A Seminar onCatalytic Hydrogenation

Submitted for credit of M. Pharm

ByS. F. Pimple

M. Pharm (P. Chemistry)Roll no. CH102

Under the guidance of Dr. S. B. Jadhav (HOD)

Progressive Education Society’s Modern College of Pharmacy, Yamunanagar, Nigdi, Pune-411044

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ContentsAim and objective

Definition

Types

Mechanism

Advantages

Limitations

Applications

References

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Aim and objective

To study the Catalytic Hydrogenation Reaction in detail and To understand its Mechanism.

REDUCTION

Definition:- An increase in hydrogen content or decrease in oxygen content of an organic compound is usually described an its reduction.

The reductive process fall into three categories:-The removal of oxygenThe addition of hydrogen andThe gain of electrons

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Types of reduction reaction

1. Reduction by desolving metals

2. Reduction by reductive cleavage

3. Reductive deoxygenation of carbonyl group

4. Reduction by hydride transfer reagent

5. Reduction by borane and dialkylborane as a hydride source

Catalytic Hydrogenation

Definition :- The addition of hydrogen to unsaturated system in the presence of catalyst OR addition of molecular hydrogen, as an reduction on metal.

Almost all the alkenes can be saturated in very high yield by treatment with hydrogen and a metal catalyst.

The hydrogenation of alkenes is an exothermic reaction.Mostly hydrogenation reactions are having high free energies of activation.

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CH3

CH

CH3

+ H2Ni

CH3

CH2

CH3

PropenePropane

Catalytic hydrogenation

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What is the need of catalyst? The reduction of alkenes with molecular hydrogen does not occur at room temperature without a catalyst but often take place at room temperature if metal catalyst is added.

Also the catalyst is able to provide a new pathway for the reaction with a lower free energy of activation.

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Catalyst used in catalytic hydrogenation reaction are of following :-

1. Palladium2. Adam’s catalyst3. Raney nickel4. Copper chromite5. Transfer hydrogenation6. Rhodium7. Ruthenium8. Triethylamine

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Types 1. Heterogeneous Catalytic Hydrogenation

•It is possible in that alkanes which are strained can be reduced catalytically by rupturing C-C cleavage would relieve the strain.

• The catalysts is not soluble in the reaction media, thus this process is referred to as a heterogenous catalysis.

• The catalyst assists in breaking the π-bond of the alkene and the H-H σ-bond.

• The reaction takes places on the surface of the catalyst. Thus, the rate of the reaction is proportional to the surface area of the catalyst.

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Heterogeneous Catalysis

• Heterogeneous catalysts dominate chemical and petrochemical industry: ~ 95% of all chemical processes use heterogenous catalysts.

• Carbon-carbon π-bond of alkenes and alkynes can be reduced to the corresponding saturated C-C bond. Other π-bond bond such as C=O (carbonyl) and C≡N are not easily reduced by catalytic hydrogenation. The C=C bonds of aryl rings are not easily reduced.

Ziegler-Natta CatalysisThe German chemist Karl Ziegler (1898-1973) discovered in 1953 that when TiCl3(s) and AlEt3 are combined together they produced an extremely active heterogeneous catalyst for the polymerization of ethylene at atmospheric pressure.

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1. Palladium An active form of palladium obtained from

palladium chloride more commonly the palladium chloride reduce in presence of charcoal or any other solid support on which the metal is deposited in a very finely divided state.

Example   

C CR

R

R

R

+Pd

C C

H

R

R

H

R

R +22

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2. Adam’s catalyst Chloroplastinic acid is fused with sodium nitrate to give a brown platinum oxide which can be stored. When required , it is treated with hydrogen to give a very finely divided black suspension of the metal.

C

C

O

O

OH

OH

H2-Pt CC

OO

OH

OH

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2. Homogeneous Catalytic Hydrogenation

1. Wilkinson`s Catalyst : Olefin Hydrogenation :- RhCl(PPh3)3 was the first highly active homogeneous hydrogenation catalyst and was discovered by Geoffrey Wilkinson (Nobel prize winner for Ferrocene) in 1964.

2. Hydroformylation :- The reaction of an alkene with carbon monoxide and hydrogen, catalyzed by cobalt or rhodium salts to form an aldehyde is called hydroformylation. Hydroformylation was discovered by Otto Roelen in 1938.

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3. Monsanto Acetic acid Process:- Monsanto developed the rhodium catalyzed process for the carbonylation of methanol to produce acetic acid.

4.Wacker Process:-This is one of the earliest industrial processes developed in Germany for the conversion of ethylene into acetaldehyde. Wacker process is more complex than other catalytic processes described above.

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• Catalytic steps in homogeneous reactionsMost catalytic process can be built up from a

small number of different types of step– Association / dissociation of a ligand requires labile complexes– Insertion and elimination reactions– Nucleophilic attack on a coordinated ligand– Oxidation and reduction of a metal center– Oxidative addition / reductive elimination

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Mechanism of Catalytic Hydrogenation:STEP 1: Hydrogen molecules react with the metal atoms at the catalyst surface. The relatively strong H-H sigma bond is broken and replaced with two weak metal-H bonds.

H H

C CA

B

X

YH H

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H

C CA

B

X

Y

H H H

STEP 2: The pi bond of the alkene interacts with the metal catalyst weakening the bond. A hydrogen atom is transferred from the catalyst surface to one of the carbons of the double bond.

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H

H H HCC

AB

XY

STEP 3: The pi bond of the alkene interacts with the metal catalyst weakening the bond.

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H HH

CC

AB

XY

H

A second hydrogen atom is transferred from the catalyst surface forming the alkane.The alkane is released from the catalyst's surface allowing the catalyst to acced pt additional hydrogen and alkene molecules.

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Advantages :

Relatively high specificityRelatively low reaction temperatures

far more easily studied from chemical & mechanistic aspects

far more active

Generally far more selective for a single product

Disadvantages :

o far more difficult for achieving product/catalyst separations

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Applications

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1. Reduction of aldehyde to alkane

2. Reduction of acid

3. Reduction of acyl chloride

4. Reduction of ester

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5.Reduction of ketone

6.Reduction of amide

7.Reduction of unhydride

References•Norman, Coxon ,“Principles of organic synthesis”, nelson thornes, Third edition, page no. 631-634.

• P.S. Kalsi, “ Organic Reaction and their mechanisms”, New age publication, Second edition, page no.453-462.

•G. R. Chatwal, “Reaction Mechanism and Reagents in Organic Chemistry”, Himalaya publishing house, page no. 1002-1006.

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Thank you