Reaction Coordinates
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Reaction Coordinates and Transition States: Hammond Postulate and Energy Changes, Study notes of Chemical Principles
An in-depth exploration of reaction coordinates, transition states, and energy changes in chemical reactions. Topics include the Hammond Postulate, activation energy, and the role of a catalyst. Reactions covered include exothermic and endothermic reactions, irreversible reactions with intermediates, and competitive irreversible reactions. The document also discusses the Curtin-Hammett Principle and the distribution of reactive molecules.
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2021/2022
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Reaction Coordinates
Reaction Coordinate
Energy
Reaction Coordinate
Energy
reactant A
product B
transition state (TS
‡ )
- ΔH
o
Exothermic reaction
Early TS
‡
The Hammond Postulate
Reaction Coordinate
Energy
reactant
product
transition state TS
‡
ΔE
act
k f
k r
- k obs
= Ae
- ΔE/RT
- k f
k r
; K
eq
= k f
/k r
= [product]/[reactant]
- ln k obs
= -ΔE/R(1/T) + ln A
- y = mx + b
- ΔH
o
rxn
• ΔG
o = ΔH
o -TΔS
o ;
• TΔS
o << ΔH
o
• ΔG
o ~ ΔH
o
Activation Energy - E
a
Reaction Coordinate
Energy
first TS
‡
A
reactant
k 1
k
k 2
k
A B C
ΔE
act
- ΔH
o
rxn
- rate = k 2
[B]
• K
eq
= [B]/[A] = k 1
/k
- rate = [A]k 2
k 1
/k
= 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
Reaction Coordinate
Energy
reactant A
product D
product C
product B
> k
red
ΔE
act
ΔH
o
red
ΔE
act
k
blue
ΔH
o
blue
ΔE
act
k
green
ΔH
o
green
Competitive Irreversible Reactions - I
Reaction Coordinate
Energy
reactant A
product D
product C
product B
ΔE
act
k
green
ΔH
o
green
ΔH
o
red
k
red
ΔE
< act
Competitive Irreversible Reactions - II
ΔE
act
k
blue
ΔH
o
blue
Reaction Coordinate
Energy
Reaction Coordinate
Energy
E
act
reactant A
product B
transition state (TS
‡ )
- ΔH
o
The Role of a Catalyst
E
act
Ethane
2 x (-98) = -196 kcal/mol Ethylene π-bond
+62 kcal/mol
Hydrogen
+104 kcal/mol
ΔH
o = -30 kcal/mol
T
1
Fraction of molecules with energy E
Energy
T
2
T
1
T
2
The Distribution of Reactive Molecules
T
4
T
4
T
3
T
3