{"id":2362,"date":"2018-06-19T20:27:31","date_gmt":"2018-06-19T20:27:31","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/3-7-names-of-formulas-of-organic-compounds\/"},"modified":"2020-09-18T03:44:13","modified_gmt":"2020-09-18T03:44:13","slug":"3-2-nomenclature-of-unsaturated-hydrocarbons","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/3-2-nomenclature-of-unsaturated-hydrocarbons\/","title":{"raw":"3.2. Nomenclature of unsaturated hydrocarbons","rendered":"3.2. Nomenclature of unsaturated hydrocarbons"},"content":{"raw":"
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Alkenes<\/h4>\r\n

The simplest alkenes are ethene<\/em>\u00a0(traditionally called ethylene)<\/em>, C2<\/sub>H4<\/sub> or CH2<\/sub>=CH2<\/sub>, and propene<\/em> (or propylene)<\/em>, C3<\/sub>H6<\/sub> or CH3<\/sub>CH=CH2<\/sub> (part (a) in the figure below<\/span>). The names of alkenes that have more than three carbon atoms use the same stems as the names of the alkanes (see table above, \"The First 10 Straight-Chain Alkanes\"<\/span>) but end in -ene<\/em> instead of -ane<\/em>.<\/p>\r\n

As with alkanes, more than one structure is possible for alkenes with four or more carbon atoms. For example, an alkene with four carbon atoms has three possible structures. One is CH2<\/sub>=CHCH2<\/sub>CH3<\/sub> (but-1-ene, formerly called 1-butene), which has the double bond between the first and second carbon atoms in the chain. The other two structures have the double bond between the second and third carbon atoms and are forms of CH3<\/sub>CH=CHCH3<\/sub> ((but-2-ene, formerly called 2-butene). All four carbon atoms in but-2-ene lie in the same plane, so there are two possible structures (part (a) in Figure 3.7.2<\/span>). If the two methyl groups are on the same side of the double bond, the compound is cis<\/em>-but-2-ene (=cis<\/em>-2-butene) (from the Latin cis<\/em>, meaning \u201con the same side\u201d). If the two methyl groups are on opposite sides of the double bond, the compound is trans<\/em>-but-2-ene (=trans<\/em>-2-butene) (from the Latin trans<\/em>, meaning \u201cacross\u201d). (NB: We will learn more complete naming for such isomers in section 4.3.)<\/a>\u00a0 These are distinctly different molecules: cis<\/em>-but-2-ene melts at \u2212138.9\u00b0C, whereas trans<\/em>-but-2-ene melts at \u2212105.5\u00b0C.<\/p>\r\n\r\n

\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"580\"]\"\" Some Simple (a) Alkenes, (b) Alkynes, and (c) Cyclic Hydrocarbons. The positions of the carbon atoms in the chain are indicated by C1<\/sub> or C2.\u00a0 Some names here are traditional names rather than modern IUPAC names.
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Just as a number indicates the positions of branches in an alkane, the number in the name of an alkene specifies the position of the first<\/em> carbon atom of the double bond. The name is based on the lowest possible number starting from either end<\/em> of the carbon chain, so CH3<\/sub>CH2<\/sub>CH=CH2<\/sub> is called but-1-ene (formerly 1-butene), not<\/em> but-3-ene. Note that CH2<\/sub>=CHCH2<\/sub>CH3<\/sub> and CH3<\/sub>CH2<\/sub>CH=CH2<\/sub> are different ways of writing the same molecule<\/em> (but-1-ene) in two different orientations.<\/p>\r\n\r\n

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The name of a compound does not<\/em> depend on its orientation.<\/strong> As illustrated for 1-butene, both condensed structural formulas and molecular models show different orientations of the same molecule. It is important to be able\u00a0to recognize the same structure no matter what its orientation.<\/em><\/p>\r\n\r\n

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Note<\/h3>\r\n

The positions of groups or multiple bonds are always indicated by the lowest number possible.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

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Alkynes<\/h4>\r\n

The simplest alkyne is ethyne<\/em>, traditionally called acetylene<\/em>, C2<\/sub>H2<\/sub> or HC\u2261CH (part (b) in the figure above). Because a mixture of acetylene and oxygen burns with a flame that is hot enough (>3000\u00b0C) to cut metals such as hardened steel, acetylene is widely used in cutting and welding torches. The names of other alkynes are similar to those of the corresponding alkanes but end in -yne<\/em>. For example, HC\u2261CCH3<\/sub> is propyne<\/em>, and CH3<\/sub>C\u2261CCH3<\/sub> is but-2-yne<\/em> because the multiple bond begins on the second carbon atom.<\/p>\r\n\r\n

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Note<\/h3>\r\n

The number of bonds between carbon atoms in a hydrocarbon is indicated in the suffix:<\/p>\r\n\r\n