8.0 gugus karbonil
TRANSCRIPT
-
8/7/2019 8.0 Gugus Karbonil
1/64
SENYAWA GUGUS KARBONIL
-
8/7/2019 8.0 Gugus Karbonil
2/64
-
8/7/2019 8.0 Gugus Karbonil
3/64
Aldehydes and Ketones
-
8/7/2019 8.0 Gugus Karbonil
4/64
The Carbonyl Group
In the remainder of the course, we will studythe physical and chemical properties of classesof compounds containing the carbonyl C=O group.
aldehydes and ketones (Chapter 16) carboxylic acids
acid halides, acid anhydrides, esters, amides
-
8/7/2019 8.0 Gugus Karbonil
5/64
The Carbonyl Group
The carbonyl group consists of one sigmabond formed by the overlap of sp2 hybridorbitals and one pi bond formed by the
overlap of parallel 2p orbitals pi bonding and pi antibonding MOs forformaldehyde
-
8/7/2019 8.0 Gugus Karbonil
6/64
Structure
The functional group of an aldehyde is acarbonyl group bonded to an H atom and acarbon atom.
The functional group of a ketone is a carbonyl
group bonded to two carbon atoms.
Propanone
(Acetone)
Ethanal
(Acetaldehyde)
Methanal
(Formaldehyde)
O O O
CH3 CHHCH CH3 CCH3
-
8/7/2019 8.0 Gugus Karbonil
7/64
Nomenclature
IUPAC names: The parent chain is the longest chain thatcontains the carbonyl group.
For an aldehyde, change the suffix from --ee
to --alal. For an unsaturated aldehyde, change the
infix from --anan-- to --enen--; the location ofthe suffix determines the numberingpattern.
For a cyclic molecule in which -CHO isbonded to the ring, add the suffix -carbaldehydecarbaldehyde.
-
8/7/2019 8.0 Gugus Karbonil
8/64
Nomenclature: Aldehydes
H
O
3-Meth ylb utanal 2-Propen al(Acrolein)
(2E)-3,7-Dimethyl-2,6-octadienal(Geranial)
1
2
3
4
5
6
7
8H
O
H
O
CHO HO CHO
Cycl p n t n -carbald hyde
trans- -Hydroxycyclo-
hexanecarbaldehyde
14
CHOC6 H5CHO
t ra s-3-Phe yl-2-prope alCi amal ehyde)Be zaldehyde
The IUPAC naming uses the common namesbenzaldehyde, cinnamaldehyde, formaldehydeand acetaldehyde.
-
8/7/2019 8.0 Gugus Karbonil
9/64
Nomenclature: Ketones
IUPAC names The parent alkane is the longest chain that
contains the carbonyl group.
For a ketone, change the suffix--ee to --oneone.
Number the chain to give C=O the smallernumber.
IUPAC uses the common names acetone,
acetophenone, and benzophenone.
Propanone(Acetone) Benzophenone 1-Phenyl-1-pentanoneAcetophenone
O O OO
-
8/7/2019 8.0 Gugus Karbonil
10/64
Order of PrecedenceFor compounds containing more than onefunctional group usually indicated by a suffix:
-
8/7/2019 8.0 Gugus Karbonil
11/64
Common Names For an aldehyde, the common name is derived
from the common name of the correspondingcarboxylic acid.
For a ketone, name the two alkyl or aryl groups
bonded to the carbonyl carbon and add theword ketone.
HCH
O
HCOH
O
CH3 CH
O
CH3 COH
O
Formaldehyde Formic acid Acetaldehyde Acetic acid
Ethyl isopropyl ketone Diethyl ketone Dicyclohexyl ketone
OO
O
-
8/7/2019 8.0 Gugus Karbonil
12/64
Physical Properties Oxygen is more electronegative than
carbon (3.5 vs 2.5) and, therefore, a C=Ogroup is polar.
C C H-H+ +
More importantcontributing
structure
C C C
Polarity ofa carbonyl
group
+C
Aldehydes and ketones are polar compounds andinteract in the pure state by dipole-dipole interaction.
They have higher boiling points and are moresoluble in water than nonpolar compounds of
comparable molecular weight.
-
8/7/2019 8.0 Gugus Karbonil
13/64
Reaction Themes
One of the most common reactionthemes of a carbonyl group is additionof a nucleophile to form a tetrahedralcarbonyl addition compound.
Tetrahedral carbonyladdition compound
+ C
R
R
CNu
RR
Nu
-
8/7/2019 8.0 Gugus Karbonil
14/64
Reaction Themes
A second common theme is reaction with aproton or other Lewis acid to form aresonance-stabilized cation.
Protonation increases the electron
deficiency of the carbonyl carbon and makesit more reactive toward nucleophiles.
B
C OR
R
H N
H B
C O
R
R
H
B C O
R
RH
CN
O H
RR
C O
R
RH
H B
+
fas ++
+
+slow
Tetredr r
+
+ +
-
8/7/2019 8.0 Gugus Karbonil
15/64
Reaction Themes
Often the addition to a carbonyl group willproduce a new stereocenter.
If none of the starting materials is chiral andthe reaction takes place in an achiral
environment, a racemic mixture will be formed.
N u- C ORR'
N u
OR'
R
N u
OR
R' + 3 O+
N u
OR
R'
N u
OR'
R
+
A racemic mixt reA new chiralcenteris created
Approach from
the bottom face
Approach from
the top face
-
8/7/2019 8.0 Gugus Karbonil
16/64
Addition of C NucleophilesAddition of carbon nucleophiles is one of themost important types of nucleophilic additionsto a C=O group.
A new carbon-carbon bond is formed in the
process. Four common types of carbon nucleophiles are:
RMgX RLi - CRC C - N
A Grig arreage t
An organolit iumreagent
An al ynean ion
Cyan i e ion
-
8/7/2019 8.0 Gugus Karbonil
17/64
Grignard Reagents
Given the difference in electronegativitybetween carbon and magnesium (2.5 - 1.3),the C-Mg bond is polar covalent, with C- andMg +.
A Grignard reagent behaves as acarbanion.
Carbanion:Carbanion: An anion in which carbon has anunshared pair of electrons and bears a
negative charge. A carbanion is a good nucleophile and adds
readily to the carbonyl group of aldehydesand ketones.
-
8/7/2019 8.0 Gugus Karbonil
18/64
Grignard Reagents
Addition of a Grignard reagent toformaldehyde followed by treatment withH3O
+ gives a 1r alcohol.
CH CH2 -MgBr
O
H-C-H
O-[MgBr]
+
CH CH2 -CH2HCl
H2 O
OH
CH CH2 -CH2 Mg2+
ether
1-Propanol(a1alcohol)
Formaldehyde
+
+
Amagnesiumalkoxide
-
8/7/2019 8.0 Gugus Karbonil
19/64
Grignard Reagents
Addition to any other RCHO gives a2r alcohol.
MgBr
-[MgBr]
+
ClH2 O
OH
Mg2+
+et er
Acetal e y e
(analdehyde)
+
A magne ium
al o i e
1-Cyclo e ylet anol
(a 2 alco ol;
(racemic)
-
8/7/2019 8.0 Gugus Karbonil
20/64
Grignard Reagents
Addition to a ketone gives a 3r
alcohol.
Ph-MgBrO
Ph
O-[MgBr]
+HCl
H2 O Ph
OH
Mg2+
+
Acetone(a ketone)
ether
+
Amagnesiumalkoxide
2-Phenyl-2-propanol(a 3 alcohol)
Phenyl-magnesiumbromide
-
8/7/2019 8.0 Gugus Karbonil
21/64
Grignard Reagents
Problem:Problem: 2-Phenyl-2-butanol can besynthesized by three different combinationsof a Grignard reagent and a ketone. Show eachcombination.
C-CH2 CH3
CH3
OH
-
8/7/2019 8.0 Gugus Karbonil
22/64
Organolithium Compounds
Organolithium compounds are generallymore reactive in C=O addition reactionsthan RMgX, and typically give higheryields.
LiO
O-
Li
HCl
H2 O
OH
3,3- imethyl-2-b ta o e
3,3- imethyl-2-phe yl-2-b ta ol(racemic)
+
Phe yl-
lithi m
A lithi m al o ide
(racemic)
-
8/7/2019 8.0 Gugus Karbonil
23/64
Salts of Terminal Alkynes
Addition of an alkyne anion followed bytreatment with H3O+ gives an -acetylenic alcohol.
C:-
N a+
HC
OC O-N a+HC
HCl
H2 O
C OHHC
1- t ynyl-cyclo e anol
A o iumal o i e
+
Cyclo e anoneSo ium
acetyli e
-
8/7/2019 8.0 Gugus Karbonil
24/64
Hydration of Terminal Alkynes
H2
H C CH H2 S 4 , H S 4
1. (sia)2 BH
2. H2 2 , Na H
H CCH3
H CH2
CH
AnE-h drox kon
AF-h drox deh de
E
F
E
-
8/7/2019 8.0 Gugus Karbonil
25/64
Addition of HCN
HCN adds to the C=O group of an aldehyde orketone to give a cyanohydrin.
Cyanohydrin:Cyanohydrin: A molecule containing an -OHgroup and a -CN group bonded to the same
carbon.
-H r r e itrile( cet l e e c ri )
+ HC N CH3
C-
C NCH3
CH
OH
H
O
-
8/7/2019 8.0 Gugus Karbonil
26/64
Addition of HCN Mechanism of cyanohydrin formation
Step 1: Nucleophilic addition of cyanide
-
+
H3 C
C
H3 C
O
O-
H3 C
H3
C
C
N
N
C
C
O-
H3
C
H3
C
C N
NH C C
C
H3
C
H3 C
O-H
N
C N++-
- Step 2: Proton transfer gives thecyanohydrin and regenerates cyanide ion
nucleophile.
-
8/7/2019 8.0 Gugus Karbonil
27/64
Cyanohydrins
Acid-catalyzed dehydration gives an alkene.
Prope e itrile
(Acrylo itrile)
+
acidcataly t
2- ydro ypropa e itrile
(Acetaldehyde cya ohydri )
CH3 CHC N NCH2 = CHC H2 O
OH
CHC
OH
N 2 H2Ni
OH
CHCH2 NH2
2-Am i o- -phe yletha ol(racem ic)
+
Be zaldehydecya ohydri
(racemic)
Catalytic reduction of the cyano group gives a 1ramine.
-
8/7/2019 8.0 Gugus Karbonil
28/64
Wittig Reaction
The Wittig reaction is a very versatilesynthetic method for the synthesis ofalkenes from aldehydes and ketones.
Tri enyl-o ine o i e
Met ylene-cyclo e ane
A o oniumyl i e
++-+
CH2 Ph3 P= OPh3 P-CH2
Cyclo e anone
O
-
8/7/2019 8.0 Gugus Karbonil
29/64
Phosphonium YlidesPhosphonium ylides are formed in two steps:
Step 1: Nucleophilic displacement of iodine bytriphenylphosphine.
Ph3P CH3-I Ph3P-CH3 I
Triphenylphosphine
++
2
Methyltriphenylphosphoniumiodide(analkyltriphenylphosphinesalt)
CH3CH2CH2CH2 Li + H-CH2 -PPh3 I
CH2 -PPh3CH3CH2CH2CH3 LiI
phosphoniumylide
Butane
Butyllithium
+++
++
Step 2: Treatment of the phosphonium salt witha very strong base, such as BuLi, NaH, or NaNH2
-
8/7/2019 8.0 Gugus Karbonil
30/64
Wittig Reaction Phosphonium ylides react with aldehydes and
ketones to give alkenes. Step 1: Nucleophilic addition of the ylide to
the electrophilic carbonyl carbon.CR2O
CH2Ph3 PCH2
-:O C
R2
Ph3 P
O CR2
Ph3 P CH2-+
An o a o etane
+
A etaine
CH2
O CR2
Ph 3 PPh 3 P= O R2 C= CH2
An al ene
+
Tri enyl o ine
o ide
- Step 2: Decomposition of the oxaphosphatane.
-
8/7/2019 8.0 Gugus Karbonil
31/64
Wittig ReactionExamples:Examples:
O Ph3
P + Ph3 P=O
2-Methyl-2-heptene
+
Acetone
HPh
O
Ph3P Ph Ph Ph3 P= O
Phenyl-
cet l ehy e
+ +
(Z)- -Phe l- -
b t e e
(87%)
(E)- -Phe l- -
b t e e
(1 %)
+
HPh
O
+ OEtPh3 P
O
PhOEt
O
Ph3 P= O
Ethyl (E)-4-phe yl-2-b te oate(only the E isomerisormed)
+
Phe yl-acetaldehyde
-
8/7/2019 8.0 Gugus Karbonil
32/64
Wittig Reaction Some Wittig reactions are Z selective, others
are E selective. Wittig reagents with an anion-stabilizing
group, such as a carbonyl group, adjacent tothe negative charge are generally E selective.
OEtPh3 P
O
OEtPh3 P
O
Resonan e cont ibutingst uctu es fo anylidestabili ed byanadjacent ca bonylg oup
Wittig reagents without an anion-stabilizinggroup are generally Z selective.
-
8/7/2019 8.0 Gugus Karbonil
33/64
Wittig Reaction
Horner-Emmons-Wadsworth modification Uses a phosphonoester.
Br-CH2 -C-OEt
O
(MeO)3PBr-CH2-C-R
O
(MeO) 2P-CH2-C-R
OO
(MeO)2P-CH2-C-OEt
OO
MeBr
MeBranE-bromoester
anE-bromoketone
+
+
AnE-phosphonoester
AnE-phosphonoketone
Trimethyl-phosphite
-
8/7/2019 8.0 Gugus Karbonil
34/64
Wittig Reaction
Phosphonoesters are prepared bysuccessive SN2 reactions.
( Me O) 3 P CH2 - C-OEO
Br
CH 3 -O- P-CH2 - C-OE
O
OMe
OMe
Br
( MeO ) 2 P-CH2 - C- OE
OO
MeBr
+
+
SN2
SN2
AnE-pho sph on oester
-
8/7/2019 8.0 Gugus Karbonil
35/64
Wittig Reaction Treatment of a phosphonoester with a strong
base followed by an aldehyde or ketone givesan alkene.
A particular value of using a phosphonoester-stabilized anion is that they are almostexclusively E selective.
( MeO) 2 P-C 2 -C- OEt
OO
O
H
OEt
O
MeO-P- O-
O
OMe
1. strongbas e
2 .O n ly th e Eiso m e r
is formed
+
D im ethy lpho sp hate
an ion
-
8/7/2019 8.0 Gugus Karbonil
36/64
Addition of H2O
Addition of water (hydration) to the carbonylgroup of an aldehyde or ketone gives a geminaldiol, commonly referred to a gem-diol. A gem-diol is also referred to as a hydrate.
C O + H2 O C
OH
OH
Carbonyl groupof an aldehyde
or ketone
A hydrate(a gem-diol)
acid orbase
-
8/7/2019 8.0 Gugus Karbonil
37/64
Addition of H2O
When formaldehyde is dissolved in water at 20rC,the carbonyl group is more than 99% hydrated.
H
H
O H2 O+
Formaldehyde
H
HOH
OH
Formaldehydehydrate(>99%)
O H2 O+
2,2-Propanediol
(0.1%)
Acetone(99.9%)
OH
OH
The equilibrium concentration of a hydratedketone is considerably smaller.
-
8/7/2019 8.0 Gugus Karbonil
38/64
Addition of Alcohols
Addition of one molecule of alcohol to theC=O group of an aldehyde or ketone givesa hemiacetal.
Hemiacetal:Hemiacetal: A molecule containing an -OHand an -OR or -OAr bonded to the samecarbon.
O H-O t OH
O t+
aci or
base
he iacetal
-
8/7/2019 8.0 Gugus Karbonil
39/64
Addition of Alcohols
Hemiacetals are only minor components of anequilibrium mixture, except where a five- orsix-membered ring can form.
(S)-4- y ro y entanal Cyclic emiacetal(majorformsresent at equilibrium)
OH
H
O
O OH O OH+
41 14 14
OH
OH
OH
HO
H
OOH
O
OH
HO
HO
H2 OH
OH
HO
H2 OH
O
OH
HO OH1
2
3
6
6
12
anomeric
carbon
FAnomerofD -glucose
cyclic hemiacetal
(pr domin t s
t quilibrium)
E Anomerof
D -glucose cyclic
hemiacetal
+
4
5
D-Glucose(op n h in form)
123
45
3
4 5
6
-
8/7/2019 8.0 Gugus Karbonil
40/64
Addition of Alcohols
At equilibrium, the b anomer of glucosepredominates because the -OH group on theanomeric carbon is equatorial.
OOH
HO
OH
HOO
OH
OH
HO
HO
CH2 OH
OHO
HO
HO
OH
O
OH
HO
HO
CH2 OH
OH
12
34 5
6
anomeric
car on
FAnomer
anomeric
car on
132
4 56
(eq atorial)
123
45
6
anomericcar on
Anomer
anomericcar on
13
2
4 56
(axial)
re rawasac air
conformation
re rawasac air
conformation
OH
OH
-
8/7/2019 8.0 Gugus Karbonil
41/64
Addition of Alcohols Formation of a hemiacetal is base catalyzed
Step 1: Proton transfer from HOR gives analkoxide.
B - H O B H - O+ +
fas tandreversible
H3 - - H3
O:O-
O:
H3 - - H3
O
+
Step 2: Attack of RO- on the carbonyl carbon.
-
8/7/2019 8.0 Gugus Karbonil
42/64
Addition of Alcohols
Step 3: Proton transfer from the alcohol toO- gives the hemiacetal and generates a newbase catalyst.
O:
CH3 -C-CH3
OR
H OR
OR
OH
CH3 -C-CH3-
OR++
-
8/7/2019 8.0 Gugus Karbonil
43/64
Addition of Alcohols
Formation of a hemiacetal is also acid catalyzed.Step 1: Proton transfer to the carbonyl oxygen.
O
CH3-C-CH3 H-A
O
CH3-C-CH3
H
A
-
+
+ +
fastandreversi le
O
CH3 -C-CH3
H
H-O- CH3 -C-CH3
O-H
O
H
++
+
Step 2: Attack of ROH on the carbonyl carbon.
-
8/7/2019 8.0 Gugus Karbonil
44/64
Addition of Alcohols
Step 3: Proton transfer from the oxonium ion toA- gives the hemiacetal and generates anew acid catalyst.
RH
CH3 -C-CH3
O
OH
OR
OH
CH3 -C-CH3 H-A
A-
+
+
-
8/7/2019 8.0 Gugus Karbonil
45/64
Addition of Alcohols
Hemiacetals react with alcohols toform acetals.
Acetal:Acetal: A molecule containing two -OR or -OAr groups bonded to the same carbon.
OH
OE
H OEH
+OE
OE
H2O
Ad iethyl acetal
+
Ahemiacetal
+
-
8/7/2019 8.0 Gugus Karbonil
46/64
Addition of AlcoholsStep 1: Proton transfer from HA givesoxonium ion.
HO
R-C-OCH3
H
H A
HHO
H
R-C-OCH3 A:-
+
Anoxoniumion
+
+
R-C OCH3
H
OHH
H
R-C OCH3 R-C
H
OCH3 H2 O+
A re onance- tabili cation
++
+
Step 2: Loss of water gives resonance-stabilized cation.
-
8/7/2019 8.0 Gugus Karbonil
47/64
Addition of AlcoholsStep 3: Reaction of the cation with methanol gives
the conjugate acid of the acetal.
CH3 -OH
H
R-C OCH3 R-C OCH3
H
OCH
3H
A rotonate acetal
++
+
A:-
R-C OCH3
H
OCH
3H OCH3
H
R-C-OCH3 H-A+(4)
Anacetal
+
+
Step 4: Proton transfer to A- gives the acetal and
generates a replacement acid catalyst.
-
8/7/2019 8.0 Gugus Karbonil
48/64
Addition of Alcohols
With ethylene glycol and other glycols, theproduct is a five-membered cyclic acetal.
+H+HO
OHOO
O
Cyclic acetal
+ H2 O
-
8/7/2019 8.0 Gugus Karbonil
49/64
Dean-Stark Trap
-
8/7/2019 8.0 Gugus Karbonil
50/64
Acetals as Protecting Groups
Suppose you want to run the Grignardreaction between these compounds.
5- ydroxy-5-phenylpentanal(racemic)
4- romobutanalenzaldehyde
??+
H
O
H
O
H
OH O
Br
-
8/7/2019 8.0 Gugus Karbonil
51/64
Acetals as Protecting Groups The Grignard reagent prepared from 4-
bromobutanal will self-destruct! To avoid this: First protect the -CHO group as an acetal.
HBr
A li et l
+H
+
H2H
H+
Br
Br1 . Mg ethe r
2 . C6 H CH
-Mg Br
+
Ahir l m gnesium lkoxide(producedasaracemicmixture)
Then do the Grignard reaction.
Hydrolysis (not shown) gives the target molecule.
-
8/7/2019 8.0 Gugus Karbonil
52/64
Acetals as Protecting Groups
Tetrahydropyranyl (THP) protecting group.
RCH 2 OH +
OO RCH2 O
Di ydro yran Atetra ydro yranylet er
H+
THPgro
The THP group is an acetal and,therefore, stable to neutral and basicsolutions, and to most oxidizing andreducing agents.
It is removed by acid-catalyzedhydrolysis.
-
8/7/2019 8.0 Gugus Karbonil
53/64
Addition of Nitrogen Nucleophiles
Ammonia, 1r aliphatic amines, and 1r aromaticamines react with the C=O group of aldehydesand ketones to give imines (Schiff bases).
CH3 CH H2 NH
+
CH3 C H =N H2 O+ +
Acet l e e Aniline Ani ine(a Schi base)
O
Ani ine(a Schi base)
AmmoniaC clohexanone
++ N H3 H2 OO N H
H+
-
8/7/2019 8.0 Gugus Karbonil
54/64
Addition of Nitrogen Nucleophiles
Formation of an imine occurs in two steps:Step 1: Carbonyl addition followed by protontransfer.
C
O
H2 N-R
H
H
C
O:-
N-R
OH
N-R
H
C+ +
O H
H
H H
C
OH
N-R N-R
H
C
OHH
OH
H
C N-R OH
H
HA i mi e
+
+
++
++ H2 O
Step 2: Loss of H2O and proton transfer
-
8/7/2019 8.0 Gugus Karbonil
55/64
Addition of Nitrogen Nucleophiles
A value of imines is that the carbon-nitrogendouble bond can be reduced to a carbon-nitrogen single bond.
+
Dicyclohexylamine
Cyclohexanone
(Animine)
Cyclohexylamine
O
N NH
-H2 O
H2 / N i
H+H 2N
-
8/7/2019 8.0 Gugus Karbonil
56/64
Addition of Nitrogen Nucleophiles
Rhodopsin (visual purple) is the imineformed in the eye between 11-cis-retinal
(vitamin A aldehyde) and the protein opsin.
1 1
1 2
11-cis-RetinalRhodopsin
(Visualpurple)
H2 N-op n
H N-op nH O
+
-
8/7/2019 8.0 Gugus Karbonil
57/64
Addition of Nitrogen Nucleophiles
Secondary amines react with the C=O group ofaldehydes and ketones to form enamines.
The mechanism of enamine formation involvesformation of a tetrahedral carbonyl additioncompound followed by its acid-catalyzeddehydration.
O H-NH
+
N H2
O
An enaminePi eri ine(aecondar amine)
++
C clohexanone
-
8/7/2019 8.0 Gugus Karbonil
58/64
Addition of Nitrogen Nucleophiles
The carbonyl groups of aldehydes and ketonesreact with hydrazine in a manner similar to their
reactions with 1r amines.
O H2 NNH2 NNH2 H2 O+
Hydrazine
+
A hydrazone
H2 N NHCNH2
H2 N OH
H2N NH
H2N NH NO2
O2 N
Reagen t,H2N-R
Hydro ylam ine O ime
Phenylhydrazine
2,4-D initrophenyl-hydrazine
S emicar azide
2,4-D initrop henylhydrazone
Semicar azone
NameofDeri ati e FormedNameofReagen t
Phenylhydrazone
O
-
8/7/2019 8.0 Gugus Karbonil
59/64
Acidity ofE-Hydrogens
Hydrogens alpha to a
carbonyl group are more
acidic than hydrogens ofalkanes, alkenes, and
alkynes but less acidic
than the hydroxyl
hydrogen of alcohols.
CH3
CH2 -H
CH3 CCH2 -H
CH2 = CH-H
CH3 CH2 - H
CH3 C C-H
TypeofBond pKa
16
20
25
44
51
-
8/7/2019 8.0 Gugus Karbonil
60/64
Acidity ofE-Hydrogens
E-Hydrogens are more acidic becausethe enolate anion is stabilized by:
1. Delocalization of its negative charge.
2. The electron-withdrawing inductive effectof the adjacent electronegative oxygen.
CH3 -C-CH2 -H
O
:A-
O
CH3 -C CH2
O-
CH3 -C= CH2 H-A
Re onance- ta ilize enolate anion
+ +
-
8/7/2019 8.0 Gugus Karbonil
61/64
Keto-Enol Tautomerism
Protonation of the enolate anion on oxygen gives
the enol form; protonation on carbon gives the keto
form.
Enolateanion
Enol formKeto form
-O
CH3 -C-CH2 CH3 -C=CH2
CH3 -C=CH2
H-A
CH3 -C-CH3 +A-
A- +
H-A OH
O-
O
-
8/7/2019 8.0 Gugus Karbonil
62/64
Keto-Enol Tautomerism
Acid-catalyzed equilibration of keto and enoltautomers occurs in two steps.
Step 1: Proton transfer to the carbonyl oxygen.
Step 2: Proton transfer to the base A-.
O
CH3 -C-CH3 H-A
O
CH3 -C-CH3
H
A-
+
+
+
Keto form
The conjugate aciofthe etone
fa t and
rever i le
O
CH3 -C-CH2 -H
H
:A-
OH
CH3 -C=CH2 H-A
nol form
+
+
low+
-
8/7/2019 8.0 Gugus Karbonil
63/64
Keto-Enol Tautomerism
Keto-enol
equilibria for
simple
aldehydes andketones lie far
toward the keto
form.
OH
O
O
CH 3 CH CH2 = CH
CH 3 CCH3
Keto form Enol form
% Enol at
Equilibrium
6 10-5
OH
CH3 C = C H2 6 10-7
O
O OH
OH
1 10-6
4 10-5
-
8/7/2019 8.0 Gugus Karbonil
64/64
Keto-Enol Tautomerism
For some molecules, the enol is the major form
present at equilibrium:
ForF-diketones, the enol is stabilized by conjugation of
the pi system of the carbon-carbon double bond and the
carbonyl group.
For acyclic F-diketones, the enol is further stabilized by
hydrogen bonding.