Cannizaro Reaction

1
Cannizaro Reaction
Overall
Restriction no a hydrogens in the aldehydes.
a hydrogens
No a hydrogens
Why the restriction? The a hydrogens are acidic
leading to ionization.
2
Mechanism
What can happen? Reactants are the aldehyde and
concentrated hydroxide. Hydroxide ion can act
both as Base, but remember we have no acidic
hydrogens (no a hydrogens). Nucleophile,
attacking carbonyl group.
Attack of nucleophilic HO-
Acid-base
Re-establish CO and eject H- which is
immediately received by second RCHO
3
Experimental Evidence
These are the hydrogens introduced by the
reaction. They originate in the aldeyde and do
not come from the aqueous hydroxide solution.
4
Kinetic vs Thermodynamic Contol of a Reaction
Examine Addition of HBr to 1,3 butadiene
5
Mechanism of reaction.
Allylic resonance
But which is the dominant product?
6
Nature of the product mixture depends on the
temperature.
Product mixture at -80 deg 80
20 Product mixture at 40
deg 20 80
Goal of discussion how can temperature control
the product mixture?
7
When two or more products may be formed in a
reaction A ? X or A ? B
Thermodynamic Control Most stable product
dominates
Kinetic Control Product formed fastest dominates
Thermodynamic control assumes the establishing of
equilibrium conditions and the most stable
product dominates.
Kinetic Control assumes that equilibrium is not
established. Once product is made it no longer
changes.
Equilibrium is more rapidly established at high
temperature. Thermodynamic control should
prevail at high temperature where equilibrium is
established. Kinetic Control may prevail at low
temperature where reverse reactions are very slow.
8
Nature of the product mixture depends on the
temperature.
Product mixture at -80 deg 80
20 Product mixture at 40
deg 20 80
More stable product
Thermodynamic Control
Kinetic Control
Product formed most quickly, lowest Ea
9
Formation of the allylic carbocation.
Can react to yield 1,2 product or 1,4 product.
10
Most of the carbocation reacts to give the 1,2
product because of the smaller Ea leading to the
1,2 product. This is true at all
temperatures. At low temperatures the reverse
reactions do not occur and the product mixture is
determined by the rates of forward reactions. No
equilibrium.
11
Most of the carbocation reacts to give the 1,2
product because of the smaller Ea leading to the
1,2 product. This is true at all
temperatures. At higher temperatures the reverse
reactions occur leading from the 1,2 or 1,4
product to the carbocation. Note that the 1,2
product is more easily converted back to the
carbocation than is the 1,4. Now the 1,4 product
is dominant.
12
Diels Alder Reaction/Symmetry Controlled Reactions
Quick Review of formation of chemical bond.
Electron donor
Electron acceptor
Note the overlap of the hybrid (donor) and the s
orbital which allows bond formation.
For this arrangement there is no overlap. No
donation of electrons no bond formation.
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Diels Alder Reaction of butadiene and ethylene to
yield cyclohexene.
We will analyze in terms of the pi electrons of
the two systems interacting. The pi electrons
from the highest occupied pi orbital of one
molecule will donate into an lowest energy pi
empty of the other. Works in both directions A
donates into B, B donates into A.
B HOMO donates into A LUMO
Note the overlap leading to bond formation
LUMO acceptor
LUMO acceptor
A HOMO donates into B LUMO
HOMO donor
HOMO donor
Note the overlap leading to bond formation
B
A
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Try it in another reaction ethylene ethylene
? cyclobutane
LUMO
LUMO
Equal bonding and antibonding interaction, no
overlap, no bond formation, no reaction
HOMO
HOMO
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Reaction Problem
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Synthesis problem
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Mechanism Problem
Give the mechanism for the following reaction.
Show all important resonance structures. Use
curved arrow notation.