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Reactions and Reaction Mechanisms
A reaction mechanism shows the actual
flow of electrons and movement of the
atoms during the reaction.
If one can understand a reaction mechanism
one can predict the course of other reactions.
Attack of a Lewis Base on a Lewis Acid
H
F
H N
B
H
F
H
F
H
H
F
N B
F
F
It is also useful to correlate changes
in energy with the movement of atoms.
To do this one creates an energy profile
of the reaction showing how the energy
changes as the atoms move.
Energy Profile of a Exothermic Reaction
H2O
+
H2O + HCl
H3O+
HCl
+
Cl-
H
+
H3O +
Potential
Energy
Cl
-
H +
O
H
Transition
State
H
Cl
Ea
H2O + HCl
H
H3O+ +
Cl-
H
H
C C +
CH3
H
H =
H
H2
 D (bonds broken) -
=
HH H
C C
H
CH3
H
 D (bonds formed)
DH-H + DC=C - 2 DC-H - DC-C
H = 432 + 614 - 2 x 413 - 347 = -127 kJ / mol
H
H
C C +
CH3
H
H2
HH H
C C
H
CH3
H
But if you mix propene and H2 nothing happens!
H H
CH3
H
C
C
H
H
Molecules repel
each other.
No good mechanism
for the reaction.
Try to find a new
mechanism
with a lower Ea
?
Ea too high thus
no reaction
How do you do
that? Use a
catalyst.
CH3CH=CH2 + H2
CH3CH2CH3
H = -127kJ/mol
Palladium metal can be used as a catalyst
for the hydrogenation of alkenes.
H2 will actually dissolve
in Pd metal. The H atoms
dissociate and go into
the octahedral holes.
The p bond of an
alkene will bond to the
surface atom of the Pd
metal.
This is all we need!
CH3
C C
H
H
Pd
H
Pd
Pd
Pd
Pd
Pd
H
H
CH3 H
CH3 C
H
H
C H
H
Pd
H
Pd
H3C
H
C H
H
Pd
H
H
H
C H
H
H
Pd
CH3 C
H
H3C
H
+
H2
CH3 C
H
C H
H
Pd
H
C H
H
Pd
H
H
CH3
H
H
H
C H
H
Pd
C H
H
Pd
H
CH3 C
H
+
H2
C H
H
Pd
H3C
H
H
H
C H
H
Palladium metal
is a catalyst for
the hydrogenation
of alkenes.
H2
Pd
H2
Pd
H2
Pd
H
H
C C
H
HH
Br
C C
HH
H
H
Addition of HBr to ethylene gives bromoethane
But what does HBr give when it reacts with
2-methylpropene?
Br
or
Br
But what does HBr give when it reacts with
2-methylpropene?
Br
or
Br
For this we need to know the mechansism.
The reaction involves a carbocation
intermediate.
Which carbocation
is formed?
CH3
H3C
C
Br
C
H
H H
H Br
CH3
H3C
C
C
H
H
or
H3C
Br
CH3
C
H
C
H
H
H
Alkyl groups stabilize
carbocations via
Hyperconjugation.
H C H
C
H
H
It involves “no bond”
resonance structures.
H
H
H
H C H
H C H
H C H
C
C
C
H
H
H
H
H
H
Empty p orbital
H
Filled sp3
hybrid
orbital
H C C
H
H
H
Electrons from the C-H bond help
stabilize the empty p orbital
Order of Carbocation Stability
Least
Stable
H
H
C
H
CH3
<
H
C
primary
H
<
CH3
C
CH3
tertiary
<
CH3
Most
Stable
CH3
H
C
CH3
secondary
Which carbocation
is formed?
CH3
H3C
C
Br
C
H
H H
Tertiary most stable
H Br
CH3
H3C
C
C
H
H
or
H3C
Br
CH3
C
H
C
H
H
How does a
carbocation react?
CH3
CH3
C
Br
Since the carbocation
is a Lewis acid
It can react with a
Lewis base.
CH3
The electron pair on
the base attacks the
electron deficient center
CH3
CH3 C
Br
CH3
General Reaction:
The addition of a hydrogen halide, HCl, HBr or
HI to an alkene gives an alkyl halide. The
regiochemistry is determined by which
carbocation is the most stable.
HBr
Br
CH3
H
more stable
H
H
I
CH3
HI
H
H
H
H
H
H
more stable
Cl
HCl
and
Cl
Both carboncation intermediates
would be secondary, little selectivity
What happens if there is no halide?
Hydration of alkenes to give alcohols.
H
H
C C
H
H
H2O
H2SO4
HO
H
H
H2SO4 is a catalyst
C C
H
H
H
What is the regiochemistry of the reaction?
H
CH3
C
H
C
H
H2O
H2SO4
HO
H
H
C
C
H
H
H
or
CH3
H
H
C
C
What is the mechanism of the reaction?
H
OH
CH3
H
CH3
The H2SO4 dissociates
to give hydronium ion.
H3C
C
O
C
H
H
H H
H
H O
CH3
H3C
C
C
H
H
H
or
H3C
CH3
C
H
C
H
H
H
O
H
H
O H
H
CH3
H3C
C
C
O H
H
H
H
H
H
O
H3C C C H
H3C
H H
O
H
H3C C C H
H3C
H H
+
+ H3O
What is the regiochemistry of the reaction?
H
CH3
C
H
C
H
H2O
H2SO4
HO
H
H
C
C
H
H
H
or
CH3
H
H
C
C
H
OH
CH3
The regiochemistry is determined by the
relative stability of the intermediate
carbocation.
Reactions of alkenes so far:
H2
Pd
X
HX
OH
H2O
H2SO4
One more reaction for the sake of completeness
Br
Br2
Br
Bromine or chlorine easily add to a double
bond to give dibromo or dichloro compounds.
Br
Br2
Br
H
C
C
H
Cl2
H
H
Cl C
C Cl
Reactions of alkenes so far:
H2
Pd
X
HX
OH
H2O
H2SO4
Br
Br2
Br
Enough of this simple stuff.
The acid catalyzed addition of water to
an alkene gives an alcohol.
OH
H2O
H2SO4
The regiochemistry is determined by the relative
stability of the intermediate carbocation.
But what if you want 1-propyl alcohol?
What could you do to “trick” the regiochemistry?
The observed regiochemistry is determined
by the first step of the reaction.
H
CH3
C
H
C
H
H+
CH3
C
H
HH
H
But we want an OH- group to add to the
primary carbon. How do we do that?
The Trick
Add some other Lewis Acid instead of H+
then convert it to an OH group.
The Trick
H
CH3
C
H
C
BH3
H
CH3
C
B H
H
H
H
H
B
H
2
tripropylborane
CH3
H H
C
H
B H
H
H
But what good is tripropylborane?
We are tryingto make 1-propanol?
It turns out that oxidation of alkyl boranes
will give boric acid plus alcohols.
H
B
H2O2
O
-
H
OH
O
B
O
H
+
3
O
H
Net Reaction
1. BH3
2. H2O2, -OH
This is called hydroboration
O
H
Thus we have a choice in regiochemistry.
1. BH3
2. H2O2, -OH
OH
H2O
H2SO4
OH
OH
H2O
H2SO4
H2O
H2SO4
1. BH3
2. H2O2, -OH
HO
Oxidation states in carbon chemistry.
Oxidation and reduction are important chemical
processes. In organic chemistry it is not always
obvious that you are carrying out an oxidation
or a reduction.
Consider average oxidation states of the
C atoms in each of the following molecules.
Remember that H is counted as +1 and O as –2.
H
CH3 CH3
-3
CH3
CH2
-2
O
H C
C C
H -2 H
C
-1
C H
-1
O
H
C
H
CH3
O
0
O
CH3
H
H
O
O H
C C
H O +3 O
Adding H2 to a double bond is a reduction.
The reverse reaction ( removing H2)
would be an oxidation.
H
H
H2
CH3 CH3
-3
C C
H -2
H
O
CH3
C
-1
H2
H
CH3
CH2
-2
O
H
Adding H2O is not an oxidation.
Removing H2O is not a reduction.
H
H
C C
H -2 H
H2O
H2SO4
CH3
CH2
-2
O
H
Adding H2O is not an oxidation.
Removing H2O is not a reduction.
H C C H
H2O
H2SO4
-1
H
O H
C
H
C
-1
H
Unstable rearranges
O
CH3
C
-1
H
To go from an alcohol to an aldehyde (or a ketone )
is an oxidation. Going from an aldehyde on to a
carboxylic acid is a further oxidation.
CH3
CH2
O
H
CH3
-2
Some
Oxidant
O
CH3
O
Some
Oxidant
C
0
O
H
C
-1
H
Primary alcohols can be oxidized to carboxylic
acids using CrO3. Chromium (VI) is a strong
oxidizing agent. It is used with an acid catalyst.
O
O
H
CrO3
H2SO4
O
CrO3
O
H
H
O
H2SO4
O
H
Alcohols can be oxidized to aldehydes or ketones
using a modified form of CrO3 called PCC for
pyridinium chlorochromate, (C5H6NCrO3Cl).
It is a milder reagent and if you use it carefully
you can stop the reaction at the intermediate
aldehyde step.
O
PCC
O
H
H
H
O
O
PCC
O
H
O
PCC
Going Backwards: Reduction of aldehydes or
ketones to an alcohol. To reduce a carbonyl
group, C=O, to an alcohol one could use H2 gas,
but a simpler way is to use a hydride as a source
of H- and a acid as a source of H+.
O
1. NaBH4
2. H3O+
The reagent of choice is NaBH4,
followed by some acid.
O
H
Na
H
B H
H
H

O

H
H
Na
H
O
H
H
B H
H
Na
O
O
H
H
O H
H
H
H
H
O
1. NaBH4
+
2. H3O
H
O
1. NaBH4
+
2. H3O
O
H
O
H
Starting with an alkene how would you make?
HBr
1. BH3
2. H2O2, OH
Br
O
H
H
H3O+
H2SO4
O
O
PCC
Can you explain an unexpected result?
Br
82%
HBr
+
?
18%
Br
HBr
82%
and
Br
H
?
H
H
H
CH3
H
18%
H
H
CH3
H