{"id":1161,"date":"2018-03-21T14:36:07","date_gmt":"2018-03-21T14:36:07","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/chapter\/cis-trans-isomers-geometric-isomers\/"},"modified":"2018-10-22T21:05:09","modified_gmt":"2018-10-22T21:05:09","slug":"cis-trans-isomers-geometric-isomers","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-monroecc-orgbiochemistry\/chapter\/cis-trans-isomers-geometric-isomers\/","title":{"raw":"13.2 Cis-Trans Isomers (Geometric Isomers)","rendered":"13.2 Cis-Trans Isomers (Geometric Isomers)"},"content":{"raw":"
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Learning Objectives<\/h3>\r\n
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  1. Recognize that alkenes that can exist as cis-trans isomers.<\/li>\r\n \t
  2. Classify isomers as cis or trans.<\/li>\r\n \t
  3. Draw structures for cis-trans isomers given their names.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

    As noted in Chapter 12 \"Organic Chemistry: Alkanes and Halogenated Hydrocarbons\"<\/a>, there is free rotation about the carbon-to-carbon single bonds (C\u2013C) in alkanes. In contrast, the structure of alkenes requires that the carbon atoms of a double bond and the two atoms bonded to each carbon atom all lie in a single plane, and that each doubly bonded carbon atom lies in the center of a triangle. This part of the molecule\u2019s structure is rigid; rotation about doubly bonded carbon atoms is not<\/em> possible without rupturing the bond. Look at the two chlorinated hydrocarbons in Figure 13.2 \"Rotation about Bonds\"<\/a>.<\/p>\r\n\r\n

    \r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1028\"]\"image\" Figure 13.2 Rotation about Bonds. In 1,2-dichloroethane (a), free rotation about the C\u2013C bond allows the two structures to be interconverted by a twist of one end relative to the other. In 1,2-dichloroethene (b), restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond are significant.[\/caption]\r\n\r\n<\/div>\r\n

    In 1,2-dichloroethane (part (a) of Figure 13.2 \"Rotation about Bonds\"<\/a>), there is free rotation about the C\u2013C bond. The two models shown represent exactly the same molecule; they are not<\/em> isomers. You can draw structural formulas that look different, but if you bear in mind the possibility of this free rotation about single bonds, you should recognize that these two structures represent the same molecule:<\/p>\r\n\r\n

    \"image\"<\/div>\r\n

    In 1,2-dichloroethene (part (b) of Figure 13.2 \"Rotation about Bonds\"<\/a>), however, restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond become significant. This leads to a special kind of isomerism.\u00a0 An isomer in which two substituent groups are attached on the same side of a double bond or ring in an organic molecule is called a cis- isomer<\/strong>.<\/span><\/span> (Latin cis<\/em>, meaning \u201con this side\u201d) The molecule shown with two chlorine atoms on the same side of the double bond is named cis-1,2-dichloroethene. \u00a0An isomer in which two substituent groups are attached to opposite sides of a double bond or ring in a molecule is called a trans isomer.\u00a0 <\/strong><\/span><\/span>(Latin trans<\/em>, meaning \u201cacross\u201d) The isomer with the two Cl atoms on opposite sides of the molecule is named trans<\/em>-1,2-dichloroethene. These two compounds are cis-trans isomers (or geometric isomers).\u00a0 T<\/span>he presence of a rigid structure such as a double bond or ring.<\/span><\/span>, compounds is necessary for cis-\/trans isomerism.\u00a0 Note that cis and trans isomers are different compounds with distinct physical and chemical properties.<\/p>\r\n

    Consider the alkene with the condensed structural formula CH3<\/sub>CH=CHCH3<\/sub>. We could name it 2-butene, but there are actually two such compounds; the double bond results in cis-trans isomerism (Figure 13.3 \"Ball-and-Spring Models of (a) Cis-2-Butene and (b) Trans-2-Butene\"<\/a>).<\/p>\r\n\r\n

    \r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1333\"]\"image\" Figure 13.3 Ball-and-Spring Models of (a) Cis-2-Butene and (b) Trans-2-Butene.\u00a0Cis-trans isomers have different physical, chemical, and physiological properties.[\/caption]\r\n\r\n<\/div>\r\n

    Cis<\/em>-2-butene has both methyl groups on the same side of the molecule. Trans<\/em>-2-butene has the methyl groups on opposite sides of the molecule. Their structural formulas are as follows:<\/p>\r\n\r\n

    \"image\"<\/div>\r\n

    Note, however, that the presence of a double bond does not necessarily lead to cis-trans isomerism. We can draw two seemingly<\/em> different propenes:<\/p>\r\n\r\n

    \"image\"<\/div>\r\n

    However, these two structures are not really different from each other. If you could pick up either molecule from the page and flip it over top to bottom, you would see that the two formulas are identical.<\/p>\r\n

    Thus there are two requirements for cis-trans isomerism:<\/p>\r\n\r\n

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    1. Rotation must be restricted in the molecule.<\/li>\r\n \t
    2. There must be two nonidentical groups on each<\/em> doubly bonded carbon atom.<\/li>\r\n<\/ol>\r\n

      In these propene structures, the second requirement for cis-trans isomerism is not fulfilled. One of the doubly bonded carbon atoms does have two different groups attached, but the rules require that both<\/em> carbon atoms have two different groups.<\/p>\r\n

      In general, the following statements hold true in cis-trans isomerism:<\/p>\r\n\r\n