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Oxidation and reduction reaction, Essays (university) of Organic Chemistry

Niger Delta UniversityOrganic Chemistry

Oxidation and reduction reaction in organic synthesis

Typology: Essays (university)

2018/2019

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Lecture Notes
Chem 51B
S. King
Chapter 12 Oxidation & Reduction
I. Introduction
In this chapter we will discuss the oxidation and reduction of akenes, alkynes,
alcohols, ethers, and epoxides. Oxidation & reduction reactions are very valuable in
organic synthesis. Recognizing whether a compound is oxidized or reduced is an
important first step in being able to successfully choose the correct reagents in a
chemical transformation.
A. Recognizing Oxidation and Reduction of Organic Compounds
Oxidation-reduction reactions involve the gain and loss of electrons, and a
change in the oxidation state of the molecules involved:
Oxidation: loss of electrons
Reduction: gain of electrons
If one molecule is oxidized, another is reduced.
Electrons are TRANSFERRED COMPLETELY from one molecule to another.
For organic compounds, oxidation-reduction reactions result in a CHANGE IN
ELECTRON DENSITY around the carbon atom rather than a complete transfer of
electrons.
Oxidation: loss of "electron density"
Carbon "loses" electrons by forming bonds with elements that
are more electronegative than it is.
loss of:
gain of:
Cu + 2Ag+ Cu2+ + Ag
shiny red metal colorless blue silver crystals
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Lecture Notes

Chem 51B

S. King

Chapter 12 Oxidation & Reduction

I. Introduction

In this chapter we will discuss the oxidation and reduction of akenes, alkynes,

alcohols, ethers, and epoxides. Oxidation & reduction reactions are very valuable in

organic synthesis. Recognizing whether a compound is oxidized or reduced is an

important first step in being able to successfully choose the correct reagents in a

chemical transformation.

A. Recognizing Oxidation and Reduction of Organic Compounds

Oxidation-reduction reactions involve the gain and loss of electrons, and a

change in the oxidation state of the molecules involved:

Oxidation: loss of electrons

Reduction: gain of electrons

 If one molecule is oxidized, another is reduced.

 Electrons are TRANSFERRED COMPLETELY from one molecule to another.

For organic compounds, oxidation-reduction reactions result in a CHANGE IN

ELECTRON DENSITY around the carbon atom rather than a complete transfer of

electrons.

Oxidation: loss of " electron density"

Carbon " loses " electrons by forming bonds with elements that

are more electronegative than it is.

loss of:

gain of:

Cu + 2 Ag

Cu

2 +

+ Ag

shiny red metal colorless blue silver crystals

Reduction: gain of " electron density "

Carbon " gains " electrons by giving up bonds to more

electronegative elements and forming bonds with hydrogen

atoms instead.

loss of:

gain of:

The following shows stepwise oxidation of methane (the most reduced form of

carbon) all the way up to carbon dioxide (the most oxidized form of carbon):

Can you recognize oxidation and reduction in the following examples?

H C

H

H

H

HO C

H

H

H

C O

H

H

C O

HO

H

O C O

O

CH

3

C

OH

O

CH

2

CH

2

OH

C C

H H

H H

3

C

C C

H

3

C

HO

H

H

H

H

H

2

O

H

3

O

Example:

Q. How does this work?

A. The catalyst is in separate solid phase - catalysis occurs at the surface of

the metal.

• The alkene is adsorbed to the metal surface, forming bonds to the metal atom

(like a Lewis acid and Lewis base interaction with an alkene and a metal)

Result:

B. Hydrogenation of Other Double Bonds

Ketones and aldehydes can be reduced with H

2

& Ni, Pd, or Pt, although it is

much more common to use LiAlH

4

or NaBH

4

for the same reduction.

linoleic acid

OH

O

H

2

, Ni

OH

O

stearic acid

✦ The carbonyl groups of esters, amides, and carboxylic acids are resistant to

hydrogenation, and will not react with H

2

, Pd, Pt or Ni. Again, LiAlH

4

or

DIBAL are the reagents typically used for this type of reduction.

C. Reduction of Alkynes

Hydrogenation of an alkyne leads to addition of H

2

to one or to both of the π-

bonds. Use of Pd, Pt or Ni as a catalyst gives full hydrogenation to an alkane.

✦ Even if you use 1 eq. of H

2

, you will still get full hydrogenation. Why?

For partial hydrogenation of alkynes, two reagents are used, depending upon the

desired configuration of the product:

CH

3

C

H

O

H

2

Ni

CH

3

C

OH

O

H

2

Ni

  • NOTICE : When we hydrogenated linoleic

acid on page 90, the carboxylic

acid was not reduced.

CCH

3

CH

3

CH

2

C

Pd, Pt or Ni

H

2

CCH

3

CH

3

CH

2

C

Lindlar's

catalyst

NH

3

( l )

-78° C

H

2

Na

III. Oxidation Reactions

A. Oxidizing Agents Oxidizing agents fall into two categories:

1. Reagents that contain an O-O bond

2. Reagents that contain a metal-oxygen bond.

B. Epoxidation

Epoxidation is the addition of a single oxygen atom to an alkene to form an

epoxide. The most common way to do this is to use a peroxyacid (AKA:

peracids)

Mechanism:

RC OOH

O

O

2

O

3

H

2

O

2

OOH

ozone

hydrogen peroxide

tert-butylhydroperoxide

peroxy acid

C

O

OOH

benzoyl peroxide

Cr(VI):

O

Cr

O O

, H

2

SO

4

Na

2

Cr

2

O

7

, H

2

SO

4

N

H

O Cr

O

Cl

O

Mn(VII): KMnO

4

, NaOH potassium permanganate

Also: OsO

4

osmium tetroxide Ag

2

O silver oxide

pyridinium chlorochromate

(PCC)

RCH CHR

RC OOH

O

Example:

  • This reaction is stereoselective! E - 2 - butene gives only trans - 2,3-

dimethyloxirane, and Z - 2 - butene gives only cis - 2,3-dimethyloxirane.

  • This gives the same product obtained with intramolecular nucleophilic

substitution of halohydrins:

C. Sharpless Asymmetric Epoxidation

One of the most important reactions discovered in the last 25 years is titanium-

catalyzed asymmetric epoxidation of primary allyic alcohols discovered by

Barry Sharpless at MIT. He won the Nobel Prize for this work! The reaction

uses tert-butyl hydroperoxide, titanium tetraisopropoxide, and either (+)-diethyl

tartrate or (−)-diethyl tartrate.

OOH

C

O

Cl

meta - Chloroperoxybenzoic acid

Common

Peroxyacid:

C C

CH

3

H

H

H

3

C

m - CPBA

(E)- 2 - butene

CH

3

CH

2

OH

C C

CH

3

H

H

H

3

C

(E)- 2 - butene

C C

HO H

H Br

CH

3

H

3

C

Br

2

H

2

O

NaH

O

4

Ti

C

HOC H

C

HO COH

OH

H

O

O

C

H COH

C

HOC OH

HO

H

O

O

(+)-( R,R )-tartaric acid (-)-( S,S )-tartaric acid

CH

3

H C

3

C

CH

3

OOH

OsO

4

works in a similar manner:

ANTI - dihydroxylation requires 2 steps:

step 1: epoxidation

step 2: nucleophilic attack with HO

−

, or reaction with H

3

O

Example:

E. Oxidative Cleavage of Alkenes: Ozonolysis

The reaction of an alkene with ozone (O

3

) to yield products of double-bond

cleavage is called ozonolysis.

Reductive workup: X = H

Oxidative workup: X = OH

CH

3

H

OsO

4

H

2

O

2

H H

m - CPBA

chloroform

NaOH

H

2

O

C O

R

H

O

C

C

R

C

R

R

R

R

H

C O

R

HO

O C

R

R

Red.

workup

Ox.

workup

O

3

, CH

3

OH

O O

O

R

H

R

- 60 °C R

Ozonolysis also works with alkynes ( With alkynes, H

2

O is used in the second step )

Example:

☛ Ozonolysis can be used to synthesize aldehydes, ketones, and carboxylic

acids from alkenes, and it can be used to break molecules into smaller pieces.

IV. Oxidation of Alcohols

By far the most famous reagent for oxidizing alcohols is Chromic acid (H

2

CrO

4

). It

comes in various different forms, as shown below. Oxidation of primary alcohols

gives aldehydes or carboxylic acids, depending on the reagent chosen. Oxidation of

secondary alcohols gives ketones. Tertiary alcohols are not oxidized!

A. Oxidation with Chromic Acid (CrO

3

/H

2

SO

4

or Na

2

Cr

2

O

7

, H

2

SO

4

or

K

2

Cr

2

O

7

, H

2

SO

4

), a strong oxidizing agent.

1° : Forms Carboxylic acids

The reaction doesn't stop here!

O

3

, CH

3

OH

- 60 °C

R C C H

H

2

O

O

3

, CH

3

OH

- 60 °C - 60 °C

(CH

3

2

S

H

2

SO

4

, H

2

O

CH

3

CH

2

CH

2

CH

2

OH

CrO

3

B. Oxidation with Pyridinium Chlorochromate (PCC), a mild oxidizing agent.

Formation of PCC:

1° : Forms aldehydes

2° : Forms ketones.

3° : No reaction.

V. Green Chemistry

Several new oxidation methods are based on green chemistry. Green chemistry is

the use of environmentally benign methods to synthesize compounds. In “greening”

a chemical reaction, efforts are made to minimize waste, by-products, and solvent,

and choose safer reagents, especially ones made from renewable resources.

☛ Many oxidation reagents, in particular, are especially toxic:

N H

CH

2

Cl

2

CH

3

(CH

2

8

CH

2

OH

CrO

3

Cl

!

OH

CH

2

Cl

2

PCC

. CrO

3

/H

2

SO

4

is so acidic it will react with these.

For molecules containing & PCC doesn't react with

One especially nice alternative to conventional chromic acid oxidation uses a

polymer supported Cr

6+

reagent, Amberlyst A-26 resin-HCrO

4

−

, that avoids the use

of strong acid, and forms a Cr

3+

by-product that can be easily removed from the

product by filtration.

Advantages:

• Alcohol and Amberlyst are heated together without solvent

• Avoids use of strong acid, H

2

SO

4

• Cr

3+

can be filtered off without added solvent

• Cr

3+

can be regenerated and reused in a subsequent reaction.

Amberlyst A-26 resin-HCrO

4

−

oxidized 1° alcohols to aldehydes and 2° alcohols to

ketones.

Example:

VI. Designing Syntheses: Part 2 (continued from chapter 11)

Recall:

In planning a synthesis, we have to consider four things:

1. Construction of the carbon skeleton

2. Functional group interconversion

3. Control of regiochemistry

4. Control of stereochemistry

NH

3

NH

3

NH

3

NH

3

NH

3

NH

3

HCrO

4

HCrO

4

HCrO

4

HCrO

4

HCrO

4

HCrO

4

Amberlyst A- 26 resin

CH

3

(CH

2

8

CH

2

OH

HCrO

4

!

Example : Outline a stereospecific synthesis of meso - 3,4-dibromohexane starting

with compounds of two carbons or fewer.

Br

Br

Et

H

H

Et