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8-SubstElimF07 - Nucleophilic Substitution Reactions of...

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Nucleophilic Substitution Reactions of Alkyl Halides Introduction Alkyl halides undergo two very important types of reactions: nucleophilic substitution reactions and β - elimination reactions. These two reactions are in competition with each other and factors such as structure of the alkyl halide, structure of the nucleophile (base), the leaving group, and solvent conditions have a strong impact on which reaction dominates. The substitution and elimination reactions can go through a unimolecular transition state (S N 1 and E1) or a bimolecular transition state (S N 2 and E2). A competition between each of the four mechanisms exists and only with practice does anyone become good at predicting which will predominate. See Loudon for a review of mechanisms and transition state structures. The goal of today’s experiment is to observe a number of substitution reactions to determine trends in relative reactivities. The following factors will be varied: 1) Structure of the alkyl halide: primary, secondary or tertiary. 2) The size and basicity of the nucleophile. 3) Solvent polarity (as measured by the dielectric constant). SAFTEY ISSUES : A lkyl halides are toxic and should be handled very carefully. Limit your exposure by transferring alkyl halides in the hood. NOTE: there is no separation scheme required for this experiment. Instead, be sure to include each of the following reactions in your notebook, predict products, and make a hypothesis for each section regarding the relative reactivities. When creating the tables in your notebook be sure to leave enough space for writing observations. ALSO NOTE: Set up the 50 o C and 30 o C water bath right away so that they have time to equilibrate at a constant temperature before you attempt Part Two and Part Three of the experiment. Reading Assignment: Loudon, G.M. Organic Chemistry, 4 th ed.; Oxford University Press: New York, 2002, Chapter 9.
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Procedure : PART ONE: The Effect of Alkyl Halide Structure and Leaving Group on the Rate of Substitution. Basic Reaction: RX + AgNO 3 CH 3 CH 2 OH ROCH 2 CH 3 + AgX (s) + HNO 3 This is an example of a solvolysis, in which the solvent acts as the nucleophile. Factors to be held constant: Solvent = ethanol Polar or non-polar? Protic or aprotic? Donor or non-donor? Does this type of solvent promote the S N 1 or S N 2 mechanism? Why? Nucleophile = ethanol Relatively strong or weak nucleophile (as compared with NaI)? How does this affect the competition between S N 1 and S N 2? Do you expect the nitrate ion to compete as the nucleophile? Factors to be varied: Structure of the alkyl halide and leaving group. Create a table like the one below in your notebook .
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