcinogens. One of the most active carcinogenic compounds, benzopyrene, occurs in coal tar and has also been isolated from cigarette smoke, marijuana smoke, automobile exhaust gases, and charcoal-broiled steaks. It is estimated that more than 1, t of benzopyrene are emitted into the air over the United States each year.
Only a few milligrams of benzopyrene per kilogram of body weight are required to induce cancer in experimental animals. Benzo[a]pyrene is metabolized to produce biologically active compounds that can form physical adducts on DNA molecules. These adducts can cause genetic mutations that cause cancer.
Photo of cigarette smoke. Substances containing the benzene ring are common in both animals and plants, although they are more abundant in the latter. Plants can synthesize the benzene ring from carbon dioxide, water, and inorganic materials.
Animals cannot synthesize it, but they are dependent on certain aromatic compounds for survival and therefore must obtain them from food. Phenylalanine, tyrosine, and tryptophan essential amino acids and vitamins K, B 2 riboflavin , and B 9 folic acid all contain the benzene ring. Many important drugs, a few of which are shown in Table 8. So far we have studied only aromatic compounds with carbon-containing rings.
However, many cyclic compounds have an element other than carbon atoms in the ring. Organic ring structures that contain an atom other than carbon are called heterocyclic compounds.
Within alkane structure there is free rotation about the carbon-to-carbon single bonds C—C. In contrast, the structure of alkenes requires that the carbon atoms form a double bond. Double bonds between elements are created using p-orbital shells also called pi orbitals.
These orbital shells are shaped like dumbbells rather than the circular orbitals used in single bonds. This prevents the free rotation of the carbon atoms around the double bond, as it would cause the double bond to break during the rotation Figure 8.
Thus, a single bond is analogous to two boards nailed together with one nail. The boards are free to spin around the single nail. A double bond, on the other hand, is analogous to two boards nailed together with two nails. In the first case you can twist the boards, while in the second case you cannot twist them.
For molecules to create double bonds, electrons must share overlapping pi-orbitals between the two atoms. This requires the dumbbell-shaped pi-orbitals show on the left to remain in a fixed conformation during the double bond formation.
This allows for the formation of electron orbitals that can be shared by both atoms shown on the right. Rotation around the double bond would cause the pi orbitals to be misaligned, breaking the double bond.
Diagram provided from: JoJanderivative work — Vladsinger talk. The fixed and rigid nature of the double bond creates the possibility of an additional chiral center, and thus, the potential for stereoisomers. New stereoisomers form if each of the carbons involved in the double bond has two different atoms or groups attached to it.
For example, look at the two chlorinated hydrocarbons in Figure 8. In the upper figure, the halogenated alkane is shown. Rotation around this carbon-carbon bond is possible and does not result in different isomer conformations.
In the lower diagram, the halogenated alkene has restricted rotation around the double bond. Note also that each carbon involved in the double bond is also attached to two different atoms a hydrogen and a chlorine. Thus, this molecules can form two stereoisomers: one that has the two chlorine atoms on the same side of the double bond, and the other where the chlorines reside on opposite sides of the double bond.
For this section, we are not concerned with the naming that is also included in this video tutorial. Article Views Altmetric -. Abstract This overhead projector demonstration utilizes two classical oxidants, permanganates and dichromate, to distinguish between alkanes, alkenes, and primary, secondary, and tertiary alcohols.
Cited By. This article has not yet been cited by other publications. Pair your accounts. The addition of water to alkynes is a related reaction, except the initial enol intermediate converts to the ketone or aldehyde. Water can be added across triple bonds in alkynes to yield aldehydes and ketones for terminal and internal alkynes, respectively. Hydration of alkenes via oxymercuration produces alcohols. This reaction takes place during the treatment of alkenes with a strong acid as the catalyst.
Privacy Policy. Skip to main content. Organic Chemistry. Search for:. Alkenes and Alkynes Naming Alkenes and Alkynes Alkenes and alkynes are named similarly to alkanes, based on the longest chain that contains the double or triple bond. Learning Objectives Translate between the structure and the name of an alkene or alkyne compound. Key Takeaways Key Points Alkenes and alkynes are named by identifying the longest chain that contains the double or triple bond.
The chain is numbered to minimize the numbers assigned to the double or triple bond. Key Terms Alkenes : An unsaturated hydrocarbon containing at least one carbon—carbon double bond. Properties of Alkenes Due to the presence of a double bond in their carbon skeletons, alkenes are more reactive than their related alkanes. Learning Objectives Recognize the properties of alkenes relative to alkanes.
Key Takeaways Key Points Alkenes are generally more reactive than their related alkanes due to the relative instability of the pi bond. The melting and boiling points of alkenes are dictated by the regularity with which they can pack and the surface area of interaction. Rotation is restricted around the double bond in alkenes, resulting in diastereoisomers with different substitution patterns around the double bond.
Key Terms diastereoisomer : A stereoisomer having multiple chiral centers; one cannot normally be superimposed on the mirror image of another.
Reactivity of Alkenes Alkenes are more reactive than their related alkanes due to the relative instability of the double bond. Applications Large amounts of ethylene are produced from natural gas via thermal cracking. Reactions of Alkenes and Alkynes Alkenes and alkynes are more reactive than alkanes due to their pi bonds. Learning Objectives Give examples of the various reactions that alkenes and alkynes undergo.
Key Takeaways Key Points Addition reactions involving alkenes and alkynes include hydrogenation, halogenation, and hydrohalogenation. Alkenes and alkynes are useful reagents in polymer synthesis—an important industrial application.
Hydrogenation reactions typically employ a metallic catalyst consisting of platinum, nickel, palladium, or rhodium.
0コメント