ENOL CONTENT

1
ENOL CONTENT
2
Keto-Enol Tautomerism
enol
keto
For most ketones, the keto form predominates in
the equilibrium
3
AS A GENERAL RULE ENOLS ARE
UNSTABLE
ol
O
H
ENOLS ( have -OH attached to a double
bond) Think of this combination as unstable.
C
C
ene
NOTE
Phenols are not enols and they are very stable
(benzene resonance).
4
TAUTOMERISM
TAUTOMERS species in equilibrium that
differ in the position of a proton or
other group.
ENOL
KETO
unstable enol
Most enols are not favored,
To interconvert tautomers, a proton is
transferred from oxygen to carbon.
E N E R G Y
enol
Mechanism, next slide..
keto
they rapidly change to keto.
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Tautomerism is Catalyzed by Traces of
Either Acid or Base
BASE CATALYSIS
..
..


enolate ion
..
-
keto
..
-

H-O
..
..

-
..




..
-

H-O

..
enol
resonance structures
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Tautomerism is Catalyzed by Traces of
Either Acid or Base
..
ACID CATALYSIS
H-O-H

..
..
H


..
keto

..


H-O-H


H
enol
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CONVERSION OF AN ENOL TO A KETONE
( catalyzed by small amounts of acid )
ENOL
KETONE
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Enol Percentages are Low in Most Ordinary Ketones
4.1 x 10-4
lt 2 x 10-4
2.5 x 10-3
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FACTORS THAT STABILIZE THE ENOL FORM
increasing the amount in the equilibrium
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MOSTLY RESONANCE
Resonance helps stabilize the enol.
1.
2.
Phenyl substituents R also help to stabilize the
enol by extending the resonance system.
Equivalent R groups results in symmetry which
also stabilizes the enol.
3.
(i.e., equivalent resonance contributors )
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Enol Percentages in 1,3-Diketones
are Higher
98
80
8
12
SOME ADDTIONAL PROPERTIES OF ENOLS
ENOLS ARE MORE ACIDIC THAN THEIR KETO FORM
ENOLS ARE NUCLEOPHILIC / REACT RAPIDLY WITH X2
ARE AN IMPORTANT INTERMEDIATE IN
ACID-CALTALYZED ALDOL CONDENSATIONS
MAKE COMPLEXES WITH FeCl3 (like phenols)
Discussed in class (if time).
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Compound Percent Enol (Neat Compound)
Compound Percent Enol (Neat Compound)
98
0.0025
89
0.035
80
lt0.004
73
lt0.002
8
Simple Ketones 18 Kcal (75kJ) or Keq 6.3 x 10-14