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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 7
7
If more than one product is formed in a reaction, what determines the
ratio of products?
It depends on the reaction!
transition
state C
‡
Thermodynamic Control:
When the reaction is reversible (reactants and products are
in equilibrium), the reaction is said to be under
thermodynamic control
, and the
product ratio is determined by the relative energies of the products.
transition
state B
‡
B
C
A
reaction coordinate
potential energy
A
C
B
[TS B]
‡
[TS C]
‡
G°
BC
[A]
[B]
[C]
G°
BC
(G
B
–G
C
)
+1.4
+4.2
+2.8
0
product ratio
([B] : [C])
1 : 10
1 : 1000
1 : 100
1 : 1
transition
state C
‡
Kinetic Control:
When the products are formed irreversibly, the reaction is said to be
under
kinetic control
, and the product ratio is determined by the difference in the
activation energies (the difference in the energies of the transition states (
∆∆
G
‡
)).
transition
state B
‡
B
C
A
reaction coordinate
potential energy
A
C
B
[TS B]
‡
[TS C]
‡
[A]
[B]
[C]
G
‡
(
G
‡
B
–
G
‡
C
)
+1.4
+4.2
+2.8
0
product ratio
([B] : [C])
1 : 10
1 : 1000
1 : 100
1 : 1
G
‡
Page 8
8
Is there any way to estimate the relative energies of transition states?
Hammond Postulate:
Related species that are similar in energy are similar in structure.
The structure of a transition state resembles the structure of the closest stable species.
•
Endothermic reactions have
late transition states.
•
In an
endothermic
reaction,
the transition state is
product-
like
(close in energy and
structure to the product).
•
In an endothermic reaction,
anything that changes the energy
of the product will also change
the energy of the transition state.
reaction coordinate
potential energy
reactants
products
‡
close in
energy and
structure
∆
G° > 0
•
Exothermic reactions have
early transition states.
•
In an
exothermic
reaction, the
transition state is
reactant-like
(close in energy and structure to
the reactant).
•
In an exothermic reaction,
anything that changes the energy
of the reactant will also change the
energy of the transition state.
reaction coordinate
potential energy
products
reactants
‡
∆
G° < 0
close in
energy and
structure
•
This will come in very handy when we start thinking about radical halogenation reactions!
Page 9
9
•
Multi-step reactions have more than one transition state.
reaction coordinate
potential energy
reactant
product
TS1
‡
TS2
‡
TS3
‡
int. 1
int. 2
rate-determining
transition state
•
The local maxima are
transition states.
•
The local minima are
reactive intermediates
(short-lived species that are
never present in high
concentration because they
react quickly as they are
formed).
•
The overall reaction rate
is determined by the
highest energy transition
state.
•
The
rate-determining
step
is the step with the
highest energy transition
state.
∆
G
‡
5.
Reaction-Energy Diagram for Radical Chorination of Methane
•
Now we have the tools to examine the energetics of a free-
radical halogenation.
•
We can use
bond dissociation energies
to estimate
∆
H°
for each step (p. 134 in Wade).
Cl
Cl
∆
or
hv
Cl
Cl
initiation step:
H° = BDE = 58 kcal/mol
bond dissociation energy:
amount of energy required to
homolytically
break a bond
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