D. Oxidation of Alcohols to Ketones 1. Introduction: In the previous ...

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D. Oxidation of Alcohols to Ketones 1. Introduction: In the previous ...

Chemistry 212-10

Retrosynthetic Analysis-3: Synthesis of Ketones

D. Oxidation of Alcohols to Ketones

1. Introduction:

In the previous section we learned how to use C-C disconnections, FGI’s of Grignard reagents and FGI dehydrations to

synthesize larger alkenes and alcohols from smaller ketones or aldehydes and organic halides. This unit introduces methods

for synthesizing ketones from alcohols, another FGI, since these reactions change functional groups, but do not alter the carbon

structure of the reactant. The addition of this reaction to our synthetic repertoire greatly increases our abilities to build larger

molecules from a number of smaller molecules.

2. Oxidation ofAlcohols to Ketones:

Chromium Trioxide (CrO 3 ) in acidic aqueous solution (H 2 SO 4 ) is a common reagent for oxidation of alcohols. This metal

oxide reagent is a relatively strong oxidizing reagent. It smoothly converts 2˚ alcohols to ketones.

HO

O

H

+

CrO 3

H 2 SO 4

H

OH

+

CrO 3

H 2 SO 4

O

3. FGI-oxidation:

The following FGI Retrosynthetic step indicates that the ketone can be synthesized from the corresponding alcohol by

oxidation.

O

FGI

H

OH

oxidation


Retrosynthetic Analysis-2: Synthesis of Ketones 2

Although this is a simple addition to our reagent list, this oxidation increases our abilities to synthesize complex molecules from

relatively simple ones. Consider the potential synthesis of the complex molecule shown below. One Grignard C-C disconnection

reveals that the target molecule can be synthesized from phenyl magnesium chloride and 2,5-dimethyl-4-heptanone.

H

O

C-C disconnection

Grignard

O

+

Mg

Cl

With only the Grignard C-C disconnection, we were able to disconnect the carbon structure of an alcohol only once.

However, we now see that the ketone precursor revealed by the above disconnection can be synthesized from a 2˚ alcohol as

indicated on page 80 and reproduced below:

O

FGI

H

OH

oxidation

This FGI reveals a compound that can be further disconnected with a Grignard C-C to reveal even simpler synthetic

precursors.

H

OH

C-C disconnection

Grignard

H

O

+

Mg

Br


Retrosynthetic Analysis-2: Synthesis of Ketones 2

If we recall that Grignard Reagents can be synthesized from alkyl halides, we can look at the entire synthetic pathway as a set

of disconnections and FGI's. The starting materials are enclosed in rectangles.

H

O

C-C disconnection

Grignard

O

FGI

Oxidation

+

Mg Cl

FGI

Grig

O

H

+

C-C disconnection

Grignard

H

OH

Cl

+ Mg o

Mg

Br

FGI

Grig

Br

+ Mg o


Retrosynthetic Analysis-2: Synthesis of Ketones 2

4. Out of Class Application:

For our next lab discussion period use retrosynthetic analysis to devise syntheses for the following compounds from

monosubstituted aromatic compounds and non-aromatic compounds with four or fewer carbon atoms.

OH

O

Then write complete synthetic paths, including reagents and reaction conditions based on your retrosynthetic analyses.

Be prepared to discuss both the retroanalysis and the synthesis.

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