Terminology
O
CH3CH2CH2CH
g b a
The reference atom is the carbonyl carbon.
Other carbons are designated a, b, g, 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.
Mechanism of Enolization
(In general)
H
••
O ••
R2C
H
•• O ••
H
H
CR'
H
O ••
••
Mechanism of Enolization
(In general)
••
O ••
R2C
CR'
H
••
•• O
R2C
CR'
H
Mechanism of Enolization
(Acid-catalyzed)
H
••
O ••
R2C
H
CR'
H
O ••
+
H
Mechanism of Enolization
(Acid-catalyzed)
H
+ ••
O
R2C
H
CR'
H
•• O •
•
H
Mechanism of Enolization
(Acid-catalyzed)
+ ••
O
R2C
H
CR'
H
Mechanism of Enolization
(Acid-catalyzed)
+ ••
O
R2C
H
•• O ••
H
H
CR'
H
Mechanism of Enolization
(Acid-catalyzed)
••
•• O
H +
•• O
H
R2C
H
CR'
H
Enol Content
OH
O
R2CHCR'
R2C
keto
CR'
enol
percent enol is usually very small
keto form usually 45-60 kJ/mol more stable
than enol
Enol Content
OH
O
CH3CH
H2C
K = 3 x 10-7
OH
O
CH3CCH3
CH
H2C
CCH3
K = 6 x 10-9
18.5
Stabilized Enols
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
enol form is aromatic
1,3-Diketones
(also called b-diketones)
Example: 2,4-pentanedione
O
O
CH3CCH2CCH3
(20%)
O
OH
CH3C
CHCCH3
(80%)
keto form is less stable than enol form
Enol form of 2,4-pentanedione
Enol form of 2,4-pentanedione
intramolecular hydrogen bond
103 pm
H
O
166 pm
O
133 pm
124 pm
C
H3C
C
C
134 pm
141 pm
CH3
H
C=C and C=O are conjugated