slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
REACTIONS OF a -HYDROGENS : ALDOL AND CLAISEN CONDENSATION REACTIONS PowerPoint Presentation
Download Presentation
REACTIONS OF a -HYDROGENS : ALDOL AND CLAISEN CONDENSATION REACTIONS

Loading in 2 Seconds...

play fullscreen
1 / 28

REACTIONS OF a -HYDROGENS : ALDOL AND CLAISEN CONDENSATION REACTIONS - PowerPoint PPT Presentation


  • 459 Views
  • Uploaded on

REACTIONS OF a -HYDROGENS : ALDOL AND CLAISEN CONDENSATION REACTIONS. TYPES OF REACTIVITY FOR ALDEHYDES AND KETONES. nucleophilic addition. Nu:. :B. removal of a -H. Good nucleophiles add. Strong bases remove a -hydrogens. . Often, both processes compete. -.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'REACTIONS OF a -HYDROGENS : ALDOL AND CLAISEN CONDENSATION REACTIONS' - Samuel


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

REACTIONS OF a-HYDROGENS :

ALDOL AND CLAISEN

CONDENSATION REACTIONS

slide2

TYPES OF REACTIVITY FOR

ALDEHYDES AND KETONES

nucleophilic

addition

Nu:

:B

removal

of a-H

  • Good nucleophiles add.
  • Strong bases remove a-hydrogens.

..

Often, both processes compete.

-

slide4

ald

+

ol

an aldol

(b-hydroxyaldehyde)

The Aldol Condensation

base

H3O+

- H2O

aldols easily lose

water to form a

double bond

a,b-unsaturated aldehyde

slide5

Aldol Condensation -- Mechanism

fast

enolate ion

slow

forms new

C-C bond

fast

slide6

The Bond Forming Step

carbonyl

(acceptor)

nucleophile

(donor)

enolate

slide8

“CROSSED” ALDOL

CONDENSATIONS

slide9

Crossed Aldol Condensations

KETONE + ALDEHYDE

-

..

aldehyde

NaOH

ketone

- H2O

Works best to if an aldehyde is

the “acceptor”, since they are

more reactive; and works really

well if the aldehyde has no a-H.

a “chalcone”

The ketone should have the a-H.

slide10

IMPORTANT GUIDELINES

Aldehyde carbonyl groups are more reactive toward

nucleophilic addition than ketone carbonyl groups.

1.

d-

d-

Alkyl groups

deactivate the

carbonyl ( +I )

to addition.

+I

+I

d+

d+

+I

Nu:

MORE

REACTIVE

slide11

O

O

O

RELATIVE REACTIVITY OF C=O GROUPS

THE EFFECT OF ALKYL SUBSTITUTION

Density - LUMO plots ( color scale 0.000 to 0.030 )

H-C-H

CH3-C-H

CH3-C-CH3

MORE

REACTIVE

LESS

REACTIVE

slide12

Ketones form enolate ions more easily than aldehydes.

2.

..

..

-

-

:

:

:

:

More

substituents

on the

double bond

aldehyde enolate

ketone enolate

more stable

-

..

..

Which enolate will

form fastest?

:

:

:

..

-

monosubstituted

-

..

..

:

:

:

..

-

disubstituted

slide13

ALDEHYDE + KETONE ?

In “mixed” reactions the ketone enolate

usually adds to the aldehyde.

The ketone forms the lower energy enolate (forms faster)

and it adds to the aldehyde (more reactive C=O).

slide14

WHAT ABOUT

TWO DIFFERENT KETONES ?

slide15

HOW MANY PRODUCTS WITH THIS ONE ?

A

B

a

b

c

d

x2

x2

four mixed products

aB, bB, cA, dA

two different self dimers

two different self dimers

aA, bA

cB, dB

8 POSSIBLE PRODUCTS !

….. which enolate do you think will form preferentially?

slide17

Formation of Rings

a1

a2

NaOH

:

-

Why don’t a2 hydrogens react ?

slide19

An Interesting Sequence

1) O3

2) H3O+

OH-

KOH

Aldol

H2SO4

- H2O

slide21

The Claisen Ester Condensation

CH3CH2OH

a b-ketoester

Notice that

the base, the solvent and the leaving group

CH3CH2O-Na+, CH3CH2OH, CH3CH2O-

all match (this is required in most cases).

slide23

Dieckmann Condensation

A CYCLIC CLAISEN CONDENSATION

NaOMe

MeOH

..

slide24

PATTERNS

Type of

Condensation

Reaction

b-hydroxy to C=O

ALDOL

3-hydroxyaldehyde or

(H)

3-hydroxyketone

(H)

-H2O

a,b-unsaturated C=O

ALDOL

2-propen-1-al or

(with loss of H2O)

2-propen-1-one

b-keto ester

CLAISEN

slide26

Synthesis of a Perfumery Compound

a2

a1

cis

cis

a3

Aldol Condensation

a4

Dehydration

Why don’t the

other sets of a-H

react?

cis-Jasmone

Scent of Jasmine in perfumes.