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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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Presentation Transcript
slide2
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

slide3
The alcohol must

have a-hydrogens.

Oxidations

no reaction

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

slide4
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.

slide6
Oxidation of Primary Alcohols

with KMnO4

REQUIRES

HEAT

twoa-hydrogens

+ MnO2

precipitate

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

slide7
Oxidation of Primary Alcohols

with K2Cr2O7

CHROMIC ACID

TEST

slide9
Oxidation of Secondary Alcohols

CHROMIC ACID

TEST

Jones Oxidation

slide11
CHROMIC ACID

EQUILIBRIA

Chromic acid

Dichromic acid

ALL OF THESE ARE CHROMIC ACID SPECIES

CrO3

H2CrO4

NaCr2O7

oxidizing

agents

H2SO4

H2SO4

H2SO4

slide12
Primary alcohol

has two a-H

MECHANISM

H2O

H2SO4

Alcohols react with

chromic acid to form

chromate esters.

:

+

..

..

..

:

..

..

Chromate ester

slide13
MECHANISM

( continued )

..

aldehyde

:

..

..

..

..

:

..

..

The two lost

electrons

end up here.

..

:

:

Loss of two

electrons

+

Loss of

a-hydrogen

Oxidation

continues

FIRST OXIDATION

(next slide)

slide14
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

slide15
MECHANISM

( continued )

:

..

Chromate

ester

:

+

..

..

..

:

..

..

:

..

Loss of a-hydrogen,

loss of 2 electrons.

hydrate

:

Carboxylic

acid

..

..

:

..

..

SECOND OXIDATION

+

slide16
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 ?

slide18
Oxidation with

Chromic Oxide and Pyridine

aldehydes do not

oxidize further

Sarett Oxidation

slide19
Oxidation with

Pyridinium Chlorochromate

aldehydes do not

oxidize further

except in DMF

which enhances

reactivity

“PCC” Oxidation

slide21
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.
slide22
HYDRIDE

DONOR

The alcohol complexes with the

aluminum isopropoxide and acts

as a hydride donor to the ketone.

slide24
The Tollens Test

metallic

silver

aldehyde

Ketones do

not react.

silver mirror

Remember that aldehydes

are easily oxidized.

slide25
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

slide26
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.

slide27
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

slide28
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.

slide30
A Simple Synthesis Problem

Make from benzyl alcohol

complete synthesis on board

slide31
A Synthesis Problem

?

complete synthesis on board

slide32
Yet Another Conversion

?

complete synthesis on board

slide33
SYNTHESIS OF TRIPHENYLMETHANOL
  • via benzoyl chloride
  • via benzaldehyde
  • via benzyl alcohol
  • via benzophenone
slide35
Another Synthesis Problem

?

complete synthesis on board

slide36
C

D

A

B

PHARMACEUTICALS

Mexican Yams

“the pill”

must be

injected

better absorbed through

the stomach and intestines

than progesterone

STEROID

progesterone

slide37
Mexican Yams

CrO3

pyridine

H+

(-H2O)

acetal

protecting group

Na

NH3(liq)

Na

H3O+

norethisterone

ad