Cannizaro Reaction Overall: 2RCHO conc. KOH heat RCO 2 - + RCH 2 OH Restriction: no hydrogens in the aldehydes. H 3 C O CHO H CHO hydrogens No hydrogens Why the restriction? The hydrogens are acidic leading to ionization.
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Cannizaro Reaction
Overall:2 RCHO
conc. KOH
heatRCO2
- + RCH2OH
Restriction: no hydrogens in the aldehydes.
H3C
O
CHO
H
CHO
hydrogens No hydrogens
Why the restriction? The hydrogens are acidic leading to ionization.
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 hydrogens).
Nucleophile, attacking carbonyl group.
R
O
H
HO-:
R
O
H
OH
R
OH
R
O
OH
+
R
OH
H
R
O
O
+R
OHH
H
Attack of nucleophilic HO-
Re-establish C=O and eject H- which is immediately received by second RCHO
Acid-base
Experimental Evidence
2 RCDO RD2OH + RCO2-
KOH, H2O
These are the hydrogens introduced by the reaction. They originate in the aldeyde and do not come from the aqueous hydroxide solution.
Kinetic vs Thermodynamic Contol of a Reaction
Examine Addition of HBr to 1,3 butadiene
HBrH
Br
+
Br
H
1,2 product 1,4 product
Mechanism of reaction.
H-Br
HH
Allylic resonance
1,2 product 1,4 product
Br Br
H
BrBr
H
But which is the dominant product?
HBrH
Br
+
Br
H
1,2 product 1,4 product
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?
Thermodynamic Control: Most stable product dominates
Kinetic Control: Product formed fastest dominates
When two or more products may be formed in a reaction A X or A B
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.
HBrH
Br
+
Br
H
1,2 product 1,4 product
Nature of the product mixture depends on the temperature.
Product mixture at -80 deg 80% 20%Product mixture at + 40 deg 20% 80%
Thermodynamic ControlMore stable product
Kinetic Control
Product formed most quickly, lowest Ea
Formation of the allylic carbocation.
Can react to yield 1,2 product or 1,4 product.
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.
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.
Diels Alder Reaction/Symmetry Controlled Reactions
Quick Review of formation of chemical bond.
HO- + H+ H - O - H
Electron donor
Electron acceptor
Note the overlap of the hybrid (donor) and the s orbital which allows bond formation.
HO- + H+ H - O H
For this arrangement there is no overlap. No donation of electrons; no bond formation.
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.
new bonds
A B
A
B
HOMOdonor
HOMOdonor
LUMOacceptor
LUMOacceptor
B HOMO donates into A LUMO
A HOMO donates into B LUMO Note the
overlap leading to bond formation
Note the overlap leading to bond formation
Try it in another reaction: ethylene + ethylene cyclobutane
new bonds
A B
A B
LUMO
HOMO
LUMO
HOMO
Equal bonding and antibonding interaction, no overlap, no bond formation, no reaction
Br
Br
excess sodium methoxide
Reaction Problem
HO
OEt
using only compoundshaving two carbons as the source of all carbons in the target molecule
Synthesis problem
Give the mechanism for the following reaction. Show all important resonance structures. Use curved arrow notation.
aq. acid
heatOEt
OHO
+ EtOH
Mechanism Problem
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