Lecture31 - Good Nucleophiles Strong Bases I Br Cl CN SH SR...

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Lecture Summary 31 November 8, 2006 Chapter 11 - Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations Summary of Substitution and Elimination Reactions Sometimes it is difficult to tell what mechanism will be the predominant pathway in a reaction. Usually the most important factor is the substrate. For competing elimination reactions, the base strength (vs. nucleophile) is key. R- X S N 1 E1 S N 2 E2 favored with good nucleophile X favored with strong base can occur best if allylic can occur with good nucleophile can occur best if allylic favored with strong base favored competes with E1 favored if basic competes with S N 1 favored with strong base Stereochem racemic 100% inversion doesn't matter antiperiplanar required X X Nucleophiles vs Bases The competition between different reaction mechanisms depends first on the substrate as summarized above. However, the type of reagent (nucleophile or base) is critical in most cases. It is improtant to recognize the difference. Alkoxides (tightly held electrons) are good bases, but terrible nucleophiles.
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Unformatted text preview: Good Nucleophiles Strong Bases I Br Cl CN SH SR N 3 C CR OH OR NH 2 bulkier is less nucleophilic Reaction Examples Below are several examples of competition between the various reaction pathways. 2006 Gregory R . Cook page 1 Chem 341 North Dakota State University Br Na CN CN HMPA 1 subtrate Good nucleophile Aprotic polar solvent Br t-BuOK BuOH S N 2 E2 1 subtrate Good Base Protic polar solvent CH 3 CO 2 Na 2 subtrate Good nucleophile very weak base NaOEt S N 2 E2 2 subtrate Good Base some subst competes Br Br O O S N 2 O 80% 20% CH 3 CH 2 OH 3 subtrate Neutral Conditions Protic polar solvent NaOEt S N 1 3 subtrate Strong Base elim wins Br Br OH EtOH E1 20% 80% E2 97% OH HCl Cl racemic S N 1 OH Tos-Cl CN inversion S N 2 Et 3 N O Tos NaCN DMSO OH PBr 3 or SOCl 2 CN Et 3 N Cl (or Br) NaCN DMSO inversion S N 2 inversion S N 2 NOTE overall retention of configuration (2 inversions) 2006 Gregory R . Cook page 2 Chem 341 North Dakota State University...
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Lecture31 - Good Nucleophiles Strong Bases I Br Cl CN SH SR...

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