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)