apter 15 Enols and Enolates 烯烯烯烯烯烯烯烯烯 () 15.1 The acidity of the αhydro gens of carbonyl compounds: eno late ions 15.2 Keto and Enol tautomers 15.3 αHalogenation of aldehydes and ketones 15.4 The Haloform Reaction 15.5 The Aldol Condensation 烯 ( 烯烯烯) 15.5.1 Mechanism for the aldol
Chapter 15 Enols and Enolates (烯醇与烯醇负离子). 15.1 The acidity of the αhydrogens of carbonyl compounds: enolate ions 15.2 Keto and Enol tautomers 15.3 αHalogenation of aldehydes and ketones 15.4 The Haloform Reaction 15.5 The Aldol Condensation (羟醛缩合) - PowerPoint PPT Presentation
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Chapter 15 Enols and Enolates(烯醇与烯醇负离子)
15.1 The acidity of the αhydrogens of carbonyl compounds: enolate ions 15.2 Keto and Enol tautomers15.3 αHalogenation of aldehydes and ketones15.4 The Haloform Reaction15.5 The Aldol Condensation (羟醛缩合)15.5.1 Mechanism for the aldol addition15.5.2 The features of aldol addition15.5.3 Mixed aldol condensation Claisen-Schmidt condensation
15.6 Nucleophlic addiotion to αβ-unsaturate aldehydes and ketones15.6.1 1,2 addition and 1,4 addition15.6.2 The Micheal reaction15.6.3 Conjugate addition of organocopper reagents15.7 Alkylation of enolate ions
αHydrogens are acidic:
βhydrogen
When a carbonyl compoundloses an αproton, the anion called enolate is produced:
The acidity of the αhydrogens of carbonyl compounds: enolate ions
R C
O
C C
H Hαhydrogen
R C
O
C
H
> HC C H
Pka: 19-20 25
R C
O
C
HB
CO
RC C C
O
RA B
Enolates ( 烯醇负离子)Enolate is stabilizedby resonance
P358P358
H --
C CO
:BC C
OC C O
p-πconjugationp-πconjugation
Lithium diisopropylamide( 二异丙基氨基锂 )
(LDA)
Sodium hydride( 氢化钠 )
Li N Na+H -
OLi NTHF
O Li
+ NH
Cyclohexanone CyclohexanoneEnolate ion(100%)
Compound pKa
CH3COOHCH2(COCH3)2
CH3COCH2CO2CH3
CH2(CO2CH3)2
H2OCH3CH2OHCH3COClCH3CHO
CH3COCH3
CH3CNCH3CON(CH3)2
NH3
59
1113
15.7416161719253035
Table 15.1 Acidity Constants for Some Organic Compounds
P361P361
Keto and enol form are called tautomers
R2CCR'
H
OR2C CR'
OH
Aldehydes and ketones containing α-hydrogen are in equilibrium with their enol isomers
Tautomerization(互变异构)
Keto and Enol tautomerism ( 酮式与烯醇式互变异构)
CH3CH
OCH2 CH
OHKeto form Enol form
Acetaldehyde(~100) (Extremely small)
Acetone(>99%) (1.5×10-4)
O OHCyclohexanone Keto Enol
(98.8%) (1.2%)
CH3CCH3
OCH2 CCH3
OH
The bond of C=Ois stronger than C=C
The bond of C=Ois stronger than C=C
P353 P353
In β-dicarbonyl compounds, two carbonylgroups are separated by one - CH2 - group
CH3CCH2CCH3
O OCH3C CHCCH3
OH O
2,4-Pentanedinone( 2,4- 戊二酮)
(24%)
Enol form(76%)
CCH
C
O OH
H3C CH3
hydrogen bond
CCH
C
O OH
H3C CH3
Resonance stabilization of the enol form
αHalogenation of aldehydes and ketones Aldehydes and ketones react with halogens by substitution of one of the αhydrogen
C
O
C
H
H
H
Br2, AlCl3Et2O, 0¡æ
C
O
C
H
H + HBr
BrFeatures of the reaction:Acid –catalyzedRegiospecificAldehydic hydrogen isn’t affected
CO
HH
+ Br2
CO
BrH
+ HBrCHCl3
(80%)
H3CC
O
CC
O
CH3
HH3C
C
O
CC
O
CH3
H
H3CC
CC
O
CH3
H
O
P35611.3P35611.3
Mechanism of αhalogenation of aldehydes and ketones
Step 1 Acid-catalyzed formation of an enol
C C
O
H
+ H:Bfast C C
OH
HB
slow C C
OH+ H:B
Step 2 Reaction of the enol with halogen
Br Br+ C C
O Hfast
C C
O
Br
+ HBr
Ch.P349Ch.P349The Haloform ( 卤仿 )Reaction
PhCCH3
O
+ X2NaOH PhCCH2X
ONaOH
X2PhCCHX2
ONaOH
X2PhCCX3
O
A methyl ketone react with a halogen in the present of base, multiple halogenations occur at the carbon of -CH3
The dissociation of the trihalomethylketone in aqueous base, to produce carboxylate(羧酸盐) and the haloform
PhC
O
OH+ PhCCX3
O
O + HCX3
CHI3 is yellowPrecipitate
( 沉淀)
CHI3 is yellowPrecipitate
( 沉淀)
The haloform reaction using I2
Indentification CH3CO
CH3 CH
OH
CH3 CH
OH
15.5 The Aldol Condensation( 羟醛缩合 )2 Molecules of aldehydes
Arthur Micheal(1853-1942) wasborn to a wealthy family in Buffalo,New York. Although he received no formal university degree,he studiedin Heidelberg,Berlin,and the Écolede Médecine,Paris.Returning to the United State, he became Professor of Chemistry at Tufts University and then at Harvard University(1912-1936).Perhaps his most impor-tant contribution to science was his instrumental role in bring the European model of gradual education to the United State.Arthur Micheal
Sir Robert Robinson United Kingdom University of Oxford
Robinson received the 1947 Nobel Prize in Chemistry for his workon the synthesis of natural products, especially the alkaloids. His 1917 landmark one-step synthesis of tropinone from three simple precursors at room temperature in dilute aqueous solution was the forerunner of modern biomimetic syntheses. He did structural and synthetic work on other alkaloids (strychnine, morphine, brucine), on steroids (cholesterol), on wood dyes (brazilin, haematoxylin), and on the coloring matter of flowers (anthocyanins). In connection with steroid synthesis, he developed a general method for constructing a six-membered ring onto a ketone with an enolizable hydrogen (Robinson annulation). In the mid-1920s, Robinson introduced his electronic theory of organic reactions, and used it to rationalize orientation in electrophilic aromatic substitution. The curved arrow used by chemists to represent electron displacements was first used in this way by Robinson (1924). Robinson wrote over 500 papersand several books on natural products but in addition he was an avid chess player who wrote "The Art and Science of Chess: A Step-by-Step Approach".http://poohbah.cem.msu.edu/Portraits/PortraitsHH_Detail.asp?HH_LName=Robinson