{"id":1080,"date":"2018-11-28T16:25:13","date_gmt":"2018-11-28T16:25:13","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/?post_type=chapter&p=1080"},"modified":"2019-01-08T14:57:19","modified_gmt":"2019-01-08T14:57:19","slug":"19-4-reduction-of-alkenes-and-alkynes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/chapter\/19-4-reduction-of-alkenes-and-alkynes\/","title":{"raw":"19.4. Reduction of alkenes and alkynes","rendered":"19.4. Reduction of alkenes and alkynes"},"content":{"raw":"
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Hydrogenation of alkenes with heterogeneous catalysts<\/span><\/h2>\r\n<\/div>\r\n<\/header>
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Further Reading<\/h3>\r\nMasterOrganic<\/em>\r\n\r\nPartial Reduction of Alkynes<\/a>\r\n\r\nCarey 5th Ed Online<\/em>\r\n\r\nAlkyne Hydrogenation<\/a>\r\n

Catalytic Hydrogenation of Alkenes<\/h2>\r\nThe double bond of an alkene<\/a> consists of a sigma (\u03c3) bond and a pi (\u03c0) bond. Because the carbon-carbon \u03c0 bond is relatively weak, it is quite reactive and can be easily broken and reagents can be added to carbon. Reagents are added through the formation of single bonds to carbon in an addition reaction.\r\n
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Introduction<\/h3>\r\nOne important alkene addition reaction is hydrogenation<\/strong>., where the alkene undergoes reduction to an alkane.\u00a0 In a hydrogenation reaction, two hydrogen atoms are added across the double bond of an alkene, resulting in a saturated alkane<\/a>. Hydrogenation of a double bond is a thermodynamically favorable reaction because it forms a more stable (lower energy) product.\u00a0In other words, the energy of the product is lower than the energy of the reactant; thus it is exothermic (heat is released).\u00a0The heat released is called the heat of hydrogenation, which is an indicator of a molecule\u2019s stability.\"Ethene.jpg\"\r\n\r\n\"Hydrogenation\r\n\r\nAlthough the hydrogenation of an alkene is a thermodynamically favorable reaction, it will not proceed without the addition of a catalyst<\/a>.\r\n\r\n\"\"\r\n\r\nCommon catalysts used are insoluble metals such as palladium in the form Pd-C, platinum in the form PtO2, and nickel in the form Ra-Ni. With the presence of a metal catalyst, the H-H bond in H2 cleaves, and each hydrogen attaches to the metal catalyst surface, forming metal-hydrogen bonds.\u00a0The metal catalyst also absorbs the alkene onto its surface.\u00a0A hydrogen atom is then transferred to the alkene, forming a new C-H bond.\u00a0 A second hydrogen atom is transferred forming another C-H bond.\u00a0At this point, two hydrogens have added to the carbons across the double bond.\r\n\r\n<\/div>\r\n
\r\n\r\n\"\"\r\n\r\nBecause this reaction takes place on a planar surface, addition of hydrogen occurs on the same<\/em> face of the double bond - a syn<\/em> addition, in other words.\u00a0 The catalytic hydrogenation of 1,2-dimethylcyclopentane will yield, for example, the cis<\/em> dimethylcycloalkane product, with little or no formation of a trans<\/em> product.\r\n\r\n\"image182.png\"\r\n

Competing reactions<\/h3>\r\nOther double bonds such as C=O can also be hydrogenated, but this reduction is usually slower than for hydrogenation of alkenes.\u00a0 This fact can be used to advantage in carrying out selective reactions. For instance, hydrogenation of a carbon-carbon double bond can be achieved without simultaneously reducing a carbonyl bond in the same molecule. For example the carbon-carbon double bond of the following aldehyde can be reduced selectively:\r\n\r\n\"\"\r\n

Common applications<\/h3>\r\nHydrogenation reactions are extensively used to create commercial goods.\u00a0 Hydrogenation is used in the food industry to make a large variety of manufactured goods, like spreads and shortenings, from liquid oils.\u00a0This process also increases the chemical stability of products and yields semi-solid products like margarine. Hydrogenation is also used in coal processing. Solid coal is converted to a liquid through the addition of hydrogen.\u00a0Liquefying coal makes it available to be used as fuel.\r\n\r\nTrans fats\r\n\r\nCatalytic hydrogenation of alkenes is currently a hot topic in food chemistry.\u00a0 Margarine is produced by partial<\/em> hydrogenation of double bonds in the unsaturated fatty acids in liquid vegetable oils, usually with a nickel catalyst.\u00a0 Complete hydrogenation would produce fully saturated fatty acids and lead to a lard-like product that is too hard to spread on toast, so conditions are adjusted to ensure that only some of the double bonds are hydrogenated while others are left in place, resulting in a soft and spreadable product.\u00a0 This process is called partial hydrogenation<\/strong>. (you may learn more about the chemical basis of the relationship between lipid saturation and melting point in Soderberg's book, section 2.4D<\/a>).\r\n\r\nRecently, however, scientists have become increasingly worried about the presence of unnatural fatty acids found in margarine and other food products made from partially hydrogenized oils. Natural unsaturated fatty acids have mainly cis<\/em> double bonds.\u00a0 In the unnatural fatty acids found in margarines, the naturally-occurring cis<\/em> stereochemistry has been converted to trans<\/em>.\u00a0\u00a0 Trans<\/em>-fatty acids have been associated with heart disease and some forms of cancer.\r\n\r\nIt appears that these unnatural trans<\/em> fatty acid isomers are unintentionally produced by the hydrogenation process. The problem is that there is no control over the regiochemistry or stereochemistry of the reverse (dehydrogenation) reaction.\u00a0 Because only a limited amount of hydrogen is used in order to achieve partial (rather then complete) hydrogenation, the process is reversible, meaning that double bonds tend to re-form - and when they do, it is often in the lower-energy trans<\/em> configuration, rather than the natural cis<\/em>\u00a0 configuration.\u00a0 The figure below shows the partial hydrogenation of a linoleic acid hydrocarbon over a nickel catalyst, resulting in oleic acid, which is the desired cis<\/em>-unsaturated product, as well as elaidic acid, the undesirable trans<\/em> fat product.\r\n\r\n\"image186.png\"\r\n\r\n\"image188.png\"\r\n\r\n\"image190.png\"\r\n\r\nFood producers are increasingly adopting alternative hydrogenation technologies and production strategies in order to offer products that are free of trans<\/em> fatty acids. (See J. Am. Diet. Assoc<\/em>. 2006<\/strong>, 106<\/em>, 867<\/a> for a detailed review of this topic).\r\n\r\n<\/div>\r\n
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References<\/h3>\r\n
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  1. \"Catalysts that improve coal hydrogenation.\"\u00a0Chemical Week\u00a0132\u00a0(1983):\u00a038.<\/li>\r\n \t
  2. List, G., M. Jackson, F. Eller, and R. Adlof. \u201cLow trans spread and shortening oils via hydrogenation of soybean oil.\u201d Journal of the Americal Oil Chemists 84 (2007): 609-612.<\/li>\r\n \t
  3. Singh, D., M. Rezac, and P. Pfromm. \u201cPartial Hydrogenation of Soybean Oil with Minimal Trans Fat Production Using a Pt-Decorated Polymeric Membrane Reactor.\u201d Journal of the American Oil Chemists Society 86 (2009): 93-101.<\/li>\r\n \t
  4. Vollhardt, K. Peter C., and Neil E. Schore. Organic Chemistry: Structure and Function. New York: W.H. Freeman and Company, 2007.<\/li>\r\n \t
  5. Zumdahl, Steven S. Chemistry. Lexington, Massachusetts: D.C. Heath and Company, 1993.<\/li>\r\n<\/ol>\r\n<\/div>\r\n
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    Problems<\/h3>\r\nComplete the following reactions.\u00a0 Provide stereochemistry if necessary.\r\n\r\n\"\"\r\n\r\n<\/div>\r\n
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    Answers<\/h3>\r\n[reveal-answer q=\"785468\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"785468\"]\r\n\r\n<\/div>\r\n
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    Contributors<\/h3>\r\n<\/div>\r\n
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