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Chemistry 353 Problem Solving Session. The problem solving session will not be held this Thursday (April 20) For this week only, the problem solving session will be moved to Wednesday, April 19, at 5:00 PM. Chemistry Building 385. Addition of Water (16.9).

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chemistry 353 problem solving session
Chemistry 353 Problem Solving Session
  • The problem solving session will not be held this Thursday (April 20)
  • For this week only, the problem solving session will be moved to Wednesday, April 19, at 5:00 PM.
  • Chemistry Building 385

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addition of water 16 9
Addition of Water (16.9)

In general, the equilibrium does not favor formation of the hydrate.

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hydrate formation
Hydrate Formation:
  • Important only for low-molecular-weight aldehydes
  • Examples:

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mechanism of hydration
Mechanism of Hydration

NOTE: The reaction is acid-catalyzed.

The reverse process must follow exactly the same pathway as the forward process.

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addition of alcohols 16 10
Addition of Alcohols (16.10)

NOTE: A hemiacetal formed from a ketone is sometimes known as a hemiketal.

Aldehydes form hemiacetals faster than ketones

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mechanism of hemiacetal formation
Mechanism of Hemiacetal Formation

NOTE: The reaction is acid-catalyzed

The reverse reaction must follow the same pathway as the forward process.

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slide8

The formation of hemiacetals is spontaneously reversible. The equilibrium between carbonyl compound and hemiacetal is constantly going back and forth.

  • Hemiacetal formation is a very important process in the chemistry of the sugars (Section 16.12)

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reaction with a second molecule of alcohol formation of acetals
Reaction With a Second Molecule of Alcohol: Formation of Acetals

heat

This reaction is also reversible. But, in this case, the equilibrium can be driven to the right by an application of Le Châtelier’s Principle.

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removal of water le ch telier s principle in action
Removal of Water: Le Châtelier’s Principle in Action

1) Remove water from the reaction as quickly as it is formed. This is accomplished by anazeotropicdistillation(see next image).

2) The acid catalyst must bedry. It cannot contain water. Catalysts that are used include:

Hydrogen chloride gas (dry)

Hydrogen chloride gas dissolved in an alcohol solvent

p-Toluenesulfonic acid (dry crystals) -- “HOTs”

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dean stark water separator
Dean-Stark Water Separator

When the azeotrope condenses, the water and benzene form separate layers (immiscible).

As water forms in the reaction, it distills out of the solution in the form of an azeotrope with benzene.

(c.f.Introduction to Organic Laboratory Techniques, Sections 10.7, 10.8)

Benzene layer

Water layer

Aldehyde or ketone, alcohol, and benzene

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slide12

Mechanism of Acetal Formation

Notice that this mechanism combines features of the SN1 reaction and carbonyl addition.

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the overall picture
The Overall Picture

heat

Technically, both steps are reversible, but only the firststep isspontaneouslyreversible. The second step requires more stringent conditions and heating to be reversed.

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formation of 2 2 dimethoxy propane
Formation of 2,2-Dimethoxy-propane

heat

Dry acid = HCl gas

HCl in methanol

HOTs

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a second example
A second example...

heat

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starting with acrolein
Starting with acrolein:

heat

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formation of a cyclic acetal
Formation of a Cyclic Acetal

heat

This reaction is important in the preparation ofprotective groups(see Section 16.11)

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do you remember this
Do you remember this?

heat

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so why not do this
So, why not do this?

heat

Because sulfur is a stronger nucleophile than oxygen, and because the products are more stable, this reaction goes much more readily toward theright.

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introduction to protective groups 16 11
Introduction to Protective Groups (16.11)
  • When a molecule has more than one functional group, it is sometimes difficult to carry out a reaction on one functional group without having the other group interfere.
  • Sometimes the non-target functional group will react at the same time as the target group, giving a mixture of products.
  • Sometimes the non-target group reacts fasterthan the target group, leading to the wrong product.
  • Sometimes the non-target group will destroy the reagent, thereby making the desired synthesis impossible.

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slide21

What we need, then, is some means of covering up the non-target functional group while we operate on the target group.

  • We need aprotective group.
  • Ideally, the protective group should be something that we can attach easily and in high yield.
  • It must be stable under the conditions of the reaction that are used to modify the target functional group. It needs to be inert during the reaction.
  • It should be easily removed without decomposing the entire structure.

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slide22

NON-TARGET

TARGET

Acetals are very important protective groups. They are inert under basic conditions, and they undergo rapid hydrolysis when heated with aqueous acid

PROTECT

NON-TARGET

TARGET

MODIFY TARGETGROUP

NON-TARGET

NON-TARGET

TARGET

TARGET

DEPROTECT

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example of an acetal used as functional group
Example of an acetal used as functional group:

A 1,3-Dioxolane

This is anacetal.

If you’re dying to know why the “dioxolane” name, I can be talked into digressing about IUPAC nomenclature of heterocyclic rings.

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use of a cyclic acetal as a protective group
Use of a Cyclic Acetal as a Protective Group

Suppose I wanted to do...

Try to make a Grignard reagent and then add ethylene oxide. BUT: If you attempt to make the Grignard reagent, it will try to react with itself!

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slide25

Protect

Use of a Protective Group

Deprotect

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a useful variation
A Useful Variation
  • Boron trifluoride is a Lewis acid -- this reaction goes under much milder conditions of acid catalysis.
  • The advantage of the 1,3-oxathiolanes is that they can be removed (deprotect step) easily without hydrolysis in an acid medium.

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deprotecting a 1 3 oxathiolane
Deprotecting a 1,3-Oxathiolane
  • Raney Nickel is a specially-prepared nickel catalyst.
  • These deprotection conditions are much milder than acid hydrolysis.

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raney nickel
Raney Nickel
  • A specially-prepared form of nickel metal, in an active, finely-divided state.
  • A nickel-aluminum alloy is treated with aqueous sodium hydroxide. The NaOH dissolves away the aluminum, leaving behind the nickel in a finely-divided state, almost as a colloidal suspension.
  • The nickel has the hydrogen already adsorbed on its surface.

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one more variation
One More Variation

What’s the 1,3-dithiolane good for? Wait until Chapter 17.

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what if i wanted to make a grignard reagent of this compound
What if I wanted to make a Grignard reagent of this compound?

This wouldn’t work. The –OH groups are too acidic to permit the formation of a Grignard reagent.

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but if we protect the oh groups
But if we protect the –OH groups…

heat

This is called an acetonide.

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and finally deprotect
… and, finally, deprotect

We get back our original diol functional groups.

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use of protective groups
Use of Protective Groups
  • The synthesis of a complex molecule requires a careful strategy.
  • When to use protective groups?
  • Which ones to use?
  • When is the best time to put them on?
  • When is the appropriate time to deprotect?

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tetrahydropyranyl ethers
Tetrahydropyranyl Ethers

The dry acid can be HCl in methanol or HOTs

The reaction is an addition of ROH across the double bond of dihydropyran.

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tetrahydropyranyl ethers1
Tetrahydropyranyl Ethers
  • Excellent protective group for alcohols
  • THP Ether is an acetal.
  • THP Ether is stable under strongly basic conditions
  • THP Ether can be deprotected by hydrolysis in aqueous acid.

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example use of thp ether as protective group
Example: Use of THP Ether as Protective Group

Problem: Devise a strategy for making the following conversion

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step three deprotect
Step Three: Deprotect

The acetal (THP ether) can be removed easily by hydrdolysis in acid.

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carbohydrates cyclic structures
Carbohydrates -- Cyclic Structures
  • Read Sections 16.15 through 16.18
  • Review Chapter 5, Sections 5.14 through 5.16

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slide43

Carbohydrates contain the functional groups of alcohols and aldehydes or ketones in the same molecule. They are polyhydroxyaldehydes or polyhydroxyketones.

  • Thus they can form acetal-type products through the intramolecular interaction of these functional groups.
  • As a model, consider the reaction:

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redrawing the linear structure as a ring
Redrawing the linear structure as a ring...

Note the hemiacetal position

The product of the cyclization is ahemiacetal.

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slide46

furan

pyran

FURANOSE AND PYRANOSE RINGS

6

a pyranose

ring

two anomers

are possible

in each case

a furanose

ring

5

for clarity no

hydroxyl groups

are shown on the

chains or rings

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slide47

ANOMERS

anomeric

carbon

(hemiacetal)

for clarity

hydroxyl groups

on the chain are

not shown

anomers differ in configuration

at the anomeric carbon -- they arediastereomers

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slide49

HAWORTH PROJECTIONS

It is convenient to view the cyclic sugars (glucopyranoses)

as a “Haworth Projection”, where the ring is flattened.

HAWORTH

PROJECTION

upper-right

back

This orientation is

always used for a

Haworth Projection

a-D-(+)-glucopyranose

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slide50

HAWORTH PROJECTIONS

HERE ARE SOME CONVENTIONS YOU MUST LEARN

1) The ring is always oriented with the oxygen

in the upper right-hand back corner.

D

2) The -CH2OH group is placed

UP for a D-sugar

3) a-Sugars have the -CH2OH group and

the anomeric hydroxyl group trans.

a

4) b-Sugars have the -CH2OH group and

the anomeric hydroxyl group cis.

b

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slide51

cis

= b

trans

= a

on right

= D

CONVERTING TO HAWORTH PROJECTIONS

D-(+)-glucose

-CH2OH

up = D

D

O

W

N

U

P

1

6

BOTH

ANOMERS OF

A D-SUGAR

(D-glucose)

2

5

3

4

1

4

3

2

5

6

HAWORTH

PROJECTIONS

FISCHER

PROJECTION

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slide52

cis

= b

trans

= a

CONVERTING TO ACTUAL CONFORMATIONS

-CH2OH

up = D

b-D-(+)-glucopyranose

HAWORTH

CONFORMATION

a-D-(+)-glucopyranose

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mutarotation
Mutarotation

We observe that when pure, crystalline a-D-(+)-glucose is placed in water, the observed rotationdecreasesover time, until if finally reaches a constant value.

+112.2° Þ +52.7°

Similarly, if we place a sample of pure, crystalline b-D-(+)-glucose in water, the observed rotationincreasesover time, until if finally reaches a constant value

+18.7° Þ +52.7°

This process is known asmutarotation.

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slide54

MUTAROTATION

+112o

pure a-D-(+)-glucopyranose1

[a]D

+57.2o

66% b

34% a

pure b-D-(+)-glucopyranose2

+19o

TIME

(min)

1 Obtained by crystallization of glucose at room temperature.

2 Obtained by crystallization of glucose at 98 °C.

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slide56

Note that the forms of glucose that were involved in mutarotation were hemiacetals.

Acetals do not show mutarotation.

In the case of an acetal, there is no longer a spontaneous equilibrium between cyclic and open-chain forms. Thus, the anomeric -OH group cannot exchange between up and down (axial and equatorial) positions.

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slide57

The sugar shown here would not undergo mutarotation.

This is an acetal.

b-Methylglucopyranoside

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slide58

FRUCTOFURANOSES

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slide59

FRUCTOSE

cis = b

up = D

1

..

:

anomeric

carbon

2

6

3

5

2

..

4

3

4

1

..

5

6

b-D-(-)-Fructofuranose

D-(-)-Fructose

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slide60

POLYSACCHARIDES

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sucrose
Sucrose

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