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In our previous article on organic chemistry we highlighted the importance of carbon and its ability to bond with almost any element. This article will expand on that by comparing the kind of bonds carbon have with other elements. With four valence electrons, the possibility of having more than a single bond with another atom exists.
But how will this affect the properties of the molecule formed? |
"An alkene, sometimes called an olefin, is a hydrocarbon that contains a carbon– carbon double bond. Alkenes occur abundantly in nature. Ethylene and propylene, the simplest alkenes, are the two most important organic chemicals produced industrially."
John McMurry, Organic Chemistry 8th ed. (2012)
Our modern society is based to a large degree on the chemicals we discuss in this article. Most are made from petroleum. In the previous article we implied that hydrocarbons—have relatively few important chemical properties other than that they undergo combustion and react with halogens. Unsaturated hydrocarbons—hydrocarbons with double or triple bonds—on the other hand, are quite reactive. In fact, they serve as building blocks for many familiar plastics—polyethylene, vinyl plastics, acrylics—and other important synthetic materials (e.g., alcohols, antifreeze, and detergents).
Aromatic hydrocarbons are defined by having 6-membered ring structures with alternating double bonds. 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.g., polyesters and polystyrene).
The two simplest unsaturated compounds—ethylene (ethene) and acetylene (ethyne)—were once used as anesthetics and were introduced to the medical field in 1924. However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in 1925. 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 1960s, when it was replaced by nonflammable anesthetics such as halothane.
Alkene and Alkyne Overview
By definition, alkenes are hydrocarbons with one or more carbon–carbon double bonds (R2C=CR2), while alkynes are hydrocarbons with one or more carbon-carbon triple bonds (R–C≡C–R). 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.
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 CH2CH2. 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 C2H4, whereas that for ethyne is C2H2.
Properties of Alkenes
The physical properties of alkenes are similar to those of the alkanes. The boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. 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 2u (equivalent to two hydrogen atoms). Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents.
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 0.1 mg of ethylene for 24 h can ripen 1 kg of tomatoes. Unfortunately, this process does not exactly duplicate the ripening process, and tomatoes picked green and treated this way don’t taste much like vine-ripened tomatoes fresh from the garden.
Other alkenes that occur in nature include 1-octene, a constituent of lemon oil, and octadecene (C18H36) found in fish liver. Dienes (two double bonds) and polyenes (three or more double bonds) are also common. Butadiene (CH2=CHCH=CH2) is found in coffee. Lycopene and the carotenes are isomeric polyenes (C40H56) 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.
Alkynes
The simplest alkyne—a hydrocarbon with carbon-to-carbon triple bond—has the molecular formula C2H2 and is known by its common name—acetylene (Fig 8.5). Its structure is H–C≡C–H.
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 International Union of Pure and Applied Chemistry (IUPAC) names for alkynes parallel those of alkenes, except that the family ending is –yne rather than –ene. The IUPAC name for acetylene is ethyne.
Benzene
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 simplest aromatic compound is benzene (C6H6) and it is of great commercial importance, but it also has noteworthy deleterious health effects.
The formula C6H6 seems to indicate that benzene has a high degree of unsaturation. (Hexane, the saturated hydrocarbon with six carbon atoms has the formula C6H14—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.
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.
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, B2 (riboflavin), and B9 (folic acid) all contain the benzene ring.
Alkene Polymers
The most important commercial reactions of alkenes are polymerizations, reactions in which small molecules, referred to in general as monomers, (from the Greek monos, meaning “one,” and meros, meaning “parts”), are assembled into giant molecules referred to as polymers (from the Greek poly, meaning “many,” and meros, meaning “parts”). A polymer is as different from its monomer as a long strand of spaghetti is from a tiny speck of flour. For example, polyethylene, the familiar waxy material used to make plastic bags, is made from the monomer ethylene—a gas.
Polyethene pellets that are produced in factories can be melted, formed into a giant bubble, and then made into a film that is used in packaging, consumer products, and food services.
There are two general types of polymerization reactions: addition polymerization and condensation polymerization. This section will focus on addition polymerization reactions. (For more information about condensation polymerization, see Chapter 10) In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. Ethylene molecules are joined together in long chains.
An interesting use of polymers is the replacement of diseased, worn out, or missing parts in the body. For example, about a 250,000 hip joints and 500,000 knees are replaced in US hospitals each year. The artificial ball-and-socket hip joints are made of a special steel (the ball) and plastic (the socket). People crippled by arthritis or injuries gain freedom of movement and relief from pain. Patients with heart and circulatory problems can be helped by replacing worn out heart valves with parts based on synthetic polymers. These are only a few of the many biomedical uses of polymers.
Aromatic hydrocarbons are defined by having 6-membered ring structures with alternating double bonds. 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.g., polyesters and polystyrene).
The two simplest unsaturated compounds—ethylene (ethene) and acetylene (ethyne)—were once used as anesthetics and were introduced to the medical field in 1924. However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in 1925. 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 1960s, when it was replaced by nonflammable anesthetics such as halothane.
Alkene and Alkyne Overview
By definition, alkenes are hydrocarbons with one or more carbon–carbon double bonds (R2C=CR2), while alkynes are hydrocarbons with one or more carbon-carbon triple bonds (R–C≡C–R). 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.
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 CH2CH2. 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 C2H4, whereas that for ethyne is C2H2.
Properties of Alkenes
The physical properties of alkenes are similar to those of the alkanes. The boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. 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 2u (equivalent to two hydrogen atoms). Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents.
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 0.1 mg of ethylene for 24 h can ripen 1 kg of tomatoes. Unfortunately, this process does not exactly duplicate the ripening process, and tomatoes picked green and treated this way don’t taste much like vine-ripened tomatoes fresh from the garden.
Other alkenes that occur in nature include 1-octene, a constituent of lemon oil, and octadecene (C18H36) found in fish liver. Dienes (two double bonds) and polyenes (three or more double bonds) are also common. Butadiene (CH2=CHCH=CH2) is found in coffee. Lycopene and the carotenes are isomeric polyenes (C40H56) 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.
Alkynes
The simplest alkyne—a hydrocarbon with carbon-to-carbon triple bond—has the molecular formula C2H2 and is known by its common name—acetylene (Fig 8.5). Its structure is H–C≡C–H.
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 International Union of Pure and Applied Chemistry (IUPAC) names for alkynes parallel those of alkenes, except that the family ending is –yne rather than –ene. The IUPAC name for acetylene is ethyne.
Benzene
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 simplest aromatic compound is benzene (C6H6) and it is of great commercial importance, but it also has noteworthy deleterious health effects.
The formula C6H6 seems to indicate that benzene has a high degree of unsaturation. (Hexane, the saturated hydrocarbon with six carbon atoms has the formula C6H14—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.
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.
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, B2 (riboflavin), and B9 (folic acid) all contain the benzene ring.
Alkene Polymers
The most important commercial reactions of alkenes are polymerizations, reactions in which small molecules, referred to in general as monomers, (from the Greek monos, meaning “one,” and meros, meaning “parts”), are assembled into giant molecules referred to as polymers (from the Greek poly, meaning “many,” and meros, meaning “parts”). A polymer is as different from its monomer as a long strand of spaghetti is from a tiny speck of flour. For example, polyethylene, the familiar waxy material used to make plastic bags, is made from the monomer ethylene—a gas.
Polyethene pellets that are produced in factories can be melted, formed into a giant bubble, and then made into a film that is used in packaging, consumer products, and food services.
There are two general types of polymerization reactions: addition polymerization and condensation polymerization. This section will focus on addition polymerization reactions. (For more information about condensation polymerization, see Chapter 10) In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. Ethylene molecules are joined together in long chains.
An interesting use of polymers is the replacement of diseased, worn out, or missing parts in the body. For example, about a 250,000 hip joints and 500,000 knees are replaced in US hospitals each year. The artificial ball-and-socket hip joints are made of a special steel (the ball) and plastic (the socket). People crippled by arthritis or injuries gain freedom of movement and relief from pain. Patients with heart and circulatory problems can be helped by replacing worn out heart valves with parts based on synthetic polymers. These are only a few of the many biomedical uses of polymers.
Ponder this
Why are hydrocarbons so chemically versatile?
Alkene and alkynes seem to change their properties depending on their degree of polymerisation. Do they have non-hydrocarbon equivalents?
Discuss
One of the key issues of our time is our over-dependence on hydrocarbons in terms of power generation and their applications in the chemical industry. Are there any viable, sustainable substitutes to hydrocarbon? Discuss on what makes hydrocarbons attractive for both functions, then find other chemical equivalents. Here's an example: sugars, starch, and cellulose are all made of the same building blocks, but have distinctive properties.
Further Reading
Carbon-carbon bonds, is a covalent bond between two carbon atoms.
Alkanes, at Understanding Chemistry
Alkenes, at Understanding Chemistry
Alkenes and alkynes at NYU, for a more detailed description of the chemical bonds involving them.
