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07.pdf-Chemistry 5.12 Spri ng 2003, Week
07.pdf-Chemistry 5.12 Spri ng 2003, Week
07.pdf-Chemistry 5.12 Spri ng 2003,...
07.pdf-Chemistry 5.12 Spri ng 2003, Week
Page 1
1
Chemistry 5.12 Spri
ng 2003,
Week 3 / Day 2
Handout #7:
Lecture 11
Outline
IX.
Free Radical Reactions
(Read Chapter 4)
A.
Chlorination of Methane
(4-2)
1.
Mechanism
(4-3)
B.
Review of Thermodynamics
(4-4,5)
C.
Review of Kinetics
(4-8,9)
D.
Reaction-Energy Diagrams
(4-10)
1.
Thermodynamic Control
2.
Kinetic Control
3.
Hammond Postulate
(4-14)
4.
Multi-Step Reactions
(4-11)
5.
Chlorination of Methane
(4-7)
Suggested Problems:
4-35–37,40,43
IX.
A.
Radical Chlorination of Methane
C
H
H
H
H
Cl
Cl
heat (
∆
) or
light (
hv
)
C
H
H
Cl
H
H
Cl
C
H
H
Cl
H
Cl
Cl
∆
or
hv
C
Cl
H
Cl
H
H
Cl
Cl
Cl
∆
or
hv
C
Cl
Cl
Cl
H
H
Cl
etc.
•
Why is light or heat necessary?
•
How fast does it go?
•
Does it give off or consume heat?
•
How fast are each of the successive reactions?
•
Can you control the product ratio?
To answer these questions, we need to:
1.
Understand the mechanism of the reaction (arrow-pushing!).
2.
Use thermodynamics and kinetics to analyze the reaction.
Page 2
2
1.
Mechanism of Radical Chlorination of Methane
(Free-Radical Chain Reaction)
Free-radical chain reactions have three distinct mechanistic steps:
•
initiation step:
generates reactive intermediate
•
propagation steps:
reactive intermediates react with stable molecules to generate
other reactive intermediates (allows chain to continue)
•
termination step:
side-reactions that slow the reaction; usually combination of two
reactive intermediates into one stable molecule
Initiation Step:
Cl
2
absorbs energy and the bond is homolytically cleaved.
(Remember to use fishhook arrows!)
Cl
Cl
∆
or
hv
Cl
Cl
Cl
Cl
•
The first-formed reactive intermediate is a chlorine radical.
reactive intermediate:
short-lived species that reacts quickly as it is formed
radical:
species with an unpaired electron (usually just draw the unpaired electron)
chlorine radicals
Page 3
3
Propagation Steps:
The Meat of the Reaction
a)
Chlorine radical abstracts a hydrogen from methane to form a methyl radical.
b)
Methyl radical abstracts a chlorine from Cl
2
to form product and another chlorine radical
Cl
Cl
Cl
C
H
H
H
H
C
H
H
H
H
Cl
methyl
radical
C
H
H
H
methyl
radical
C
Cl
H
H
H
Cl
a)
b)
Sum:
C
H
H
H
H
C
Cl
H
H
H
H
Cl
Cl
Cl
keeps the
chain going
•
The sum of the propagation steps is the overall chlorination reaction.
Termination Steps:
Unproductive Steps
•
Consumption of reactive intermediates (radicals) without generation of new ones.
Cl
Cl
Cl
C
H
H
H
C
Cl
H
H
H
•
Termination steps slow down and eventually stop the chain reaction.
•
Termination reactions become most important at the end of a reaction, when
there are very few reactant molecules left.
Cl
Cl
C
H
H
H
C
H
H
H
C
C
H
H
H
H
H
H
Before we investigate the energetics of this particular reaction, let's review
some general thermodynamics and kinetics.
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