1057-RG_5-09

1057-RG_5-09 - Chem 1057 J Walcott LEARNING STRATEGIES...

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Chem 1057 J. Walcott LEARNING STRATEGIES CENTER Fall 2009 Review Guide #5 Lectures: 9/21/09 – 9/25/09: Chapter 4 I. Alkene Stability Alkyl substiuents bonded to the sp 2 carbons of an alkene have a stabilizing effect on the alkene due to hyperconjugation. The more alkyl substituents bonded to the sp 2 carbons of an alkene, the greater is its stability. Furthermore, trans isomers tend to be more favorable than cis because the substituents are farther apart. When large substituents are on the same side of the double bond, they can interfere with one another causing steric strain. C C R R R R C C R R R H C C H R R H C C H R H R C C H R H H C C H H H H > > > > > tetra-substituted tri-substituted di-substituted di-substituted mono-substituted E Z II. Alkene Addition Reactions A. Addition of Hydrogen Halides (HX where X = Cl, Br or I) (Electrophilic Addition Reactions ) The π bond serves as a nucleophile which reacts with an electrophile (H + ) forming a carbocation intermediate. The carbocation intermediate is a Lewis acid (electrophile) that then reactions with the halogen anion serving as a nucleophile. 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
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1057-RG_5-09 - Chem 1057 J Walcott LEARNING STRATEGIES...

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