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ALKENES AND ALKYNES II. OXIDATION AND REDUCTION OF ALKENES. With metals chemists usually define oxidation and reduction in terms of the loss or gain of electrons. Here are some new ways to look at oxidation and reduction from the viewpoint of organic compounds. OXIDATION.

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ALKENES AND ALKYNES II

OXIDATION AND REDUCTION

OF ALKENES


With metals chemists usually define

oxidation and reduction in terms of

the loss or gain of electrons.

Here are some new ways to look at

oxidation and reduction from the

viewpoint of organic compounds.



What is oxidation

CH4 + 2 O2

CO2 + 2 H2O

What is Oxidation?

gained 2 oxygens

lost 4 hydrogens

  • Loss of electrons

  • Gain of oxygen atoms

  • Loss of hydrogen atoms

+ O

- 2e

+ O

+ O

- 2e

- 2e

- 2e

- 2H+

INCREASING OXIDATION

d-

d+

Each oxygen, or an other

electronegative element,

takes electrons from

carbon, oxidizing it.

C

H

C

O


What is Oxidation?

more examples

(oxidized)

Co2+ - 1e-

Co3+

C O - 2H+ - 2e- C=O

(oxidized)

(= H2)

H H

(oxidized)

C H + O2 C=O + H2O

H

(= +O -H2)

C + O2 CO2

(oxidized)



What is Reduction?

  • Gain of electrons

  • Gain of hydrogen atoms

  • Loss of oxygen atoms

REDUCED

H2 can be viewed as

2H+ + 2e- , i.e., as

adding two electrons

lost (a bond to) oxygen

gained two hydrogens


What is Reduction?

more examples

(reduced)

Fe3+ + 1e-

Fe2+

C=O + 2H+ + 2e- C O

(reduced)

(= H2)

H H

C=O + 4H+ + 4e- C H + H2O

(= 2H2)

H

(reduced)


OVERVIEW OF

ALKENE OXIDATIONS


OXIDATIONS OF ALKENES

glycol

formation

epoxidation

complete

oxidation

cleavage

n

+ H2O


OVERVIEW

The first part of Chapter 14 deals with oxidations of alkenes.

glycol

cleavage

syn

over-oxidation

ozonide

anti

complete

oxidation

epoxide

glycol

SOME OXIDATION REACTIONS OF ALKENES


EPOXIDATION AND ANTI GLYCOL FORMATION

First we will look at

1) the formation of epoxides, and

2) the opening of epoxides to form

glycols

generally

“trans”

anti

unless opened in acid

and a good carbocation

can form

epoxide

glycol



R

R

R

R

EPOXIDES (OXIRANES)

cis

trans

O

O

planar

ring

1,2-Disubstituted oxiranes can have cis - trans isomerism.


MAKING EPOXIDES

TWO METHODS

1)

via bromohydrin

(Sections 8.7 and 12.8)

cis

cis

bromohydrin

alkene

epoxide

( peracid )

2)

using peracid

(Section 14.1)


METHOD ONE

SYNTHESIS OF EPOXIDES BY

CYCLIZATION OF BROMOHYDRINS


EPOXIDES FROM BROMOHYDRINS

carbonic

acid

unstable,

decomposes

bicarbonate

[ H2CO3 ]

..

Na+

:

..

CO2 + H2O

:

..

..

..

..

:

:

:

+ NaBr

NaHCO3

:

:

..

internal SN2


METHOD TWO

SYNTHESIS OF EPOXIDES

USING PERACIDS


PERACIDS

Carboxylic

Acid

Hydrogen peroxide

Organic Peroxide

A PERACID


TWO COMMONLY-USED PERACIDS

peracetic acid

m-chloroperbenzoic acid


PERACIDS ARE NOT REALLY ACIDS !

resonance

-H+

Acid

doesn’t

happen

Peracid

-H+

no resonance

resonance

Peracids are subject to

nucleophilic attack that

breaks the O-O bond


EPOXIDATION WITH A PERACID

A CONCERTED REACTION

both bonds

formed at the

same time

epoxide

STEREOSPECIFIC


.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

STEPWISE ANALYSIS OF THE

CONCERTED REACTION

An open carbocation

would allow cis - trans

interconversion.

_

:

..

:

:

+

:

..

All these steps

are merged

..

..

..

+

:

:

:

_

..

..

REMEMBER: “Concerted” means everthing

happens in one step.


THE REACTION IS STEREOSPECIFIC

RCO3H

RCO3H

cis

cis

trans

trans

This stereospecificity argues against the reaction being a

stepwise process.

An “open” carbocation

would have a lifetime and

would be free to rotate,

thereby interchanging

the cis and trans

arrangement of the

R groups before the

ring closes.

..

..

..

..

+

+

cis

trans



OPENING EPOXIDES IN AQUEOUS MEDIUM

GIVES GLYCOLS (1,2-DIOLS)

“anti”

glycol

“syn”

glycol

Ring opening can be

either syn or anti

depending on conditions.

1,2-diols

“glycols”

1,2-diols are frequently called “glycols”


SN2

anti only

OPENING EPOXIDES

basic solution

(Section 12.8)

:

..

..

..

:

:

:

:

:

:

:

:

:

..

:

:

SN2 : hydroxyl attacks at

leastsubstituted carbon,

STEREOSPECIFIC

and on the opposite side


ACID SOLUTION

In acid solution epoxides can open by

either SN1 or SN2 depending on the

substitution pattern.

Epoxides with only primary or secondary cabons

react primarily via the SN2 mechanism.

attack occurs at the

least-substituted

carbon

1o

2o

Epoxides with tertiary carbons (or the possibility

of resonance) will react via the SN1 mechanism.

3o

attack occurs at the

most-substituted

carbon

resonance


opens only

if tertiary or

conjugated

asymmetric

bridged ion

anti only

OPENING EPOXIDES

water attacks at most

substituted carbon

acidic solution

(Section 12.8)

SN1

:

:

..

syn + anti

..

:

:

:

:

:

..

+

:

SN2

:

:

water attacks at least

substituted carbon

NOT STEREOSPECIFIC

opening in acid frequently

gives “anti” but more often

gives a mixture: syn + anti


anti GLYCOL FORMATION

SUMMARY


anti -Glycol Formation

accomplished by opening of epoxides

H2O

(H+ or OH-)

BASE : always anti (SN2) STEREOSPECIFIC

ACID : usually anti ,

unless a stable carbocation can form

then you get syn + anti(SN1)

NOT RELIABLY

STEREOSPECIFIC


SYN GLYCOLS AND CLEAVAGE

glycol

cleavage

syn

“syn”

over-

oxidation

Next we will look at

1) the syn formation of glycols

(aldehydes)

2) cleavage of syn glycols

3) oxidation of aldehydes


syn GLYCOL FORMATION

trans - Glycols and anti -glycols are made

by opening epoxides …..

is there a stereospecific method to

make cis -glycols or syn -glycols ?


YES :

syn -Glycol Formation

syn glycols are made with OsO4 or KMnO4

OsO4

osmium tetroxide

KMnO4

potassium permanganate


:

:

:

:

:

:

:

:

O

O

O

O

..

..

..

O

s

Mn

O

O

O

O

:

:

:

:

..

..

..

..

-

CYCLIC 5-RING INTERMEDIATES

These reagents react via cyclic five-membered

ring intermediates

The ring controls the stereochemistry to be syn.

Both oxygens attach to the same side of the C=C bond.



H

O

O

s

O

N

a

H

S

O

4

3

H

O

2

O

s

O

N

a

H

S

O

4

3

SYN ADDITION GIVES CISGLYCOLS

C

H

3

C

H

H

3

2

O

H

H

O

H

cis

C

H

3

C

H

H

3

C

H

C

syn

conformation

C

H

C

H

3

3

C

H

O

H

C

H

O

H

meso

cis -2-butene

remember: addition is syn

result is cis

What is the result with

trans -2-butene?


MECHANISM

Notice the

transfer of

2e- onto Os

= REDUCTION

of Osmium

NaHSO3 /

Both of the hydroxyl

oxygens in the glycol

come from OsO4

OXIDIZED

further reduced

REDUCED

Os + 4H2O

NaHSO3



MECHANISM

K+

KMnO4

oxidized

purple

reduced

brown sludge


KMn

O

4

KMn

O

4

C

H

3

C

H

H

3

O

H

dilute, neutral, 0oC

H

O

H

cis

C

H

3

C

H

H

3

C

H

C

C

H

C

H

3

3

C

H

O

H

C

H

dilute, neutral, 0oC

O

H

meso

What is the result with

trans -2-butene?

Same result as with OsO4.



PERMANGANATE TEST FOR A C=C DOUBLE

BOND

POSITIVE TEST

Baeyer Test

KMnO4 reacts

implies the presence

of a double bond

KMnO4

brown

sludge

MnO2

(+)

purple

NEGATIVE TEST

(-)

KMnO4 is not

decolorized

compound with a

suspected double bond

implies that a double

bond is not present


BROMINE TEST FOR A C=C DOUBLE BOND

POSITIVE TEST

DO YOU REMEMBER ?

Br2 reacts

implies the presence

of a double bond

Br2 / CCl4

(+)

purple

colorless

NEGATIVE TEST

(-)

Br2 is not decolorized

(does not react)

compound with a

suspected double bond

implies that a double

bond is not present


KMnO4

MORE VIGOROUS CONDITIONS


MORE VIGOROUS OXIDATION WITH KMnO4

Given enough time, or if the temperature is increased,

or if the solution is made strongly acidic or basic, KMnO4

will oxidize almost any organic compound to carbon

dioxide and water.

partial

oxidation

Notice how

changing the

conditions

gives three

different

results.

glycol

formation

the compound is destroyed

Notice that the sidechain oxidizes more easily than the benzene ring.


Cleavage of syn Glycols

glycol

cleavage

“syn”

Cleavage of glycols

over-

oxidation

(aldehydes)


CLEAVAGE OF GLYCOLS

HIO4

periodic acid


CLEAVAGE OF syn - GLYCOLS

HIO4

O

O

H

H

HO

OH

H

H

two carbonyl compounds

Once again … 5-ring cyclic intermediates are involved.


PERIODIC ACID WILL CLEAVE syn - GLYCOLS

H

H

H

H

HO

OH

HIO4

..

cyclic

intermediate

..

:

:

:

:

O

O

:

:

..

O

O

I

I

O

O

:

:

:

:

:

:

O

O

..

..

..

..

-

-

hydrolysis

H

O

2

O

O

H

H

H

O

O

H

..

HIO4 is a

mild reagent

aldehydes do

not oxidize.

I

:

:

O

O

:

..

..

-


OH

HO

HIO4 CLEAVES ONLY syn - GLYCOLS

CYCLIC

GLYCOLS

HIO4

no reaction

HIO4

ACYCLIC

GLYCOLS

anti glycols will not cleave

unless they can rotate to syn


POTASSIUM PERMANGANATE

Will also cleave syn glycols …..


PERMANGANATE ALSO CLEAVES syn-GLYCOLS

H

H

H

H

HO

OH

KMnO4

..

cyclic

intermediate

..

:

:

:

:

O

O

:

:

..

O

O

Mn

Mn

O

O

:

:

:

:

:

:

O

O

..

..

..

..

-

-

hydrolysis

H

O

2

O

O

This is not

dilute, neutral, cold.

Different conditions

are used.

HO

OH

H

O

O

H

..

harsh method

aldehydes

oxidize

Mn

:

:

O

O

:

..

..

-


ALDEHYDES ARE EASY TO OXIDIZE !

non-specific oxidizing agent

[O]

Almost any oxidizing agent will work.

reagents include : O2 (air), H2O2, KMnO4 etc etc


gentle

harsh

EXAMPLE : HIO4 vs KMnO4

aldehyde

survives

HIO4

cis

aldehyde is

oxidized to

carboxylic

acid

KMnO4

cis

Remember: given enough time

KMnO4 will oxidize completely.


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