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.
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 Download 3D 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. Cyclohexene 3D Download 3D Cyclohexene rings are considerably less strained than the smaller cycloalkenes, and occur commonly in nature.
Cyclooctene 3D cis -cyclooctene Download 3D Cyclooctene has a large enough ring that it can exist in both the cis and trans stereoisomers. 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 Download 3D 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.
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. 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. 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 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-methylbutenyl 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.
Myrcene is found in bay leaves and oil of bay. Geraniol 3D Download 3D Geraniol is found in roses with 2-phenylethanol , citronella oil, pomerosa oil, geraniums, and other flowers.
Citral Geranial 3D Download 3D 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. 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 Download 3D Thujene is found in oil of thuja, sage, tansy, and wormwood. Thujone 3D Download 3D Thujone is found in oil of thuja, sage, tansy, and wormwood. Menthol 3D Download 3D Menthol is found in peppermint and other mint oils. It is used in cough drops, shaving lotion, and mentholated tobacco.
Menthone is the oxidized form of menthol, and has a similar taste and physiological effect. It is the active component of medicinal eucalyptus oils. Camphene 3D Download 3D Camphene is found in turpentine oil, rosemary, cypress oil, and oil of citronella.
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. Caryophyllene 3D Download 3D Caryophyllene is found in oil of cloves.
Vitamin A also known as retinol is a fat-soluble vitamin, which is produced by the breakdown of the carotenes especially b -carotene. 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. The cis-trans naming system can be used to distinguish simple isomers, where each carbon of the double bond has a set of identical groups attached to it.
For example, in Figure 8. The cis and trans system, identifies whether identical groups are on the same side cis of the double bond or if they are on the opposite side trans of the double bond.
For example, if the hydrogen atoms are on the opposite side of the double bond, the bond is said to be in the trans conformation. When the hydrogen groups are on the same side of the double bond, the bond is said to be in the cis conformation.
Notice that you could also say that if both of the chlorine groups are on the opposite side of the double bond, that the molecule is in the trans conformation or if they are on the same side of the double bond, that the molecule is in the cis conformation.
To determine whether a molecule is cis or trans , it is helpful to draw a dashed line down the center of the double bond and then circle the identical groups, as shown in figure 8. Both of the molecules shown in Figure 8. Thus, the cis and trans designation, only defines the stereochemistry around the double bond, it does not change the overall identity of the molecule.
However, cis and trans isomers often have different physical and chemical properties, due to the fixed nature of the bonds in space.
Cis-trans isomerism also occurs in cyclic compounds. In ring structures, groups are unable to rotate about any of the ring carbon—carbon bonds. Therefore, groups can be either on the same side of the ring cis or on opposite sides of the ring trans. For our purposes here, we represent all cycloalkanes as planar structures, and we indicate the positions of the groups, either above or below the plane of the ring.
It relates to our consumption of dietary fats. Inappropriate or excessive consumption of dietary fats has been linked to many health disorders, such as diabetes and atherosclerosis, and coronary heart disease. The sp 2 carbon is much more electron-withdrawing than the sp3 hybridize orbitals, therefore, creates a weak dipole along the substituent weak alkenly carbon bond. The two individual dipoles together form a net molecular dipole.
In trans-subsituted alkenes, the dipole cancel each other out. In cis-subsituted alkenes there is a net dipole, therefore contributing to higher boiling in cis-isomers than trans-isomers. Jim Clark Chemguide. Structural Isomerism All the alkenes with 4 or more carbon atoms in them show structural isomerism.
For example, with C4H8, it isn't too difficult to come up with these three structural isomers: There is, however, another isomer. Butene also exhibits geometric isomerism. Geometric cis-trans Isomerism The carbon-carbon double bond doesn't allow any rotation about it. Physical state Ethene, Propene, and Butene exists as colorless gases. Density Alkenes are lighter than water and are insoluble in water due to their non-polar characteristics.
Solubility Alkenes are virtually insoluble in water, but dissolve in organic solvents. Boiling Points The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms.
Melting Points Melting points of alkenes depends on the packaging of the molecules. If you're seeing this message, it means we're having trouble loading external resources on our website.
To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Donate Login Sign up Search for courses, skills, and videos.
Science Organic chemistry Alkenes and alkynes Alkene nomenclature. Alkene structure and classification. Next lesson. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript - [Instructor] Let's compare the structures of ethane and ethene.
For two carbons, six hydrogens is the maximum number that you can have, so we say that ethane is completely saturated with hydrogens. If we look at ethene, we only have four hydrogens for two carbons, so we say that ethene is unsaturated. So, for two carbons, it's possible to have more, so this is unsaturated.
Next, let's look at the carbons present in both molecules. We'll start with ethane. This carbon is sp3 hybridized, and so is this one, so ethane contains two sp3 hybridized carbons. And we know the geometry around sp3 hybridized carbon is tetrahedral. So we should have tetrahedral geometry around both of these carbons. For ethene, let me use a different color here, so this carbon and this carbon are both sp2 hybridized.
So sp2 hybridized carbons. And we know the geometry around an sp2 hybridized carbon is trigonal planar. So, there's planar geometry around both of these carbons. Finally, let's look at the bonding between the two carbons.
0コメント