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OXIDATIONS OF ALCOHOLS PowerPoint PPT Presentation


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OXIDATIONS OF ALCOHOLS. OXIDATION OF AN ALCOHOL. ( LOSS OF 2H + and 2e - ). a- carbon. - 2H. a. OXIDATION. C. O. C. O. REDUCTION. + 2H. H. H. LOSS OF TWO HYDROGENS. a- hydrogen. one an a -H. The alcohol must. have a - hydrogens. Oxidations. no reaction.

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OXIDATIONS OF ALCOHOLS

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OXIDATIONS OF ALCOHOLS


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OXIDATION OF AN ALCOHOL

( LOSS OF 2H+ and 2e- )

a-carbon

- 2H

a

OXIDATION

C

O

C

O

REDUCTION

+ 2H

H

H

LOSS OF TWO

HYDROGENS

a-hydrogen

one an a-H


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The alcohol must

have a-hydrogens.

Oxidations

no reaction

Remember: A dehydrogenation (loss of hydrogen) is also a form of oxidation!


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MANTRA

  • Primary alcohols oxidize to give

  • Carboxylic acids(via aldehyde)*

  • Seconday alcohols oxidize to give

  • Ketones

  • Tertiary alcohols do not oxidize

  • no oxidation

  • Aldehydes oxidize easily to give

    • Carboxylic acids

*With special reagents, the oxidation of a primary alcohol

can be stopped at the aldehyde.


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PRIMARY ALCOHOLS


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Oxidation of Primary Alcohols

with KMnO4

REQUIRES

HEAT

twoa-hydrogens

+ MnO2

precipitate

C=C Double bonds are also oxidized by this reagent.


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Oxidation of Primary Alcohols

with K2Cr2O7

CHROMIC ACID

TEST


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SECONDARY ALCOHOLS


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Oxidation of Secondary Alcohols

CHROMIC ACID

TEST

Jones Oxidation


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JONES REAGENT


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CHROMIC ACID

EQUILIBRIA

Chromic acid

Dichromic acid

ALL OF THESE ARE CHROMIC ACID SPECIES

CrO3

H2CrO4

NaCr2O7

oxidizing

agents

H2SO4

H2SO4

H2SO4


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Primary alcohol

has two a-H

MECHANISM

H2O

H2SO4

Alcohols react with

chromic acid to form

chromate esters.

:

+

..

..

..

:

..

..

Chromate ester


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MECHANISM

( continued )

..

aldehyde

:

..

..

..

..

:

..

..

The two lost

electrons

end up here.

..

:

:

Loss of two

electrons

+

Loss of

a-hydrogen

Oxidation

continues

FIRST OXIDATION

(next slide)


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MECHANISM

( continued )

Requires

water and acid

hydrate

(an alcohol !)

Oxidation continues because

the aldehyde forms a hydrate.

The hydrate is an alcohol (diol)

that has an a-hydrogen.

Oxidation

continues


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MECHANISM

( continued )

:

..

Chromate

ester

:

+

..

..

..

:

..

..

:

..

Loss of a-hydrogen,

loss of 2 electrons.

hydrate

:

Carboxylic

acid

..

..

:

..

..

SECOND OXIDATION

+


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HOW CAN WE

STOP OXIDATION OF THE ALDEHYDE ?

Oxidation of the aldehyde requires the hydrate to form.

Formation of the hydrate requires acid and water.

What if we do the reaction in

basic medium with no water ?


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SARRETT AND PCC REAGENTS


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Oxidation with

Chromic Oxide and Pyridine

aldehydes do not

oxidize further

Sarett Oxidation


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Oxidation with

Pyridinium Chlorochromate

aldehydes do not

oxidize further

except in DMF

which enhances

reactivity

“PCC” Oxidation


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MEERWEIN-PONNDORF-VERLEY

REDUCTION


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MEERWEIN-PONNDORF-VERLEY REDUCTION

aluminum

isopropoxide

Al(OiPr)3

+

+

isopropyl alcohol

acetone

EQUILIBRIUM

  • Use an excess of isopropyl alcohol to reduce a ketone.

  • Use an excess of acetone to oxidize an alcohol.


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HYDRIDE

DONOR

The alcohol complexes with the

aluminum isopropoxide and acts

as a hydride donor to the ketone.


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TOLLEN’S TEST


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The Tollens Test

metallic

silver

aldehyde

Ketones do

not react.

silver mirror

Remember that aldehydes

are easily oxidized.


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CARBOHYDRATES

AND THE TOLLEN’S TEST

aldehyde

The open chain form

is an aldehyde and

gives a Tollen’s test.

“REDUCING SUGAR”

b-D-(+)-glucopyranose

Pyranose and open-chain

forms are in equilibrium

in solution.

D-(+)-glucose


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CARBOHYDRATES

AND THE TOLLEN’S TEST

ketone

TAUTOMERIZATION

aldehyde

*

D-(-)-fructose

“enediol”

* both diastereomers

“REDUCING SUGAR”

a-D-(-)-fructofuranose

Ketoses also give the test because they tautomerize! in solution.


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CARBOHYDRATES

AND THE TOLLEN’S TEST

hemiacetals open in solution

= REDUCING SUGAR

aldehyde

ketoses

tautomerize

to reducing

sugars

X

acetals do not open in solution unless hydrolyzed in acid

= NONREDUCING SUGAR


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CARBOHYDRATES

AND THE TOLLEN’S TEST

Any polysaccharide which has a hemiacetal ring will

give a positive Tollen’s test = reducing sugar.

(+)-Sucrose

non-reducing

Neither sugar is in a

hemiacetal link.

Maltose

reducing sugar

Hemiacetal link at a.


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SYNTHESIS PROBLEMS


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A Simple Synthesis Problem

Make from benzyl alcohol

complete synthesis on board


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A Synthesis Problem

?

complete synthesis on board


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Yet Another Conversion

?

complete synthesis on board


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SYNTHESIS OF TRIPHENYLMETHANOL

  • via benzoyl chloride

  • via benzaldehyde

  • via benzyl alcohol

  • via benzophenone


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CONVERSIONS


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Another Synthesis Problem

?

complete synthesis on board


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C

D

A

B

PHARMACEUTICALS

Mexican Yams

“the pill”

must be

injected

better absorbed through

the stomach and intestines

than progesterone

STEROID

progesterone


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Mexican Yams

CrO3

pyridine

H+

(-H2O)

acetal

protecting group

Na

NH3(liq)

Na

H3O+

norethisterone


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