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

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

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

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.

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

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