Lesson26 - Stereoisomers and Addition Reactions Many...

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Unformatted text preview: Stereoisomers and Addition Reactions Many addition reactions have important stereochemical consequences and as we have previously seen, new details about a reaction mechanism will be unveiled when we examine the stereochemical relationship between reactants and products. For example, the hydrogenation to alkenes normally results in both hydrogen atoms adding to the same side, or face of the planar double bond. Same-face addition of atoms to a double bond is known as syn addition . An example of syn addition is illustrated for the hydrogenation of cis-2,3-dideuterio-2-pentene (deuterium, D is the nuclear isotope of hydrogen with an atomic mass of 2). Syn addition can take place from either the bottom or top. Because both top and bottom face addition are equally possible, an equimolar mixture of an enantiomer pair results. An equamolar mixture of enantiomers is known as a racemic mixture. Note that different products result from the trans alkene. Anti vs. Syn Stereoselectivity Although hydrogenation generally follows syn addition, special conditions exist that favor anti addition. In anti addition, one hydrogen atom adds to one face while the other adds to the other. The hydrogenation conditions shown in the reaction below exhibit anti stereoselectivity . You should draw the implied hydrogen atoms in the two products, showing their proper stereochemial orientation. The anti and syn products are diastereomers. The enantiomer of the anti product is not shown but is produced with equal preference to the anti isomer that is shown (i.e., of the 73% anti product, half of this amount is one enantiomer and half is the other). The syn product is a meso molecule. Can you find the plane of symmetry? CH 3 CH 3 CH 3 CH 3 anti CH 3 CH 3 syn 73% 27% + H 2 Pd / Al 2 O 3 CH 3 CO 2 H these specialized reaction conditions favor the anti addition product The specific set of conditions written above and below the reaction arrow describe to a chemist important details about how the reaction was performed. In this particular case, palladium metal was deposited on solid aluminum oxide and the reaction was conducted in a solvent of acetic acid (CH 3 CO 2 H) in the presence of gaseous hydrogen. Please note that it is NOT important for you to memorize these specialized conditions or even understand the mechanistic details of how these conditions produce anti products. What is important is that you have the ability to examine a Anti vs. Syn Stereoselectivity Although hydrogenation generally follows syn addition, special conditions exist that favor anti...
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Lesson26 - Stereoisomers and Addition Reactions Many...

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