Redoks Alcohol HO

8/3/2019 Redoks Alcohol HO

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REDOX REACTIONS


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Oxidation is a loss of electrons a more positive oxidation

number a loss of hydrogen atoms the addition of oxygenatoms. more bonds to oxygen

CH3 CH3 CH3 CH2

OH

CH3 C H

O

CH3 C OH

O

CO2 H2O+

Alkane Alcohol (1) Aldehyde Carboxylic acid

1 Bond to O 2 Bonds to O 3 Bonds to O

[O] [O] [O][O]

Reduction is a gain of electrons a less positive oxidation

number a gain of hydrogen atoms the loss of oxygen atoms the loss of bonds tooxygen.

REDOX


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Oxidation numbers

Each C-H is a -1

Each C-O is a +1

Sum total C-H and C-O to determineOxidation Number


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Oxidation numbers

Each C-H is a -1

Each C-O is a +1

What are the oxidation numbers for

CH3

OH CH2

O

HCO2H

Hint: Write the expandedformula to determine thenumber of bonds to C


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1, 2, 3 Carbons

Chapter 11 5

=>


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OXIDATION REACTIONS

PrimarySecondaryTertiary

ALCOHOLS


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1 Alcohols are oxidized to either aldehydes or carboxylic acids,depending on the reagent.

Oxidation of 1 Alcohols

PCC = Pyridinium chlorochromate


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Oxidation of Secondary Alcohol

Secondary alcohol Ketone[O]

R1 CH

OH

R2[O]

R1 C

O

R2 H2O

R1, R2: alkylgroups

2 AlcoholKetone

Double bondsformsTwo hydrogen atoms removed


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Oxidation and Reduction

Any of the Cr6+ oxidants effectively oxidize 2 alcohols to ketones.



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Metabolic example

Lactic aciddehyrogenase

Lactic acid Pyruvicacid

C H 3 C H

O H

C O H

O

C H 3 C

O

C O H

O


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Oxidation of Tertiary Alcohol

Tertiary alcohol No reaction[O]

R1, R2, R3: alkyl groups

3 Alcohol

ly one hydrogen atom, not the two required to be removed

R1-C-R2

R3

OH[O]

NR


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Alcohols are oxidized to a variety of carbonyl compounds.

Oxidation of Alcohols


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REDUCTION REACTIONSALCOHOLS


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Reduction of Alcohols

Dehydrate with conc. H2SO4, then add

H2

Tosylate, then reduce with LiAlH4

CH3CHCH3

OHH2SO4

CH2 CHCH3

H2

PtCH3CH2CH3

alcohol alkene alkane

alcohol

CH3CHCH3OH TsCl

CH3CHCH3OTs LiAlH4

alkane

CH3CH2CH3

tosylate


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Dehydration Mechanisms

CH3CHCH3

OHH2SO4

alcohol

CH3CHCH3

OH

H

CH3CHCH3

CH2 CHCH3H2O


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Dehydration of Alcohols to form Alkenes

During the dehydration of an alcohol, a Hand OH are removed from adjacentcarbon atoms of the same alcohol toproduce a water molecule. A double bond

forms between the same two carbonatoms to produce an alkene.

H OH

H+, heat

H-C-C-H H-C=C-H + H2O

H H H H

alcohol (ethanol) alkene (ethene)


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Saytzeffs Rule

The dehydration of a secondary alcoholcan result in the formation of either of twoproducts.

Saytzeffs rule

THE POOR GET POORERrule


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Saytzeffs Rule

The major product is the one that results

when the hydrogen is removed from thecarbon atom with the smallest number ofhydrogen atoms. The poor get poorer

H

OH

H+

Heat

+H2O

+H2O

2-Butene (major product: 90%)

1-Butene (minor product: 10%)2-Butanol

2H

3H

THE MOST SUBSTITUTED C=C IS THE MAJOR PRODUCT


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Alkenes

Oxidative Cleavage of Alkenes

Oxidative Without Cleavage of Alkenes


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Addition of Water to Alkenes:Hydroboration

Herbert Brown (HB) invented hydroboration (HB)

Borane (BH3) is electron deficient is a Lewis acid.

Borane adds to an alkene to give an

organoborane.

C CB

H

H H

Borane

+BH2H

Organoborane


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BH3 Is a Lewis Acid

Six electrons in outer shell

Coordinates to oxygen electron pairs in ethers

O id ti With t Cl f Alk


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Addition of H-BH2 (from BH3-THF complex) to threealkenes gives a trialkylborane

Oxidation with alkaline hydrogen peroxide in waterproduces the alcohol derived from the alkene

Hydroboration-Oxidation Alcohol Formation fromAlkenes

Oxidative Without Cleavage of Alkenes


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Orientation in Hydration viaHydroboration

Regiochemistry is opposite to Markovnikovorientation

OH is added to carbon with most Hs

H and OH add with syn stereochemistry, to thesame face of the alkene (opposite of anti addition)

CH3BH3

THF

H

CH3

H

B

HH -

OH

H2O2

H

CH3

H

OH

1-methylcycopentene Alkylborane intermediate trans-2-methylcyclopentanol(85%)


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Mechanism of Hydroboration

Borane is a Lewisacid

Alkene is Lewis base

Transition stateinvolves anionicdevelopment on B

The components of

BH3 are across C=C


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Hydroboration, Electronic Effects Give Non-Markovnikov

More stable carbocation is also consistentwith steric preferences


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Hydroboration - Oxygen InsertionStep

H2O2, OH- inserts OH in place of B

Retains syn orientation


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Epoxidation is the addition of a single oxygen atom to an

alkene to form an epoxide.Epoxidation is typically carried out with a peroxyacid.

Epoxidation

E id ti


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Epoxidation occurs via syn addition of an O atom to either side of aplanar double bond. Thus, a cis alkene gives an epoxide with cissubstituents. A trans alkene gives an epoxide with trans substituents.

Epoxidation

Epoxidation is stereospecific because cis and trans alkenes yield differentstereoisomers as products.


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Dihydroxylation is the addition of two hydroxy groups to a double bond,forming a 1,2-diol or glycol.

Depending on the reagent, the two new OH groups can be added to theopposite sides (anti addition) or the same side (syn addition) of thedouble bond.

Dihydroxylation

Dih d l ti


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Anti dihydroxylation is achieved in two stepsepoxidation,

followed by ring opening with OH or H3O+.

Dihydroxylation


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Syn hydroxylation results when an alkene is treatedwith either KMnO4 or OsO4.

Dihydroxylation

Dih dro lation


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Each reagent adds two oxygen atoms in a syn fashion.

Hydrolysis of the cyclic intermediate cleaves the metal oxygenbonds, forming a cis-1,2-diol.

Dihydroxylation


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Dihydroxylation can also be carried out by using a catalytic

amount of OsO4, if the oxidant N-methylmorpholine N-oxide(NMO) is also added.

In the catalytic process, dihydroxylation of the double bondconverts the Os8+ oxidant into an Os6+ product, which is thenre-oxidized by NMO to Os8+.

Dihydroxylation


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Oxidative cleavage of an alkene breaks both the and bonds

of the double bond to form two carbonyl compounds. Cleavagewith ozone (O3) is called ozonolysis.



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Addition of O3 to the bond of an alkene forms an unstableintermediate called a molozonide, which rearranges to anozonide in a stepwise process.

The unstable ozonide is reduced to afford carbonylcompounds. Zn (in H2O) or dimethylsulfide (CH3SCH3) are two

common reagents used to convert the ozonide into carbonylcompounds.



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Ozonolysis of dienes or other polyenes results in oxidativecleavage of all C=C bonds.

It is important to note that when oxidative cleavage involves adouble bond that is part of a ring, the ring opens up affording asingle chain with two carbonyls at the carbons where the

double bonds were originally.



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Examples of Ozonolysis of Alkenes

Used in determination of structure of anunknown alkene


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Permanganate Oxidation of Alkenes

Oxidizing reagents other than ozone alsocleave alkenes

Potassium permanganate (KMnO4) can produce

carboxylic acids and carbon dioxide if Hs arepresent on C=C

+ KMnO4

O

O

H

O

O H

+


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Cleavage of 1,2-diols

Reaction of a 1,2-diol with periodic (per-iodic)acid, HIO4 , cleaves the diol into two carbonyl

compounds

Sequence of diol formation with OsO4 followed

by diol cleavage is a good alternative toozonolysis


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Mechanism of Periodic Acid Oxidation

Via cyclic periodate intermediate


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Reduction of Alkenes: Hydrogenation

Addition of H-H across C=C

Reduction in general is addition of H2 or its

equivalent

Requires Pt or Pd as powders on carbon andH2

Hydrogen is first adsorbed on catalyst

Reaction is heterogeneous (process is not in

solution)


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Hydrogen Addition- Selectivity

Selective for C=C. No reaction with C=O, C=N

Polyunsaturated liquid oils become solids


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Mechanism of Catalytic Hydrogenation

Heterogeneous reaction between phases

Addition of H-H is syn


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Other Oxidation Reagents

Collins reagent: Cr2O3 in pyridine

Jones reagent: chromic acid inacetone

KMnO4 (strong oxidizer)

Nitric acid (strong oxidizer)

CuO, 300C (industrialdehydrogenation)

Swern oxidation: dimethylsulfoxide,with oxalyl chloride and hindered

base, oxidizes 2alcohols to ketones45


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ethanol orange Acetic acid green

Breathalyzer test forsuspected drunk driversCH3 CH2

OH

CH3 C H

O

Cr6+[O]

+ + Cr3+O

CH3 CH2

OH

CH3 C H

O

CO2H

2O

+

Oxidation of Alcohol in the body[O][O]


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Figure 12.10

Blood alcohol screening

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Redoks Alcohol HO