9-NucSubsF09 - Nucleophilic Substitution Reactions of Alkyl...

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10/29/09 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). SAFETY 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, please complete this worksheet – note that some of the work required is pre-lab work ! Answer all questions indicated in this handout and return to your TA in lieu of a lab report. 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 , 5 th ed.; Roberts & Company Publishers: Greenwood Village, Colorado, 2009, Chapter 9.
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10/29/09 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.
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10/29/09 BEFORE YOU COME TO LAB : Complete the table and additional work as indicated below. Draw the
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This note was uploaded on 02/13/2010 for the course EGR 012 taught by Professor Witfield during the Spring '10 term at Aarhus Universitet.

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9-NucSubsF09 - Nucleophilic Substitution Reactions of Alkyl...

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