1057-RG_11-09 - Chem 1057 J. Walcott LEARNING STRATEGIES...

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Chem 1057 J. Walcott LEARNING STRATEGIES CENTER Fall 2009 Review Guide #11 Lectures: 11/9/09 – 11/20/09: Chapters 11 and 13 I. Free Radical Reactions A. Free-radical addition of HBr to an alkene. HBr and peroxides add to an alkene with anti-Markovnikov regiochemistry. The stability order of alkyl free radicals is the same as the stability order of carbocations, for the same reasons. Free radical reaction mechanisms involve movement of single electrons rather than pairs of electrons. “Fishhook arrows” are used to represent movement of single electrons in a free-radical reaction in a 3-step mechanism: initiation , propagation , or termination . This free radical addition reaction does not occur with HCl or HI. B. Chlorination and Bromination of Alkanes Alkanes react with chlorine or bromine in the presence of light or at high temperatures to form alkyl chlorides or alkyl bromides. The reaction is a chain reaction often leading to a mixture of products - it is not a useful reaction. C. Reactions Involving Allylic and Benzylic Radicals An allylic group is a group on a carbon adjacent to a double bond, whereas a benzylic group is a group on a carbon adjacent to a benzene ring or substituted benzene ring. Allylic and benzylic groups can be unusually reactive. 1. An allylic radical has an unpaired electron at an allylic position and a benzylic radical has an unpaired electron at the benzylic position. Because the resonance structures are especially stable, allylic radicals are more stable than tertiary radicals and are more readily formed than other alkyl radicals. 2. N-Bromosuccinimide (NBS) in CCl 4 is used to carry out allylic brominations. This reaction is selective because of the stability of the allylic free-radical intermediates that are involved. Note the roles of HBr and Br 2 in the reaction mechanism covered in class. 3.
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1057-RG_11-09 - Chem 1057 J. Walcott LEARNING STRATEGIES...

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