Reactions and Reaction Mechanisms

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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.
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Attack of a Lewis Base on a Lewis Acid
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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.
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Energy Profile of a Exothermic Reaction
Potential Energy
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Transition State
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But if you mix propene and H2 nothing happens!
Molecules repel each other.
No good mechanism for the reaction.
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? Ea too high thus no reaction
Try to find a new mechanism with a lower Ea
How do you do that? Use a catalyst.
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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!
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Palladium metal is a catalyst for the
hydrogenation of alkenes.
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Addition of HBr to ethylene gives bromoethane
But what does HBr give when it reacts
with 2-methylpropene?
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But what does HBr give when it reacts
with 2-methylpropene?
For this we need to know the mechansism.
The reaction involves a carbocation intermediate.
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Which carbocation is formed?
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Alkyl groups stabilize carbocations
via Hyperconjugation.
It involves no bond resonance structures.
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Empty p orbital
Filled sp3 hybrid orbital
Electrons from the C-H bond help stabilize the
empty p orbital
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Order of Carbocation Stability
Least Stable
primary
Most Stable
secondary
tertiary
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Which carbocation is formed?
Tertiary most stable
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How does a carbocation react?
Since the carbocation is a Lewis acid It can
react with a Lewis base.
The electron pair on the base attacks
the electron deficient center
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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.
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more stable
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more stable
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Both carboncation intermediates would be
secondary, little selectivity
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What happens if there is no halide?
Hydration of alkenes to give alcohols.
H2SO4 is a catalyst
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What is the regiochemistry of the reaction?
What is the mechanism of the reaction?
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The H2SO4 dissociates to give hydronium ion.
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What is the regiochemistry of the reaction?
The regiochemistry is determined by the relative
stability of the intermediate carbocation.
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Reactions of alkenes so far
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One more reaction for the sake of completeness
Bromine or chlorine easily add to a double bond
to give dibromo or dichloro compounds.
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Reactions of alkenes so far
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Enough of this simple stuff.
The acid catalyzed addition of water to an alkene
gives an alcohol.
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?
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The observed regiochemistry is determined by the
first step of the reaction.
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.
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The Trick
tripropylborane
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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.
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Net Reaction
This is called hydroboration
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Thus we have a choice in regiochemistry.
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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.
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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.
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Adding H2 to a double bond is a reduction. The
reverse reaction ( removing H2) would be an
oxidation.
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Adding H2O is not an oxidation. Removing H2O is
not a reduction.
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Adding H2O is not an oxidation. Removing H2O is
not a reduction.
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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.
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Primary alcohols can be oxidized to carboxylic
acids using CrO3. Chromium (VI) is a strong
oxidizing agent. It is used with an acid
catalyst.
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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.
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Going Backwards Reduction of aldehydes or
ketones to an alcohol. To reduce a carbonyl
group, CO, 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.
The reagent of choice is NaBH4, followed by some
acid.
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Starting with an alkene how would you make?
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Can you explain an unexpected result?
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