1. Transition State Theory - The Sequel
The potential-energy surface for a reaction R-X+ Y  R-Y + X
(• denotes start of reaction, T denotes transition state, and contours are constant potential-energy surfaces)
The activated complex, or transition state, decomposes to products through a vibration (translation) whose energy is plotted as the reaction coordinate. DG DG
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2. Forward and Reverse Reactions
The principle of microscopic reversibility suggests that a reaction and its reverse proceed by the same mechanism.
Forward and reverse reactions must have the same intermediates and rate-determining transition states.
Example: Alcohol Dehydration
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3. Thermodynamic Properties of an Activated Complex
For the reaction:
A + B X‡ C
we can write:
where, X‡ DG
A + B DG C
According to transition state theory, both the enthalpy and entropy of activation affect the rate of an elementary reaction
Enthalpy of Activation:
Entropy of Activation:
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4. Properties of an Activated Complex: Enthalpy
Enthalpy of Transition Enthalpy of formation for
activation State enthalpy Reactants A and B
Transition states of high enthalpy of activation represent slow reactions, an example of which is a bimolecular nucleophilic substitution reaction:
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5. Properties of an Activated Complex: Entropy
Entropy of Transition Entropy of formation for
activation State entropy Reactants A and B
Although we often focus on the “activation energy” or enthalpy of activation, entropy can dominate elementary reaction kinetics.
large-scale ordering to form a transition state (low entropy of activation) reduces the rate of reaction
Consider hydrogen atom abstraction from methane by a variety of radical species.
CH4 + R•  CH3• + R-H
Radical A (M-1s-1) EA (kcal mole-1)
F• *
Cl• *
CH3• *
CH3O• *
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6. Early and Late Transition States
Reactant
Product ‡
In endothermic reactions, the transition state resembles the product in terms of energy and structure.
This is called a “late” transition state or product-like t.s.
DG‡
DGo
Reactant ‡
Product
Exothermic reactions have a transition state more closely resembling the reactants in terms of both energy and structure.
This is called an “early” transition state or reactant-like t.s.
DG‡
DGo
CHEE 323
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7. Hammond Postulate
The use of transition state theory to describe chemical kinetics requires us to consider the structure of the transition state.
By definition, the transition state cannot be isolated. How can we make meaningful inferences regarding its structure?
While there is no universal relationship between the stability of a reaction product and its rate of formation, many reactions can be characterized by the Hammond Postulate.
The position of the transition state along the reaction coordinate, its energy, and its geometry are related, and depend on the relative stabilities of the reactant and the product.
McMurray states in his text:
The structure of a transition state resembles the structure of the nearest stable species.
CHEE 323
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8. Hammond Postulate: Examples
Classify these reaction
profiles in terms of:
A. Product stability
B. Transition State energy
C. Position of the transition
state (early/late)
What generalizations can be made regarding the position of the transition state and the rate of reaction?
CHEE 323
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9. Hammond Postulate: Inappropriate Use
Consider the polymerization of methylmethacrylate to produce a transparent, glassy polymer (tradename plexiglass)
the reaction proceeds with “head-to-tail” regioselectivity to give linear polymer chains
as is the case for most polymerizations, it is strongly exothermic.
What would the reaction profile look like for these reactions?
Can product stability arguments (Hammond Postulate) be used to explain the head-to-tail preference?
CHEE 323
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10. Case Study: Ethylene Hydrogenation Cycle
Consider the catalytic hydrogenation of ethylene by Wilkinson’s catalyst RhCl(PPh3)3. Assuming
this sequence is comprised of
elementary reactions, what
would the reaction profile
look like?
We will examine
the migratory insertion
reaction in greatest detail.
CHEE 323
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11. Complex 6
4-centred transition state
Complex 5
Dihydrido-olefin
Complex 8A
Hydrido-alkyl

12. Case Study: Potential Energy Diagram
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