{"id":1114,"date":"2018-03-21T14:33:31","date_gmt":"2018-03-21T14:33:31","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/chapter\/alkenes-structures-and-names\/"},"modified":"2018-10-30T13:01:38","modified_gmt":"2018-10-30T13:01:38","slug":"alkenes-structures-and-names","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-monroecc-orgbiochemistry\/chapter\/alkenes-structures-and-names\/","title":{"raw":"13.1 Alkenes: Structures and Names","rendered":"13.1 Alkenes: Structures and Names"},"content":{"raw":"
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Learning Objectives<\/h3>\r\n
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  1. Name alkenes given formulas and write formulas for alkenes given names.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nAs we noted in <\/span>Chapter 4 \"Covalent Bonding and Simple Molecular Compounds\"<\/a>, <\/span>alkenes<\/span><\/span> are hydrocarbons with one or more carbon-to-carbon double bonds (R<\/span>2<\/sub>C=CR<\/span>2<\/sub>) and <\/span>alkynes<\/span><\/span> are hydrocarbons with at least one carbon-to-carbon triple bonds (R\u2013C\u2261C\u2013R). Collectively, they are called <\/span>unsaturated hydrocarbons<\/span><\/span> because they have fewer hydrogen atoms than does an alkane with the same number of carbon atoms, as is indicated in the following general formulas:<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n
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    \"image\"<\/div>\r\n

    Some representative alkenes\u2014their names, structures, and physical properties\u2014are given in Table 13.1 \"Physical Properties of Some Selected Alkenes\"<\/a>.<\/p>\r\n\r\n

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    Table 13.1<\/span> Physical Properties of Some Selected Alkenes<\/th>\r\n<\/tr>\r\n
    IUPAC Name<\/th>\r\nMolecular Formula<\/th>\r\nCondensed Structural Formula<\/th>\r\nMelting Point (\u00b0C)<\/th>\r\nBoiling Point (\u00b0C)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    ethene<\/td>\r\nC2<\/sub>H4<\/sub><\/td>\r\nCH2<\/sub>=CH2<\/sub><\/td>\r\n\u2013169<\/td>\r\n\u2013104<\/td>\r\n<\/tr>\r\n
    propene<\/td>\r\nC3<\/sub>H6<\/sub><\/td>\r\nCH2<\/sub>=CHCH3<\/sub><\/td>\r\n\u2013185<\/td>\r\n\u201347<\/td>\r\n<\/tr>\r\n
    1-butene<\/td>\r\nC4<\/sub>H8<\/sub><\/td>\r\nCH2<\/sub>=CHCH2<\/sub>CH3<\/sub><\/td>\r\n\u2013185<\/td>\r\n\u20136<\/td>\r\n<\/tr>\r\n
    1-pentene<\/td>\r\nC5<\/sub>H10<\/sub><\/td>\r\nCH2<\/sub>=CH(CH2<\/sub>)2<\/sub>CH3<\/sub><\/td>\r\n\u2013138<\/td>\r\n30<\/td>\r\n<\/tr>\r\n
    1-hexene<\/td>\r\nC6<\/sub>H12<\/sub><\/td>\r\nCH2<\/sub>=CH(CH2<\/sub>)3<\/sub>CH3<\/sub><\/td>\r\n\u2013140<\/td>\r\n63<\/td>\r\n<\/tr>\r\n
    1-heptene<\/td>\r\nC7<\/sub>H14<\/sub><\/td>\r\nCH2<\/sub>=CH(CH2<\/sub>)4<\/sub>CH3<\/sub><\/td>\r\n\u2013119<\/td>\r\n94<\/td>\r\n<\/tr>\r\n
    1-octene<\/td>\r\nC8<\/sub>H16<\/sub><\/td>\r\nCH2<\/sub>=CH(CH2<\/sub>)5<\/sub>CH3<\/sub><\/td>\r\n\u2013102<\/td>\r\n121<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

    We used only condensed structural formulas in Table 13.1 \"Physical Properties of Some Selected Alkenes\"<\/a>. Thus, CH2<\/sub>=CH2<\/sub> stands for<\/p>\r\n\r\n

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    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. Note that the molecular formula for ethene is C2<\/sub>H4<\/sub>, whereas that for ethane is C2<\/sub>H6<\/sub>.<\/p>\r\n

    The first two alkenes in Table 13.1 \"Physical Properties of Some Selected Alkenes\"<\/a>\u2014ethene and propene (Figure 13.1 \"Ethene and Propene\"<\/a>)\u2014are 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. (For more information about polymers and plastics, see Section 13.5 \"Polymers\"<\/a>.) Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products. (For more information about alcohols, see Chapter 14 \"Organic Compounds of Oxygen\"<\/a>, Section 14.2 \"Alcohols: Nomenclature and Classification\"<\/a>.)<\/p>\r\n\r\n

    \r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1209\"]\"image\" Figure 13.1 Ethene and Propene.<\/em> The ball-and-spring models of ethene\/ethylene (a) and propene\/propylene (b) show their respective shapes, especially bond angles.[\/caption]\r\n\r\n<\/div>\r\n
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    Note<\/h3>\r\n

    Although there is only one alkene with the formula C2<\/sub>H4<\/sub> (ethene) and only one with the formula C3<\/sub>H6<\/sub> (propene), there are several alkenes with the formula C4<\/sub>H8<\/sub>. Section 13.2 \"Cis-Trans Isomers (Geometric Isomers)\"<\/a> begins a discussion of butenes.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

    Here are some basic rules for naming alkenes from the International Union of Pure and Applied Chemistry (IUPAC):<\/p>\r\n\r\n

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    1. The longest chain of carbon atoms containing the double bond<\/em> is considered the parent chain. It is named using the same stem as the alkane having the same number of carbon atoms but ends in -ene<\/em> to identify it as an alkene. Thus the compound CH2<\/sub>=CHCH3<\/sub> is propene<\/em>.<\/li>\r\n \t
    2. If there are four or more carbon atoms in a chain, we must indicate the position of the double bond. The carbons atoms are numbered so that the first of the two that are doubly bonded is given the lower of the two possible numbers. The compound CH3<\/sub>CH=CHCH2<\/sub>CH3<\/sub>, for example, has the double bond between the second and third carbon atoms. Its name is 2-pentene (not 3-pentene).<\/li>\r\n \t
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      Substituent groups are named as with alkanes, and their position is indicated by a number. Thus,<\/p>\r\n\r\n

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      is 5-methyl-2-hexene. Note that the numbering of the parent chain is always done in such a way as to give the double bond the lowest number, even if that causes a substituent to have a higher number. The double bond always has priority in numbering.<\/em><\/p>\r\n<\/li>\r\n<\/ol>\r\n

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      Example 1<\/h3>\r\n

      Name each compound.<\/p>\r\n\r\n

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        [reveal-answer q=\"489179\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"489179\"]<\/p>\r\n\r\n

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        1. The longest chain containing the double bond has five carbon atoms, so the compound is a pentene (rule 1). To give the first carbon atom of the double bond the lowest number (rule 2), we number from the left, so the compound is a 2-pentene. There is a methyl group on the fourth carbon atom (rule 3), so the compound\u2019s name is 4-methyl-2-pentene.<\/li>\r\n \t
        2. The longest chain containing the double bond has four carbon atoms, so the parent compound is a butene (rule 1). (The longest chain overall has five carbon atoms, but it does not contain the double bond, so the parent name is not pentene.) To give the first carbon atom of the double bond the lowest number (rule 2), we number from the left, so the compound is a 1-butene. There is an ethyl group on the second carbon atom (rule 3), so the compound\u2019s name is 2-ethyl-1-butene.[\/hidden-answer]<\/li>\r\n<\/ol>\r\n<\/div>\r\n
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          Skill-Building Exercise<\/h3>\r\n

          Name each compound.<\/p>\r\n\r\n

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            CH3<\/sub>CH2<\/sub>CH2<\/sub>CH2<\/sub>CH2<\/sub>CH=CHCH3<\/sub><\/p>\r\n\r\n<\/div><\/li>\r\n \t

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            \"image\"<\/div>\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nJust as there are cycloalkanes, there are cycloalkenes<\/em>. These compounds are named like alkenes, but with the prefix cyclo<\/em>- attached to the beginning of the parent alkene name.\r\n\r\n<\/div>\r\n
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            Example 2<\/h3>\r\n

            Draw the structure for each compound.<\/p>\r\n\r\n

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            1. 3-methyl-2-pentene<\/li>\r\n \t
            2. cyclohexene<\/li>\r\n<\/ol>\r\n

              [reveal-answer q=\"806794\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"806794\"]1. First write the parent chain of five carbon atoms: C\u2013C\u2013C\u2013C\u2013C. Then add the double bond between the second and third carbon atoms:<\/p>\r\n\r\n

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              Now place the methyl group on the third carbon atom and add enough hydrogen atoms to give each carbon atom a total of four bonds.<\/p>\r\n\r\n

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              2. First, consider what each of the three parts of the name means. Cyclo<\/em> means a ring compound, hex<\/em> means 6 carbon atoms, and -ene<\/em> means a double bond.<\/p>\r\n\r\n

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              [\/hidden-answer]<\/div>\r\n<\/div>\r\n
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              Skill-Building Exercise<\/h3>\r\nDraw the structure for each compound.\r\n
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                2-ethyl-1-hexene<\/p>\r\n\r\n<\/div><\/li>\r\n \t

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                cyclopentene<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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                Concept Review Exercises<\/h3>\r\n<\/div>\r\n
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                  Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

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                  Briefly identify the important distinctions between an alkene and an alkane.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

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                  Classify each compound as saturated or unsaturated. Identify each as an alkane, an alkene, or an alkyne.<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n

                  a.\u00a0\"image\"<\/div>\r\nb. CH3<\/sub>CH2<\/sub>C\u2261CCH3<\/sub>\r\n
                  c.\u00a0\"image\"<\/div>\r\n<\/div>\r\n
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                  [reveal-answer q=\"493576\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"493576\"]<\/p>\r\n

                  1. Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather unreactive.<\/p>\r\n

                  2. An alkene has a double bond; an alkane has single bonds only.<\/p>\r\n

                  3. a. saturated; alkane\u00a0 b. unsaturated; alkyne\u00a0 c. unsaturated; alkene[\/hidden-answer]<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

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                  Key Takeaway<\/h3>\r\n