Transcript CH 6-1 Power Point

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
Ea
(RLS)
Since the “Nuc-Lg” step is
concerted–one step
mechanism, there is one
Transition State.
Reactants
E
Products
Progress of Reaction
The SN2 Reaction: Transition State
•One step mechanism:
ts
E
R
“Structure” of the
transition state
P
Reaction Progress
“Pentavalent Carbon”
in the TS!
(very crowded)