{"id":2325,"date":"2017-01-29T12:58:03","date_gmt":"2017-01-29T20:58:03","guid":{"rendered":"http:\/\/www.wou.edu\/chemistry\/?page_id=2325"},"modified":"2017-05-12T16:25:37","modified_gmt":"2017-05-12T23:25:37","slug":"ch105-chapter-8","status":"publish","type":"page","link":"https:\/\/wou.edu\/chemistry\/courses\/online-chemistry-textbooks\/ch105-consumer-chemistry\/ch105-chapter-8\/","title":{"rendered":"CH105: Chapter 8 – Alkenes, Alkynes and Aromatic Compounds"},"content":{"rendered":"

Chapter 8 – Alkenes, Alkynes and Aromatic Compounds<\/span><\/strong><\/h2>\n

This chapter is also available as a downloadable PDF file. Please click here to download:<\/span> CH105 Chapter 8 PDF file<\/a><\/strong><\/em><\/p>\n

This text is published under creative commons licensing, for referencing and adaptation, please click<\/span> here. <\/em><\/strong><\/a><\/span><\/p>\n

Opening Essay<\/span><\/strong><\/a><\/h3>\n

8.1 Alkene and Alkyne Overview<\/span><\/span><\/strong><\/a><\/h3>\n

8.2 Properties of Alkenes<\/span><\/strong><\/span><\/a><\/h3>\n

Looking Closer: Environmental Note<\/strong><\/span><\/a><\/h4>\n

8.3<\/span> Alkynes<\/strong><\/span><\/a><\/h2>\n

8.4<\/span> Aromatic Compounds: Benzene<\/span><\/strong><\/a><\/h2>\n

Polycyclic Aromatic Hydrocarbons<\/span><\/strong><\/a><\/h4>\n

8.5 Geometric Isomers<\/span><\/strong><\/a><\/h3>\n

Cis-Trans Nomenclature<\/strong><\/span><\/a><\/h4>\n

E-Z<\/em> Nomenclature<\/span><\/b><\/a><\/h4>\n

8.6 Reactions of Alkenes<\/strong><\/span><\/a><\/h2>\n

Addition Reactions<\/strong><\/span><\/a><\/h4>\n
Hydrogenation<\/span><\/em><\/strong><\/a><\/h5>\n
Halogenation<\/span><\/em><\/strong><\/a><\/h5>\n
Hydrohalogenation<\/span><\/em><\/strong><\/a><\/h5>\n
Hydration<\/span><\/em><\/strong><\/a><\/h5>\n
Markovnikov’s Rule<\/span><\/em><\/strong><\/a><\/h5>\n

Elimination Reactions<\/strong><\/span><\/a><\/h4>\n

Rearrangement Reactions<\/strong><\/span><\/a><\/h4>\n

Substitution Reactions<\/strong><\/span><\/a><\/h4>\n

8.7 Alkene Polymers<\/span><\/strong><\/a><\/h3>\n

The Production of Polyethylene<\/span><\/strong><\/a><\/h4>\n

8.8 Chapter Summary<\/strong><\/a><\/h3>\n

8.9 References<\/span><\/strong><\/a><\/h3>\n

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Opening Essay<\/strong><\/span><\/h3>\n

Our modern society is based to a large degree on the chemicals we discuss in this chapter. Most are made from petroleum. In Chapter\u00a07, we noted that alkanes\u2014saturated hydrocarbons<\/em><\/strong>\u2014have relatively few important chemical properties other than that they undergo combustion and react with halogens. Unsaturated hydrocarbons<\/em><\/strong>\u2014hydrocarbons with double or triple bonds\u2014on the other hand, are quite reactive. <\/span><\/p>\n

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In fact, they serve as building blocks for many familiar plastics\u2014polyethylene, vinyl plastics, acrylics\u2014and other important synthetic materials (e.g., alcohols, antifreeze, and detergents).<\/span><\/p>\n

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Figure 8.1 Common polymers made\u00a0using alkene building blocks.\u00a0<\/strong> Upper left<\/em>, a stainless steel and ultra high molecular weight polyethylene hip replacement.\u00a0The polyethylene repeating unit is shown in the lower left<\/em>. Upper middle<\/em>, shatterproof acrylic plexiglas used to build a large indoor aquarium. The methylacrylate repeating unit is shown in the lower middle<\/em>. Upper right<\/em>, common PCV piping used as material being used for sewage and drains. The polyvinylchloride repeating unit is shown in the lower left<\/em>.<\/span><\/p>\n

Hip replacement photo provided by:<\/span> The Science Museum London \/ Science and Society Picture Library<\/a>. Plexiglas aquarium photo provided by:<\/span> Leonard G.<\/a>\u00a0PVC pipe installation photo provided by:<\/span> Steve Tan<\/a>.<\/p>\n

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Aromatic hydrocarbons are defined by having 6-membered ring structures with alternating double bonds (Fig 8.2). <\/span><\/p>\n

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Figure 8.2: Aromatic Hydrocarbons.<\/strong><\/span> Aromatic hydrocarbons contain the 6-membered benzene ring structure (A) that is characterized by alternating double bonds. Ultradur, PBT is a plastic polymer that contains an aromatic functional group. 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.<\/span><\/p>\n

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Thus, they have formulas that can be drawn as cyclic alkenes, making them\u00a0 unsaturated.\u00a0 However,\u00a0due to the cyclic structure,\u00a0the\u00a0properties of aromatic rings are generally quite different,\u00a0and they\u00a0do not behave as typical alkenes. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene).<\/span><\/p>\n

The two simplest unsaturated compounds\u2014ethylene (ethene) and acetylene (ethyne)\u2014were 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 (CHBrClCF3<\/sub>).<\/span><\/p>\n

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(Back to the Top)<\/em><\/strong><\/span><\/a><\/h5>\n
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8.1 Alkene and Alkyne Overview<\/span><\/span><\/strong><\/h3>\n

By definition, alkenes<\/em><\/strong> are hydrocarbons with one or more carbon\u2013carbon double bonds (R2<\/sub>C=CR2<\/sub>), while alkynes<\/em><\/strong> are hydrocarbons with one or more carbon-carbon triple bonds (R\u2013C\u2261C\u2013R). Collectively, they are called unsaturated<\/em><\/strong> hydrocarbons<\/em><\/strong>, 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:<\/span><\/p>\n

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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 CH2<\/sub>CH2<\/sub>. 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 C2<\/sub>H4<\/sub>, whereas that for ethyne is C2<\/sub>H2<\/sub>. 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:<\/span><\/p>\n

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(Back to the Top)<\/em><\/strong><\/span><\/a><\/h5>\n
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8.2 Properties of Alkenes<\/span><\/strong><\/span><\/h3>\n
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The physical properties of alkenes are similar to those of the alkanes. Table 8.1\u00a0shows that 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 2 u (equivalent to two hydrogen atoms). Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents.<\/span><\/p>\n

Some representative alkenes\u2014their names, structures, and physical properties\u2014are given in Table 8.1.<\/span><\/p>\n

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Table 8.1<\/span> Physical Properties of Some Selected Alkenes<\/span><\/strong><\/p>\n<\/div>\n

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The first two alkenes in Table\u00a08.1 \u2014ethene and propene, are most often called by their common names\u2014ethylene and propylene, respectively. 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. <\/span><\/p>\n

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Figure 8.3. Ethene and Propene.<\/strong><\/span> The ball-and-spring models of ethene\/ethylene (a) and propene\/propylene (b) show their respective shapes, especially bond angles.<\/span><\/p>\n<\/div>\n

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Looking Closer: Environmental Note<\/strong><\/span><\/h4>\n

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\u2019t taste much like vine-ripened tomatoes fresh from the garden.<\/span><\/p>\n

Other alkenes that occur in nature include 1-octene, a constituent of lemon oil, and octadecene (C18<\/span><\/sub>H36<\/span><\/sub>) found in fish liver. Dienes (two double bonds) and polyenes (three or more double bonds) are also common. Butadiene (CH2<\/span><\/sub>=CHCH=CH2<\/span><\/sub>) is found in coffee. Lycopene and the carotenes are isomeric polyenes (C40<\/span><\/sub>H56<\/span><\/sub>) 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.<\/span><\/p>\n

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Figure 8.4 The bright red color of tomatoes is due to lycopene.\u00a0<\/strong> <\/span><\/p>\n

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Photo from : \u00a9 Thinkstock; Lycopene structure from:<\/span> Jeff Dahl<\/a><\/p>\n

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Concept Review Exercises<\/span><\/em><\/strong><\/h5>\n
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  1. \n
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    Briefly describe the physical properties of alkenes. How do these properties compare to those of the alkanes?<\/span><\/p>\n<\/div>\n<\/li>\n

  2. \n
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    Without consulting tables, arrange the following alkenes in order of increasing boiling point:<\/span><\/p>\n

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    Answers<\/span><\/em><\/strong><\/h5>\n
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    1. \n
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      Alkenes have physical properties (low boiling points, insoluble in water) quite similar to those of their corresponding alkanes.<\/span><\/p>\n<\/div>\n<\/li>\n

    2. \n
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      ethene < propene < 1-butene < 1-hexene<\/span><\/p>\n<\/div>\n<\/li>\n<\/ol>\n<\/div>\n

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      Key Takeaway<\/span><\/em><\/strong><\/h5>\n