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