1057_RG_10_08

1057_RG_10_08 - Chem 1057 J Walcott LEARNING STRATEGIES...

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Unformatted text preview: Chem 1057 J. Walcott LEARNING STRATEGIES CENTER Fall 2008 Review Guide #10 Lectures: 11/3/08 – 11/7/08: Chapters 9 and 10 I. Elimination and Substitution Reactions A. The S N 2 Reaction 1. The S N 2 reaction is a bimolecular, second order reaction. The reaction rate depends on the concentration of both the nucleophile and the substrate. 2. The nucleophile replaces the leaving group in a concerted, one-step mechanism (backside attack) that always occurs with inversion. 3. S N 2 reactions will occur with methyl, primary and secondary alkyl halides but not tertiary ones. B. The S N 1 Reaction 1. The S N 1 reaction is a unimolecular, first order reaction. The reaction rate depends only on the concentration of the haloalkane substrate. 2. The bond between a carbon and the leaving group breaks, and a carbocation intermediate is formed. In the second step, some nucleophile (frequently the solvent) reacts with the carbocation by forming a bond to the positively charged C atom. This reaction is nonstereoselective, since a carbocation is planar, and the new bond can form from either side of the plane. 3. The S N 1 pathway is favored for substrates with a high degree of branching at the α carbon, since more highly branched carbocations are more stable. Generally, S N 1 reactions occur with teriary and secondary but not with primary or methyl halides. C. E1 Reactions 1. Both S N 1 and E1 reactions have first order rate laws whose rate depends only on the concentration of the haloalkane substrate. Since a carbocation intermediate is formed in the first step, the S N 1/E1 pathway is favored for substrates with a high degree of branching, 2. A base (frequently the solvent) abstracts a β-hydrogen forming the new pi bond. This reaction is nonstereoselective because of the planar geometry of the carbocation intermediate. The regioselectivity is determined by Zaitzeff's rule which says that the most stable alkene...
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This note was uploaded on 12/17/2008 for the course CHEM 3570 taught by Professor Ganem, b during the Fall '06 term at Cornell.

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1057_RG_10_08 - Chem 1057 J Walcott LEARNING STRATEGIES...

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