![]() For example, alkynes undergo many of the typical addition reactions of alkenes. 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.Īlkynes are similar to alkenes in both physical and chemical properties. The flame from such a torch can be very hot. Its structure is H–C≡C–H.Īcetylene is used in oxyacetylene torches for cutting and welding metals. 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). The world would be a much less colorful place without alkenes. Vitamin A, essential to good vision, is derived from a carotene. 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. ![]() Butadiene (CH2=CHCH=CH2) is found in coffee. Dienes (two double bonds) and polyenes (three or more double bonds) are also common. Other alkenes that occur in nature include 1-octene, a constituent of lemon oil, and octadecene (C18H36) found in fish liver. 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. 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. Ripening fruits and vegetables give off ethylene, which triggers further ripening. Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents.Īlkenes occur widely in nature. 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). The boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. The physical properties of alkenes are similar to those of the alkanes. Note that the molecular formula for ethene is C2H4, whereas that for ethyne is C2H2. The double or triple bond nature of a molecule is even more difficult to discern from the molecular 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 CH2CH2. 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. ![]() Collectively, they are called unsaturated hydrocarbons, which are defined as hydrocarbons having one or more multiple (double or triple) bonds between carbon atoms. Even so, it remained an important anesthetic into the 1960s, when it was replaced by nonflammable anesthetics such as halothane.īy 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). Ethylene was thought to be safer, but it too was implicated in numerous lethal fires and explosions during anesthesia. However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in 1925. The two simplest unsaturated compounds-ethylene (ethene) and acetylene (ethyne)-were once used as anesthetics and were introduced to the medical field in 1924. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene). However, due to the cyclic structure, the properties of aromatic rings are generally quite different, and they do not behave as typical alkenes. ![]() Thus, they have formulas that can be drawn as cyclic alkenes, making them unsaturated. 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).Īromatic hydrocarbons are defined by having 6-membered ring structures with alternating double bonds. Unsaturated hydrocarbons-hydrocarbons with double or triple bonds-on the other hand, are quite reactive. In the previous article we implied that hydrocarbons-have relatively few important chemical properties other than that they undergo combustion and react with halogens. ![]() Our modern society is based to a large degree on the chemicals we discuss in this article. ![]()
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