elimination of hydrogen

TOPIC: OXIDATION

ASSIGNED BY: DR.SAJID MEHMOOD

ASSIGNED TO: 01,02,04

AMNA , AZWAH AND SIDRA MARYUM

ORGANIC CHEMISTRY PAPER 2

PREPARED BY: AZWAH ALI

UNIVERSITY OF EDUCATION ,TOWNSHIP CAMPUS LAHORE

CONTENTS:

DEFINITION OF DEHYDROGENATION:

Aromatization of Six-Membered rings in presence of

hydrogenation catalysts.(Pt,S,Se or Quionones):

Dehydrogenations yielding Carbon-Carbon double Bonds:

Dehydrogenation of Alcohals to Aldehydes and Ketones:

Oxidation of Phenols and Aromatic Amines to Quinones:

Dehydrogenation of Amines:

Oxidation of Hydrazines,Hydrazones,and Hydroxylamines:

DEFINITION

“A chemical reaction involving elimination of hydrogen

from a compound to produce a less saturated analog”

Dehydrogenation reactions fall into six

classes

1. Aromatization of Six-Membered rings in presence of hydrogenation

catalysts.(Pt,S,Se or Quionones)

2. Dehydrogenations yielding Carbon-Carbon double Bonds

3. Dehydrogenation of Alcohals to Aldehydes and Ketones

4. Oxidation of Phenols and Aromatic Amines to Quinones

5. Dehydrogenation of Amines

6. Oxidation of Hydrazines,Hydrazones,and Hydroxylamines

1. Aromatization of six membered

rings

Hexahydro-terelimination

Six membered alicyclic rings can be

aromatized in number of ways

Six membered alicyclic rings can be aromatized in number of ways.

1. Hydrogenation catalysts such as palladium ,platinium,and nickel.

CYCLOHEXENE has been detected as an intermediate in the conversion

of CYCLOHEXANE to BENZENE,using Pt.

Pt

300-350

intermediate

2.The elements SULPHUR and SELENIUM which

combine with hydrogen evolved to give

respectively H2S and HSe.

3.Quionones which become reduced to corresponding Hydroquinones.

Chloranil(2,3,5,6 tetrachloro-1,4-benzoquinone) and DDQ (2,3-dichloro-5,6-

dicyano-1,4-benzoquinone) are important quinones used for aromatizations.It

is likely that the mechanism involves a transfer of hydride to the quinone

oxygen,followed by the transfer of a proton to the phenolate ion

.

2:Dehydrogenations yielding Carbon-

Carbon double Bonds

Dihydro-elimination

It is a synthesis developed by Leonard and Co.workers in which tertiary amines give Enamines

when treated with mercuric acetate,in this case the initial product is the iminium ion 1 which

loses a proton to give the enamines.

2-In another example ,the oxidizing agent SeO2 can in certain

cases convert a carbonyl compound to an à,ß-unsaturated

carbonyl compound by removing H2.

The reaction has been most often applied in

Steroid series

3:Dehydrogenation of Alcohals to

Aldehydes and Ketones

C,O-Dihydro-elimination

RCHOcopper

chromite

RCH2OH

RCHOHR~K2Cr2O7

H2SO4

RCOR`

Primary alcohals can be converted to aldehydes and

Secondary alcohals to ketones in seven main ways,whose

names are as following but explanation here is not of concern.

1. Oppenauer oxidation

2. With DMSO based reagents

3. TEMPO and related reagents

4. With hypervalent Iodine reagent

5. By catalytic dehydrogenation

6. Miscellaneous reagents

4:Oxidation of Phenols and Aromatic Amines to

Quinones

1/O,6/O-Dihydroelimination:

O

OOH

OHcyclohexa-2,5-diene-1,4-dione

hydroquinone

K2CrO7

H2SO4

Ortho and para diols are easily oxidized to Ortho- and para –

quinones respectively

Many oxidizing agents have been

used:

1. Acid dichromate

2. Silver oxide

3. Silver carbonate

4. Lead tetraacetate

5. HIO4

6. NBS_H2O_H2SO4

7. MnO2 on bentonite with microwave irriadiation

8. Dimethyl dioxarane

In below case,it seems to vary with the oxidizing agent.For

oxidation of catechol with NaIO4,it was found that the reaction

conducted in H2 18O gave unlabeled quinone,so the following

mechanism was proposed .

OH

OH

NaIO4

O

OH

I3O

O

O

pyrocatechol2-hydroxyphenoxy diiodohypoiodite

cyclohexa-3,5-diene-1,2-dione

5:Dehydrogenation of Amines

1/1/N,2/2/C-Tetrahydrobielimination

RCH2NH2 RCN

Primary amines at a primary carbon can be dehydrogenated to

nitriles.The reaction has been carried out with a variety of reagents

among others.

1. Lead acetate

2. NaOCl

3. K2S2O8/NiSO4

4. Me3N/O/OsO4

5. Ru/Al2O3/O2

6. CuCl/O2/pyridine

6:Oxidation of Hydrazines,Hydrazones,and

Hydroxylamines

1/N,2/N-Dihydroelimination

NaOBrNH NH

1,2-diphenylhydrazine

N N

1,2-diphenyldiazene

N,N`-Diarylhydrazines(hydrazo compounds) are oxidized to azo

compounds by several oxidizing agents including

NaOBr,HgO,K3Fe(CN)6under phase transfer conditions or with

galvinoxyl.

Hydrazines (both alkyl and aryl) substituted on only one side also

give azo compounds ,but these are unstable and decompose to

nitrogen and the hydrocarbon

Hydrazines (both alkyl and aryl) substituted on only one side also

give azo compounds ,but these are unstable and decompose to

nitrogen and the hydrocarbon

NH NH2

1-phenylhydrazine

N NH

1-phenyldiazene

H

+ N2

Benzene

Hydrazones are oxidized with HgO,Ag2O,MnO2,lead acetate .or

certain other agents, Diazo compounds are obtained.

R2C N NH2

HgO

R2C N N

+ _

Aromatic hydroxylamine are easily oxidized to nitroso compounds

most commonly by acid dichromate.

NH OH

N-hydroxybenzenamine

N O

1-nitrosobenzene

Acid dichromate

Thank you