1057-RG_9-09 - Chem 1057 J. Walcott LEARNING STRATEGIES...

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Chem 1057 J. Walcott LEARNING STRATEGIES CENTER Fall 2009 Review Guide #9 Lectures: 10/26/09 – 10/30/09 : Chapters 8 and 9 I. Reactions of Alkyl Halides ( Please see Review Guide #8 for S N 2 and E2 Reactions) A. The S N 2 Reaction 1. The nucleophile replaces the leaving group in a concerted, one-step mechanism (backside attack) that always occurs with inversion. S N 2 reactions will occur with methyl, primary and secondary alkyl halides but not tertiary ones. 2. The S N 2 reaction is overall second order (rate depends on the concentration of both the nucleophile and the substrate). It is a bimolecular reaction (both the nucleophile and the substrate are involved in formation of the rate-determining transition state). 3. S N 2 reactions give complete stereochemical control and will produce one product from a single reactant. 4. The rate of the S N 2 reaction is affected by nucleophilicity and leaving group ability. a. Nucleophilicity correlates roughly with Bronsted basicity. b. The best leaving groups are the weakest bases. 5. The solvent effect is important. a. In aprotic solvents, a nucleophile does not become well solvated or stabilized and is therefore more reactive. b. A protic solvent stabilizes an anion through formation of hydrogen bonds. The stronger the stabilizing interaction between an anion and the solvent is, the less reactive (and less nucleophilic) the anion will be. c. For S N 2 reactions, a polar aprotic solvent is desirable (eg. Acetone, DMSO, DMF). B. The E2 Reaction 1. The E2 reaction is a single-step, concerted, bimolecular reaction. It is a second order reaction with a bimolecular rate-determining step. The reaction involves the removal of a β -proton by a base, with a concerted loss of a leaving group. 2.
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This note was uploaded on 11/27/2009 for the course CHEM 3570 at Cornell University (Engineering School).

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1057-RG_9-09 - Chem 1057 J. Walcott LEARNING STRATEGIES...

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