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Chapter 6: Substitution & Elimination Reactions of Alkyl Halides

Chapter 6: Substitution & Elimination Reactions of Alkyl Halides. CH 6-1: Introduction SN2 Reaction Key Terms: substitution, elimination, nucleophile, leaving group, electrophilic carbon, SN1, SN2, E1, E2, energy diagrams, transition state, rate limiting step, carbocation, racemization.

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Chapter 6: Substitution & Elimination Reactions of Alkyl Halides

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  1. Chapter 6: Substitution & Elimination Reactions of Alkyl Halides CH 6-1: Introduction SN2 Reaction • Key Terms: substitution, elimination, nucleophile, leaving group, electrophilic carbon, SN1, SN2, E1, E2, energy diagrams, transition state, rate limiting step, carbocation, racemization. • The SN2 Substitution Reaction • Transition State Theory • The SN1 Substitution Reaction • The E2 Elimination Reaction • The E1 Elimination Reaction • Identifying Mechanisms and Predicting Products

  2. Substitution Reactions of Alkyl Halides weak base • Nuc = Nucleophile (strong or weak base) • Lg = leaving group (“Lg”) is part of the “substrate” -withdraws electrons in sigma bonds (“electronegative”) -polarizes the C-Lg bond -Nucleophile is attracted to the partial + charge on C -forms a weak base, Lg-) Note that the Nucleophile Substitutes for the Leaving Group

  3. Learning Objectives: Substitution Reactions GOAL: Mechanism of Substitution (SN2 or SN1) • Identify the nucleophile (weak/strong base; small/large?) • Identify the electrophilic carbon (d+) in the substrate (contains the Leaving Group) • Identify the Leaving Group - must form a weak base! • Evaluate substrate structure: is the Lg bonded to a 1o, 2o, 3o, vinyl, or phenyl carbon; are there large, bulky alkyl groups? • What type of solvent is being used? • Structure and stereochemistry of the products

  4. The SN2 Reaction • SN2: “Substitution….Nucleophilic….Bimolecular” • Rate = k [nucleophile] [substrate] (2nd order reaction) • “Concerted” Mechanism: • The nucleophile “grabs” the electrophilic carbon from the side 180o opposite the Lg.

  5. The SN2 Reaction: Energy Diagram (RLS) TS Since the “Nuc-Lg” step is concerted–one step mechanism, there is one Transition State. Ea (RLS) Reactants Products

  6. The SN2 Reaction: Transition State • One step mechanism: “Structure” of the transition state “Pentavalent Carbon” in the TS!(very crowded)

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