Transcript Enolization

Enols
Enols
1
Keto-Enol Tautomerism?
O
OH
R
R'
R
R'
H
R
Keto form
R
Enol form
Tautomerism v. - an interconversion between two
structures that differ by the placement of an atom or
group; in this case, an
Enols
α hydrogen atom (H).
2
The -Carbon Atom and
its Hydrogens
Enols
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Terminology
H

H

O
O

R
Reference
Atom (ipso)
CH3CH2CH2CH
  
H

The reference atom is the carbonyl carbon.

Other carbons are designated , , , etc. on the basis
of their position with respect to the carbonyl carbon.

Hydrogens take the same Greek letter as the carbon to
which they are attached.
Enols
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Enolization
(tautomerism)
Enols
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Mechanism of Enolization
(In general)
H
••
O ••
R2C
H
•• O ••
H
O ••
••
CR'
H
H
••
•O
•
Enols
R2C
CR'
H
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Mechanism of Enolization
(Acid-catalyzed)
••
O ••
R2C
H
Enols
CR'
H
H
+ ••
O
H O ••
+
H
R2C
CR'
H
•• O •
•
H
H
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Mechanism of Enolization
(Acid-catalyzed)
+ ••
O
R2C
H
• O ••
•
H
Enols
H
CR'
••
•• O
H
H +
•• O
R2C
H
CR'
H
H
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Enol Content
OH
O
R2CHCR'
keto
R2C
CR'
enol
percent enol is usually very small
keto form usually 45-60 kJ/mol more stable
than enol
Enols
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Enol Content
Acetaldehyde
OH
O
CH3CH
H2C
CH
K = 3 x 10-7
Acetone
OH
O
CH3CCH3
Enols
H2C
CCH3
K = 6 x 10-9
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Stabilized Enols
Enols
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2,4-Cyclohexadienone
O
OH
H
H
H
H
H
H
H
H
H
H
H
 keto form is less stable than enol form
 keto form is not aromatic
Enols enol form is aromatic
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Enols
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SUMMARY: Keto-Enol Tautomerism
O
OH
R
R'
R
R'
H
R
Keto form
Enols
R
Enol form
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SUMMARY: Enol-Keto Tautomerism
O
OH
R
R
R'
R
Enol form
Enols
R'
H
R
Keto form
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O
H
O
O
H
H
HO
C2H5
C2H5
O
O
O
keto
keto
most stable
keto
keto
most stable
HO
H
HO
O
H
H
HO
C2H5
C2H5
OH
OH
OH
keto
no α-H
Enols
enol
enol
most stable
enol
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HO
H
HO
O
OH
H
H
HO
C2H5
C2H5
OH
OH
OH
enol
Enols
keto
enol
keto
enol
keto
enol
keto
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O
H
O
O
H
H
HO
C2H5
C2H5
O
O
O
Enols
keto
keto
keto
keto
enol
enol
keto
enol
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