Energy Diagrams
A Review
ENERGY
• Energy Diagrams are a plot of the reaction
steps, or “Reaction Coordinate” (X-axis) versus
the Energy (Kcal or KJ)
REACTION COORDINATE
ENERGY
• In a spontaneous reaction, the product(s) are
more stable than the reactant(s), thus the
products are at a lower energy than the
reactants. For a single step reaction, the
energy diagram would look like:
REACTION COORDINATE
• “Spontaneous” refers to the total Gibbs Free
Energy (DG) during a reaction, taking into
account bond energies (Enthalpy, DH) and
disorder changes (Entropy, DS).
DG = DH - TDS
• When enthalpy is focused on, the term is
“exothermic” for a reaction/reaction step.
• In an exothermic reaction or reaction step, the
product bonds are more stable than the
reactant bonds , thus the products are at a
lower energy than the reactions.
• Energy Diagram for Exothermic:
SPONTANEOUS OR EXOTHERMIC
ENERGY
• Note the drop in Energy!
DG
REACTION COORDINATE
• The energy is determined by: E(products)E(reactants).
The
sign
for
spontaneous/exothermic reaction steps is
always negative, when energy is being given
off.
ENERGY
SPONTANEOUS OR EXOTHERMIC
DG
REACTION COORDINATE
• Exothermic Reaction – heat is given off –
Product bonds are more stable than reactant
bonds
SPONTANEOUS OR EXOTHERMIC
ENERGY
– DH value is negative,
– heat coming out of system
DG
REACTION COORDINATE
ENERGY
• In a non-spontaneous reaction, the products
are less stable than the reactants, thus the
products are at a higher energy than the
reactants. For a single step reaction, the
energy diagram would look like:
REACTION COORDINATE
• As before, “Non-Spontaneous” refers to the
total Gibbs Free Energy during a reaction,
taking into account bond energies (Enthalpy)
and disorder changes (Entropy).
• When enthalpy is focused on, the term is
“endothermic” for a reaction/reaction step.
• In an endothermic reaction or reaction step,
the product bonds are less stable than the
reactant bonds , thus the products are at a
higher energy than the reactions.
• Energy Diagram for Endothermic:
NON-SPONTANEOUS OR ENDOTHERMIC
ENERGY
• Note the climb in energy!
DG
REACTION COORDINATE
ENERGY
• Again, the energy is determined by:
E(products)-E(reactants). The sign for nonspontaneous/endothermic reaction steps is
always positive, when energy must be added
SPONTANEOUS OR EXOTHERMIC
in.
DG
REACTION COORDINATE
• Endothermic Reaction – heat is required –
Reactant bonds are more stable than product
bonds
– DH value is positive
– heat must be added to system
ENERGY
SPONTANEOUS OR EXOTHERMIC
DG
REACTION COORDINATE
On a side note…
• Very often, organic chemists estimate overall
DG as being approximately the same as DH.
• Values are a close approximation but not
exactly the same (missing DS factor)
• Bonds energies are calculated for gas phase
reactions (but we do everything in solutions)
and do not indicate rate of reaction (may
seem favorable mathematically but could take
two months!!)
Entropy (DS)
• Entropy – DS (Disorder)
• A B + C
– DS increasing as one becomes two or more pieces
• A + B
C
– DS decreasing as two become one in a reaction
• DS decreases when the world is less chaotic,
as in the reaction shown below, as two
molecules add together to become one
molecule:
H
H
H
H
+ HBr
CH3CH2Br
• Entropy is not viewable on an energy
diagram.
So… Energy Diagrams… What to
recognize…?
• Spontaneous steps are Exothermic and NonSpontaneous steps are Endothermic.
NON-SPONTANEOUS OR ENDOTHERMIC
ENERGY
ENERGY
SPONTANEOUS OR EXOTHERMIC
REACTION COORDINATE
REACTION COORDINATE
• Notice the high-energy points in a diagram:
ENERGY
HIGH ENERGY!
REACTION COORDINATE
Transition States
• This high-energy point in the diagram step is
what is called a “transition state” in the step
ENERGY
TRANSITION STATE OF STEP
REACTION COORDINATE
Transition States
• Transition States are high energy, unstable
species which cannot be isolated, therefore
they are only theoretical
• Being “theoretical”, their structures are not
physically proven, but thought to exist based
on evidence in reaction, what the starting
material looks like as well as the product.
Transition States
• Every mechanistic step in a reaction process
has a transition state
• On an energy diagram, every transition state is
recognized as every high point in the diagram.
• On the following energy diagram, how many
transition states are present? (or alternatively,
how many mechanism steps are in the
process? Same answer for both!)
Transition States
ENERGY
• How many transition states?
REACTION COORDINATE
ENERGY
• How many transition states?
• Every high point is a transition state. 4 total
• Every transition state = a step. 4 steps shown.
REACTION COORDINATE
• Notice the low points, between the high
points:
ENERGY
LOW POINT
REACTION COORDINATE
Intermediates
• The low points are energy values for
intermediates.
ENERGY
INTERMEDIATE
REACTION COORDINATE
Intermediates
• Intermediates are species like anions, cations
(cations on carbons are called carbocations) or
radicals.
• These are also higher in energy, in general, as
they are an unstable species (too many
electrons, not enough electrons or oddnumbered, unpaired electrons).
• Intermediates, unlike transition states, are
species that can be physically isolated.
Intermediates
• The definition of an intermediate is “a species
that forms during a reaction, that then
continues to react to form something else”.
• How many intermediates are shown on the
following energy diagram?
Intermediates
ENERGY
• How many intermediates are shown?
REACTION COORDINATE
Intermediates
ENERGY
• How many intermediates are shown on the
following energy diagram? There are 3 (notice
all of the “valleys”, between the “hills”).
REACTION COORDINATE
Label them all now
ENERGY
• Label the reactant (R), product (P), transitions
states (TS) and intermediates (I).
REACTION COORDINATE
Label them all now
• The transition states and intermediates are
numbered here for each step.
TS₁
TS₂
TS₃
ENERGY
I1
TS₄
R
I2
P
I3
REACTION COORDINATE
Label them all now
• Notice that Step 1 starts with the reactant, R,
and ends with I1. Step 2: I1  I2. Step 3: I2 
I3. Step 4: I3  P.
TS₁
TS₂
TS₃
ENERGY
I1
R
TS₄
Step 1
Step 2
I2
P
Step 3
REACTION COORDINATE
I3
Step 4
Let’s talk energy now…
• Now, the higher in energy a species is, the
more unstable it is.
• Transition states are the high points in energy
in the middle of each step.
• A certain amount of energy is required in
order for that transition state to form.
• This energy value is called the Activation
Energy or Activation Barrier, or DG‡.
Activation Barrier
• The activation barrier is the increase in energy
from the start of the step to the TS of the step
ENERGY
ENERGY NEEDED TO REACH TS
DG‡
REACTION COORDINATE
Activation Barriers
• Every step has an activation barrier
TS₁
ENERGY
DG ₁‡
TS₂
DG₂‡
TS₃
I1
DG₃‡
R
TS₄
I2
DG ₄‡
I3
REACTION COORDINATE
P
Activation Barriers
• The step with the largest Activation Barrier
requires the most energy and is the slowest
step in the process. Which step is that?
TS₁
ENERGY
DG ₁‡
TS₂
DG₂‡
TS₃
I1
DG₃‡
R
TS₄
I2
DG ₄‡
I3
REACTION COORDINATE
P
The Slowest Step…
• Starting at the beginning of the step and rising
up to each TS, you can see that Step 1 has the
largest rise, thus this is the slowest step.
TS₁
ENERGY
DG ₁‡
TS₂
DG₂‡
TS₃
I1
DG₃‡
R
TS₄
I2
DG ₄‡
I3
REACTION COORDINATE
P
The RDS…
• The slowest step is always referred to as the
RATE-DETERMINING STEP or RDS.
TS₁
ENERGY
DG ₁‡
TS₂
DG₂‡
TS₃
I1
DG₃‡
R
TS₄
I2
DG ₄‡
I3
REACTION COORDINATE
P
Other Energy Values to Find?
• The overall reaction, or each individual step,
has a change in energy as the reactant for the
start of the step is converted into the product.
• We alluded to this when we initially talked
about exothermic and endothermic steps
(spontaneous or non-spontaneous, if you like).
Other Energy Values to Find?
• These values are usually referred to as DG, the
Gibbs Free Energy of the step.
• You should be able to find DG for any single
step or the overall reaction, on an Energy
Diagram.
ENERGY
• Label all values of DG (for each step and the
overall reaction):
REACTION COORDINATE
• DG begins at each reactant for a step/reaction
and ends at each product for a step/reaction.
• Every step has an energy value and the overall
reaction has an energy value:
ENERGY
Step 2
DG1
DG2
DGoverall
Step 1
Step 3
DG3
Step 4
REACTION COORDINATE
DG4
• Which steps are exothermic/spontaneous?
• Which steps are endothermic/nonspontaneous?
• Is the overall reaction exo or endo?
ENERGY
Step 2
DG1
DG2
DGoverall
Step 1
Step 3
DG3
Step 4
REACTION COORDINATE
DG4
• If energy increases  endothermic
• If energy decreases  exothermic
EXOTHERMIC
ENERGY
Step 2
DG1
DG2
EXOTHERMIC
DGoverall
Step 1
ENDOTHERMIC
Step 3
EXOTHERMIC
DG3
Step 4
ENDOTHERMIC
REACTION COORDINATE
DG4
Application to reactions?
• Consider the addition of HBr to an alkene.
H
H-Br
H
Br
Br
Application to reactions?
• Consider the addition of HBr to an alkene.
H
H
H-Br
R
Br
Br
I
P
• This is a two-step process, R  I and I  P.
Step 1: Reactant forms Intermediate
• In Step 1, the alkene attacks the HBr to form a
carbocation intermediate:
H
H
Br
+
Br
• The molecules must approach each other.
Enough energy must be present to overcome
electron cloud repulsions between the neutral
alkene and the neutral HBr. Energy rises…
H
H
Br
+
Br
• Energy rises as the pi bond begins to break
and attack the HBr (think high energy - TS!),
and then collision occurs!
• If the orientation of the alkene with the HBr is
correct, (i.e. the alkene finds the H, not the
Br), the pi bond and H-Br break and a new
sp3C-H forms! Intermediate forms…
H
H
Br
+
Br
• The intermediate that forms will be higher in
energy than the starting materials, as it is a
charged species, not neutral.
• It needs to react to become stable again… so
here comes Step 2…
Step 2: Intermediate forms Product
• In Step 2, the bromide attacks the carbocation
intermediate:
H
H
Br
Br
• Again, the molecules must approach each other and
enough energy must be present to overcome electron
cloud repulsions but this step has less electron
repulsions as the TS forms. Lower Activation barrier!
Step 2: Intermediate forms Product
H
H
Br
Br
• This time one species is electron-poor (the
carbocation) and the other one is electronrich (the Br-) so this has a lower energy barrier
to overcome! sp3 C-Br Bond Formation!
Step 2: Intermediate forms Product
H
H
Br
Br
• Formation of a stable, neutral product will
lower the energy in the system again.
• What would the energy diagram look like?
Energy Diagram – Adding HBr to
Alkenes
• Here’s the basic energy diagram for this type of
reaction:
TS₁
TS₂
ENERGY
DG ₁
DG₂
I
R
P
REACTION COORDINATE
• Note the larger activation barrier for step 1.
Endothermic.
• Step 2, small activation barrier. Exothermic.
• This particular reaction is exothermic, overall.
Overview:
• For any Energy Diagram, you should be able to
find:
• DG for each individual step or for overall reaction
– Difference in Energy between the Product and
Reactant of each step, or overall reaction
– Spontaneous/Non-spontaneous or
Exothermic/Endothermic
• DG‡ for each step (fastest step, slowest step)
– Difference in Energy between reactant of a step and
the Transition State for the step
• Identify all transition states (TS) and
intermediates (I)
All the basic pieces:
TS1 DG1
TS2 DG2
ENERGY
DGºoverall
REACTANT
DG1
INTERMEDIATE
DG2
REACTION COORDINATE
PRODUCT