057_Ho_9_RG_6_07 - Chem 057 J. Walcott CENTER FOR LEARNING...

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Chem 057 CENTER FOR LEARNING AND TEACHING Handout #9 J. Walcott LEARNING STRATEGIES CENTER Fall 2007 Review Guide #6 Lectures: 9/24/07 – 928/07: Chapters 4 and 5 I. Three types of Alkene Addition Reactions (continued from Review Guide #5) A. Syn-Anti Addition The ± bond serves as a Lewis base which reacts with a Lewis acid forming a carbocation intermediate. The order of carbocation stability is 3 ° > 2 ° >> 1 ° >>>Me. Consequently, the addition of hydrogen halides to alkenes is regioselective ie. one in which more than one constitutional isomer is possible as product, but not all possible products are formed. Markovnikov ± s rule applies because H adds to the less substituted carbon placing the positive charge on the more substituted carbon so the more stable carbocation is formed. When the carbocation intermediate is formed, one of the sp 2 hybridized alkene carbons becomes sp 3 hybridized and can rotate freely. The carbocation is sp 2 hybridized and flat, so groups can add from either the top or the bottom resulting in either syn or anti addition. If no stereocenter is formed in the product, no stereoisomers are formed. If one stereocenter is formed, 2 1 or 2 stereoisomers (enantiomers) are possible. If 2 stereocenters are formed in the product, 2 2 or 4 stereoisomers are formed. However, when the two possible carbocation intermediates are both tertiary, neither is favored and structural isomers are formed.
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This note was uploaded on 09/12/2008 for the course CHEM 057 taught by Professor Walcott, j during the Fall '07 term at Cornell University (Engineering School).

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057_Ho_9_RG_6_07 - Chem 057 J. Walcott CENTER FOR LEARNING...

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