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Chapter 15 Enols and Enolates (烯醇与烯醇负离子) PowerPoint 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 tautomers 15.3 αHalogenation of aldehydes and ketones 15.4 The Haloform Reaction 15.5 The Aldol Condensation (羟醛缩合)

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slide1

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

15.4The Haloform Reaction

15.5The Aldol Condensation(羟醛缩合)

15.5.1Mechanism for the aldol addition

15.5.2The features of aldol addition

15.5.3 Mixed aldol condensation

Claisen-Schmidt condensation

slide2

15.6 Nucleophlic addiotion to αβ-unsaturate

aldehydes and ketones

15.6.1 1,2 addition and 1,4 addition

15.6.2 The Micheal reaction

15.6.3 Conjugate addition of organocopper

reagents

15.7 Alkylation of enolate ions

slide3

αhydrogen

βhydrogen

The acidity of the αhydrogens of

carbonyl compounds: enolate ions

P358

αHydrogens are acidic:

Pka: 19-20 25

When a carbonyl compound

loses an αproton, the anion called enolate is

produced:

A

B

Enolate is stabilized

by resonance

Enolates(烯醇负离子)

slide4

-

H

-

-

Na+H

Cyclohexanone

Cyclohexanone

Enolate ion(100%)

p-πconjugation

Sodium hydride

(氢化钠)

Lithium diisopropylamide

(二异丙基氨基锂)

(LDA)

slide5

Table 15.1 Acidity Constants for

Some Organic Compounds

Compound pKa

5

9

11

13

15.74

16

16

17

19

25

30

35

CH3COOH

CH2(COCH3)2

CH3COCH2CO2CH3

CH2(CO2CH3)2

H2O

CH3CH2OH

CH3COCl

CH3CHO

CH3COCH3

CH3CN

CH3CON(CH3)2

NH3

P361

slide6

Tautomerization

(互变异构)

Keto form Enol form

Acetaldehyde

(~100) (Extremely small)

Acetone

(>99%) (1.5×10-4)

Keto Enol

(98.8%) (1.2%)

Cyclohexanone

P353

Keto and Enol tautomerism

(酮式与烯醇式互变异构)

Aldehydes and ketones containing α-

hydrogen are in equilibrium with their

enol isomers

Keto and enol form are called tautomers

The bond of C=O

is stronger than

C=C

slide7

2,4-Pentanedinone

(2,4-戊二酮)

(24%)

Enol form

(76%)

In β-dicarbonyl compounds, two carbonyl

groups are separated by one-CH2-group

Resonance stabilization of the enol form

slide8

αHalogenation of aldehydes and ketones

Aldehydes and ketones react with halogens

by substitution of one of the αhydrogen

P356

11.3

Features of the reaction:

Acid –catalyzed

Regiospecific

Aldehydic hydrogen isn’t affected

slide9

Mechanism of αhalogenation of aldehydes

and ketones

Step 1 Acid-catalyzed formation of

an enol

Step 2 Reaction of the enol with halogen

slide10

Indentification

The haloform

reaction using I2

The Haloform (卤仿)Reaction

Ch.P349

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

CHI3 is yellow

Precipitate

(沉淀)

slide11

Acetaldehyde

3-hydroxybutanal

15.5 The Aldol Condensation(羟醛缩合)

2 Molecules of aldehydes

To form hydroxy

aldehyde

Dilute sodium hydroxide

P366

11.8

aldehyde + alcohol = aldol

Aldol addition , aldol reaction or aldol

condensetion

15.5.1 Mechanism for the aldol addition:

Step 1 Base-catalyzed formation of enolate

ion:

Enolate ion

slide12

Step 2 The nucleophilic addition of enolate

to carbonyl group:

Step 3 The alkoxide ion abstracts a proton

from water to form aldol:

slide13

15.5.2 The features of aldol additions:

  • Carbon-carbon bond formation between
  • theα-carbon atom of one aldehyde and
  • the carbonyl group of another.

2. Dehydration of addition product

The addition product ( aldol + OH-)is

heated,hydration occurs to form α,β-

unsaturated aldehyde:

π-π

conjugation

slide14

3. Reversible reaction. the equilibrium of

aldol reation for ketones is unfavorable.

Intra-molecular aldol condensation:

Bicyclo[5.3.0]dec-1(7)-

en-2-one(96%)

(二环[5.3.0]-1-癸烯-2-酮)

1.6-Cyclodecanedione

(1,6-环癸二酮)

slide15

4. The product with two functional groups:

Insect

repellent

15.5.3 Mixed aldol condensation

  • Only one of the reactant can form an enolate.
  • One of the reactant is more reactive toward
  • nuelophilic addition than other.
slide16

4-Phenyl-3-buten-2-one

(4-苯基-3-丁烯-2-酮)(70%)

benzaldehyde

acetone

Claisen-Schmidt condensation

Ketones react with aromatic aldehyde

in the presence of base, to give mixed aldol

condensation product:

The enolate of ketone as a nucleophile

attacks the carbonyl group of aromatic

aldeyhyde.

2-Hydroxymethyl-

3-methylbutanal

(3-甲基-2-羟甲基丁醛)

slide17

Claisen, LudwigBorn: Köln (Germany), 1851 Died: Godesberg near Bonn

(Germany), 1930

http://www.chemsoc.org/networks/

enc/fecs/Claisen.htm

slide18

15.6 Conjugation addition to α,β-

unsaturated aldehydes and ketones

15.6.1 1,2 addition and 1,4 addition

The nucleophilic addition of α,β- unsaturated

aldehydes or ketones

may be in two way:

1,2-addition

Direct addition

(直接加成)

The resonance

structure:

1,4-addition

Conjugate addition(共轭加成)

slide19

(95%)

  • General roles:
  • When the nucleophile is stronger base,
  • 1,2 addition is often observed: RMgX,
  • RLi, LiAlH4.

2. When the nucleophile is weaker base,

conjugate addition is observed:

slide20

3. 1,2 addition-Kinetic control

1,4 addtion-Thermodynamic control

1,2 addition product retains C=C bond,

1,4 addition product retains C=O bond.

carbon-oxygen double bonds are more stable

than carbon-carbon double bonds

Ch.P413

15.6.2 The Micheal Reaction

Conjugate additions of enolate ions

(or carbanions)to α,β-unsaturate carbonyl

compounds-Micheal addition(迈克尔加成)

or Micheal reaction.

Enolate ions or carbanions: derived from

β- dicarbonyl compounds.

slide21

Methyl

vinyl ketone

(甲基乙烯基甲酮)

2-Methyl-2-

(3’-oxobutyl)-

1,3-cyclohexanediol)

[2-甲基2-(3’-氧代丁基)-

1,3-环己二酮](85%)

2-Methyl-1,3-

Cyclohexanedione

(2-甲基-1,3-环己二酮)

Intromecular aldol

addition

Micheal addition +

(65%)

Robinson annulation

Intramolecular

Aldol addition

product

slide22

2-Cyclohexenone

(2-环己烯酮)

Diethyl,3-oxocyclohexyl-

malonate

(3-氧代环己基丙二酸二乙酯)

(90%)

Problem:

Micheal addition of stabilized anions

3-oxocyclohexylacetic acid

(3-氧代环己基乙酸)

slide23

Arthur Micheal(1853-1942) was

born to a wealthy family in Buffalo,

New York. Although he received no

formal university degree,he studied

in Heidelberg,Berlin,and the École

de 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

http://books.nap.edu/books/0309022401/html/331.html#pagetop

slide24

Sir Robert Robinson

United Kingdom University of Oxford Oxford, United Kingdom

b. 1886 d. 1975

The Nobel Prize in Chemistry 1947

http://www.nobel.se/chemistry/laureates/1947/robinson-bio.html

slide25

Robinson received the 1947 Nobel Prize in Chemistry for his work

on 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 papers

and 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

slide26

15.6.3 Conjugate addition of organo-

copper reagents

Organocopper reagents (CuLiR2) undergo

conjugate addition toα,β-unsaturated

carbonyl compounds:

98% 2%

Lithium dialkylcuprates adds predominantly

in the less-hindered way to give the product

with the alkyl groups trans to each other.

slide27

2,4-Pentanedione

(2,4-戊二酮)

Iodo-

methane

3-Methyl-

2,4-pentanedione

15.7 Alkylation of enolate ions

Enolate ions derived from β- dicarbonyl

compounds react with alkyl halide by SN2

mechanism :

P362

11.6

Enolate ions of β- dicarbonyl compounds

are more stables than aldehydes or ketones.

p-π conjugation

2. Alkyl halide: CH3X,

RCH2X,