Review Of Organic Chemistry Page 20
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34Organic Chemistry I Review: Highlights of Key Reactions, Mechanisms, and Principles
20
Section 15.10 S
2 on Allylic, Benzylic Systems Are Really Fast
N
Ex.
NaOCH
3
Slow, and contaminated by competing E2
10 hours
Br H
H OCH
3
80% yield
NaOCH
Fast and Clean
3
15 min
Br H
H OCH
3
100% yield
Why? Because the backside-attack transition-state is stabilized by conjugation!
(Transition state-stability-reactivity principle).
Br
Br H
H
H OCH
H
CO
3
3
1. Neither the product nor the reactant has conjugation, so it’s hard to see why conjugation should
apply
2
2. However, in the 5-coordinate T-state the reactive carbon is sp
hybridized
the nucleophile and the electrophile are essentially on opposite ends of a temporary p-orbital.
2
3. That transient sp
hybridization in the transition-state is stabilized by π-overlap with the adjacent
p-bond.
4. This stabilization of the transition-state lowers the activation barrier and greatly accelerates
reaction
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