Energy Diagrams and kinetics
Major concepts
 A reaction will only occur if the rate determining step is kinetically possible
 A catalyst speeds up a reaction by lowering the activation energy of key step(s) in the
mechanism
 Intermediates that are lower in energy will tend to form faster because the activation energy
needed to form that intermediate is lower.
 The major product formed is generally the one that forms faster (the one that goes through the
lower energy rate-determining step)

Vocabulary
 Transition state
 Reaction intermediate
 Rate determining step
 Kinetics
 Carbocation stability
Students should be able to:
 Draw an energy diagram for a multistep reaction mechanism, and indicate intermediates and
transition states
 Use an energy diagram to show how an acid catalyst makes the addition of water to an alkene
faster
 Use an energy diagram to show why the Markovnicov product is the major product in an
addition reaction
Daily Problems
1. What are the relative energies of the starting materials, products, and intermediates in this
mechanism? Draw an energy diagram for this mechanism, assuming the first step is rate limiting.
highest due to charge
lowest/ most sigma bonds
intermediate
E
starting
material
products
rxn coord
2. Draw an energy diagram for a 2-step mechanism with a high-energy intermediate that is
endothermic. (Assume its first step is the rate determining step.)
The intermediate is higher than the starting
materials or products. The highest transition
state is the first one, representing the rate
determining step. The products are higher
than the starting material, indicating the
exothermic reaction.
intermediate
E
products
starting
material
rxn coord
3. Draw an energy diagram for a spontaneous three-step reaction mechanism in which the
intermediates are high energy intermediates and the second transition state is the highest energy TS.
Which step is rate determining?
Spontaneous: starting
materials are higher in
energy than the
products.
The rate determining
step is from the first
intermediate to the
second intermediate
because it is the highest
transition state.
E
rxn coord
4. Draw energy diagrams for mechanisms A and B for the addition of water to propene. (Be sure to
consider the relative stabilities of all the compounds and draw them with correct relative energies.)
Which reaction works? What effect does the catalyst have on the reaction? Refer to your energy
diagrams to answer these questions.
Mechanism B works
while mechanism A does
not because the rate
determining step in A is
too high so the reaction
is very slow! The
addition of a catalyst in
B lowers the RDS
transition state/Ea and
speeds up the reaction
Mechanism A:
Mechanism B:
E
E
rxn coord
rxn coord
5. A. Draw an energy diagram for just the first step (the rate determining step) of the addition of HCl to
propene to form the primary carbocation. B. Draw a similar diagram for formation of the secondary
carbocation. C. Put parts A and B together on the same diagram, taking into consideration that the
starting material is identical in both cases. D. How does this energy diagram explain why the
Markovnicov product is formed rather than the primary alkyl chloride?
This diagram shows you that the formation of
the primary carbcation (red line) takes more
energy because it is less stable, while the
formation of the secondary carbocation (blue
line) takes less energy because it is more
stable.
E
rxn coord
Cumulative problems
6. Draw mechanisms for the formation of Product A and Product B. Explain why Product A is formed,
but not product B using an energy diagram.
Product A is formed over
product B because the
activation energy (height of
the RDS transition state) is
lower to form a resonance
stabilized carbocation (red)
over a nonstabilized
carbocation (blue).
E
rxn coord
7. When HBr is added to 1,1,1-trifluorobut-2-ene, two possible carbocation intermediates could form.
Draw both possible intermediates. Which one is more stable? Draw the major product of the reaction.
8. When treating a diene with HCl, only one potential intermediate cation is formed, and not the other.
Explain.
HCl
This carbocation is
resonance
stabilized
not formed
Once this intermediate is formed, it can lead to two products. Explain, and give a mechanism.
Extension problem
9. Based on the bonding in this reaction, in which direction would you predict the equilibrium of this
reaction to lie?
This is a non-ionic reaction, so you must evaluate the bonds made to the bonds broken.
Broken: C-OH σ and C-H σ
Made: C=C π and H-OH σ
Sigma bonds are more stable than pi bonds; the reaction would lie to the side with more sigma bonds.
The equilibrium lies to the left.
If, during the course of this reaction, water is constantly removed so that it can’t react to go back to
starting material, the equilibrium of the reaction would be force to the right.
This is based on La Chatlier’s principle (see your C117 notes or textbook). If there is no water to add
across the pi bond, there is no H-X reactant and no reverse reaction.