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1. Reaction Coordinates

2. Reaction Coordinate
E
n
e
r
g
y
Reaction Coordinate
E
n
e
r
g
y
reactant A
product B
transition state (TS
‡
)
-H
o
Exothermic reaction
Early TS
‡
The Hammond Postulate

3. E
n
e
r
g
y
Reaction CoordinateReaction Coordinate
reactant A
Endothermic reaction
Late TS
‡
+H
o
product B
The Hammond Postulate
transition state (TS
‡
)

4. Reaction Coordinate
E
n
e
r
g
y
reactant
product
transition state TS
‡
E
act
k
f
k
r
•k
obs = Ae
-E/RT
•k
f
>> k
r
; K
eq
= k
f
/k
r
= [product]/[reactant]
•ln k
obs
= -E/R(1/T) + ln A
• y = mx + b
H
o
rxn
•G
o
= H
o
-TS
o
;
•TS
o
<< H
o
•G
o
~ H
o
Activation Energy - E
a

5. Reaction Coordinate
E
n
e
r
g
y
first TS
‡
A
reactant
k
1
k
-1
k
2
k
-2
A B C
E
act
H
o
rxn
• rate = k
2
[B]
• K
eq
= [B]/[A] = k
1
/k
-1
• rate = [A]k
2
k
1
/k
-1
= k
2
K
eq
[A]
• k
obs
= k
2
K
eq
C
product
intermediate
B
second TS
‡
rate limiting step
A Preliminary Equilibrium, Exothermic Reaction

6. Reaction Coordinate
E
n
e
r
g
y
Areactant
C
product
intermediate
B
first TS
‡
second TS
‡
rate limiting step
k
1
k
-1
k
2
k
-2
E
act
H
o
rxn
rate = k
1
[A]
A B C
An Irreversible Reaction with an Intermediate

7. Reaction Coordinate
E
n
e
r
g
y
reactant A
product D
product C
product B
<
>k
red

E
act
H
o
red
>
E
act
k
blue
<
>
H
o
blue
>
E
act
k
green
H
o
green
Competitive Irreversible Reactions - I

8. Reaction Coordinate
E
n
e
r
g
y
reactant A
product D
product C
product B
E
act
k
green
H
o
green
H
o
red
k
red

E
act<
>
>
Competitive Irreversible Reactions - II
E
act
k
blue>
H
o
blue
<
<

9. Reaction Coordinate
E
n
e
r
g
y
E
act
(E
act
)
E
act
(H
o
)
H
o
C
H
o
D
D
B
A
A B
C
D’
The Curtin - Hammett Principle

10. Reaction Coordinate
E
n
e
r
g
y
Reaction Coordinate
E
n
e
r
g
y
E
act
reactant A
product B
transition state (TS
‡
)
-H
o
The Role of a Catalyst
E
act
+
Ethane
2 x (-98) = -196 kcal/molEthylene -bond
+62 kcal/mol
Hydrogen
+104 kcal/mol
H
o
= -30 kcal/mol

11. T
1
F
r
a
c
t
io
n

o
f

m
o
le
c
u
le
s

w
it
h

e
n
e
r
g
y

E

Energy
T
2
T
1
>
T
2
The Distribution of Reactive Molecules
T
4
>
T
4
T
3
>
T
3

12. The End
F. E. Ziegler 2009