Example 1:
Copyright, Arizona State University
• this reaction "goes" from left to right because it gets 2 electrons into a bond, lowering their energy, the reaction is exothermic
Energy Diagram
Example 2:
• this reaction "goes" because although 1 bond is broken, 2 are formed
• the acid transfers a proton to the base, this is thus BOTH a Lewis and a Brønsted acid/base reaction
Energy Diagram
Organic Reactions 1 Copyright, Arizona State University

Example 3: not all acid/base reactions have to be exothermic:
• this reaction is fairly neutral as far as the energies of the electrons are concerned, however, it is driven by entropy (we will revisit entropy later...)
• this reaction is also a Brønsted acid/base reaction
Energy Diagram
Example 4:
H
3
C
C C
H
+
H O
H H
3
C
C C
H
H +
O
H only reasonable reaction
H
3
C H
LB and BB
H
LA and BA
H
3
C H
H
• the products of this reaction are higher in energy than the reactants (the reaction is endothermic)
• nevertheless, we will see later that reactions such as these are important
Energy Diagram
Organic Reactions 2 Copyright, Arizona State University

rate = k = A exp(-Ea/RT)
• thus, a smaller Ea results in a larger k , i.e. a faster rate
• a larger Ea results in a smaller k , i.e. a slower rate step 1 step 2
H H Br
C C H makes 1 bond
H H H C C H
Br
H C
H
C H
H Br
H
H
H
H
H
Br
‡
‡
H H
Br
H
H
H H
H
‡
H H
‡
Energy
Ea(1)
Ea(2) partial bonds
11 kcal/mol reaction coordinate
• the diagram shows the energy is of the ENTIRE SYSTEM (not just the alkene and fragments) as a function of progression along the "reaction coordinate"
• Each step (1 and 2) has its own energy barrier, Ea
• Ea(step 1) > Ea(step 2)
• step 1 is thus slower than step 2
• step 1 is the RATE DETERMINING STEP , i.e. the slowest step
• The value of the activation energy to make the first ionic intermediates is sufficiently large to result in a reaction that is not spontaneous, but is not too large that the reaction is precluded from happening
• So, this reaction "GOES" BOTH BECAUSE IT IS FAVORED THERMODYNAMICALLY AND BECAUSE IT IS
NOT TOO SLOW KINETICALLY
• There are actually many reacti8ons that are favored thermodynamically that are too slow to go because of thermodynamics, which isn’t always a bad thing!
Example, which of the following two reactions A or B is faster??
LA/BA
H Br step 1
Br
LB
H step 2
Br H
A slower
H H
H
C C
LB/BB
H
H C
H
C
H
H H C
H
C
H
H
LA/BA
H Br
Br
LB
B faster
H
3
C
H
3
C C
H
C H step 1
H
3
C C
H
3
C
H
C
H
H
LA step 2
H
3
C
H
3
Br
C
C
H
C
H
• the rate determining step in A involves formation of a higher energy, less stable primary cation
• the rate determining step in B involves formation of a lower energy, more stable tertiarty cation
H
Organic Reactions 3 Copyright, Arizona State University

• the rate determining step in B is faster than that in A, thus reaction B is overall faster than reaction A
The Hammond postulate: all other things being equal (i.e. for similar or related reactions..........
• More endothermic reactions have later, higher energy transition states (‡)
• More exothermic reactions have earlier, lower energy transition states (‡)
‡
Br
Relative
Energy
‡
Ea 1 (A)
‡
‡
Ea 2 (A)
Ea 2 (B)
Me H
Me H
Br
H
H H
H H
H
Ea 1 (B)
(Me,H)
H–Br
H
(Me,H) H
"earlier"
"later"
Br H
(Me,H)
(Me,H) H
H reaction coordinate
• The rate determining step, step 1, for reaction (A) is more endothermic than the same step for reaction (B)
• The more endothermic step has the LARGER Ea and the LATER transition state, Ea1(A) > Ea1(B)
• The more endothermic step also has a LATER transition state
• The first step in reaction A is SLOWER, because the intermediate cation is less stable
• The second step for reaction (A) is more exothermic than the second step for reaction (B)
• The more exothermic step has a SMALLER Ea, Ea2(A) < Ea2(B), and an earlier transition state
• The second step in reaction (A) is faster because the intermediate is less stable
• Reactions that FORM less stable intermediates are SLOWER, Reactions OF less stable intermediates are faster
Example Problem:
H Br
LA/BA
HBr
Br
LB/BB
2 ° cation
Free
Energy
H
3
C
H
Br
LB
H
3
C methyl shift
(generically, an alkyl shift)
‡
‡
‡
LA
3 ° cation
Ea(step 1) rate determining
 G rxn exothermic and exergonic reaction coordinate
Organic Reactions 4 Copyright, Arizona State University