Enols
18
Enols Enols 1 Enols Enols
description
Enols. Keto-Enol Tautomerism?. Tautomerism v . - an interconversion between two structures that differ by the placement of an atom or group; in this case, an α hydrogen atom ( H ). The -Carbon Atom and its Hydrogens. O. H. O. g. a. CH 3 CH 2 CH 2 CH. b. R. H. H. Terminology. - PowerPoint PPT Presentation
Transcript of Enols
EnolsEnols 11
EnolsEnols
EnolsEnols 22
Keto-Enol Tautomerism?Keto-Enol Tautomerism?
O
R'R
R
OH
R'R
RH
Keto form Enol form
Tautomerism Tautomerism vv. - an interconversion between two . - an interconversion between two structures that differ by the placement of an atom or structures that differ by the placement of an atom or
group; in this case, group; in this case, an an αα hydrogen atom ( hydrogen atom (HH).).
EnolsEnols 33
The The -Carbon Atom and -Carbon Atom and
its Hydrogensits Hydrogens
EnolsEnols 44
The reference atom is the carbonyl carbon.The reference atom is the carbonyl carbon.
Other carbons are designated Other carbons are designated , , , , , , etcetc. on the . on the
basis of their position with respect to the carbonyl carbon.basis of their position with respect to the carbonyl carbon.
Hydrogens take the same Greek letter as the carbon Hydrogens take the same Greek letter as the carbon
to which they are attached.to which they are attached.
TerminologyTerminology
CHCH33CHCH22CHCH22CHCH
OO
Reference Reference Atom (Atom (ipsoipso))
O
R
H
H
H
EnolsEnols 55
Enolization Enolization
(tautomerism)(tautomerism)
EnolsEnols 66
Mechanism of EnolizationMechanism of Enolization(In general)(In general)
OO••••
CR'CR'
••••
RR22CC
HHOOHH
HH
••••••••
OO
HH
HH ••••••••
RR22CC CR'CR'
OO••••
•••• HH
EnolsEnols 77
Mechanism of EnolizationMechanism of Enolization(Acid-catalyzed)(Acid-catalyzed)
OO
HH
HH ••••
HH++
RR22CC
HH
OO••••
CR'CR'
•••• OO••••
RR22CC CR'CR'
••••
HH
OO
HH
HH ••••
HH
++
EnolsEnols 88
Mechanism of EnolizationMechanism of Enolization(Acid-catalyzed)(Acid-catalyzed)
OO••••
RR22CC CR'CR'
HH
HH++
OOHH
HH
•••••••• HHOOHH
HH
••••++
OO••••
RR22CC CR'CR'
HH••••
EnolsEnols 99
percent enol is usually very smallpercent enol is usually very small
keto form usually 45-60 kJ/mol more stableketo form usually 45-60 kJ/mol more stablethan enolthan enol
Enol ContentEnol Content
RR22CHCR'CHCR'
OO
RR22CC CR'CR'
OHOH
enolenolketoketo
EnolsEnols 1010
Enol ContentEnol Content
CHCH33CHCH
OO
HH22CC CHCH
OHOH
KK = 3 x 10 = 3 x 10-7-7
AcetaldehydeAcetaldehyde
CHCH33CCHCCH33
OO
HH22CC CCHCCH33
OHOH
KK = 6 x 10 = 6 x 10-9-9
AcetoneAcetone
EnolsEnols 1111
Stabilized EnolsStabilized Enols
EnolsEnols 1212
2,4-Cyclohexadienone2,4-Cyclohexadienone OO
HH
HH
HH
HH
HH
HH
OHOH
HH
HH HH
HH
HH
keto form is less stable than enol formketo form is less stable than enol form keto form is not aromaticketo form is not aromatic enol form is aromaticenol form is aromatic
EnolsEnols 1313
EnolsEnols 1414
SUMMARY: Keto-Enol TautomerismSUMMARY: Keto-Enol Tautomerism
O
R'R
RH
Keto form
OH
R'R
R
Enol form
EnolsEnols 1515
SUMMARY: Enol-Keto TautomerismSUMMARY: Enol-Keto Tautomerism
O
R'R
RH
Keto form
OH
R'R
R
Enol form
EnolsEnols 1616
C2H5 OH
HOH
C2H5 OH
HOHO
H
OH
OH
C2H5 O
HO
C2H5 O
HOHO
H
O
O
keto keto keto keto
keto enol enol enolno α-H most stable
most stable most stable
EnolsEnols 1717
C2H5 OH
HOH
C2H5 OH
HOHO
H
OH
OH
OH
keto keto keto keto
enol enol enol enol
EnolsEnols 1818
C2H5 O
HO
C2H5 O
HOHO
H
O
O
keto keto keto keto
keto enol enol enol
