catalytic reduction

1. CATALYTIC HYDROGENATION OF ALKENES
Dr. Alok Kumar Verma
Assistant Professor
KNIPSS, Sultanpur

2. • Addition of H-H across C=C
• Reduction in general is addition of H2 or its equivalent
• Catalytic reduction: 1. Heterogenous 2. Homogenous
• Heterogenous catalytic reduction requires metals like
Pt, Pd, or Ni as powders adsorbed on carbon and H2
Reduction of Alkenes
C C + H2 C C
H H
M
M = Pt, Pd, or Ni
2

3. C C
CH3
H
CH3
H
H2
Pt CH3CH2CH2CH3
C C
H
CH3
CH3
H
H2
Ni CH3CH2CH2CH3
H2
Ni CH3CH2CH2CH3
CH3CH2CH CH2
3
Examples

4. + H—H
s
p s
s
exothermic DH° = –136 kJ/mol
4
C C H C C
H H
H H
H
H
H
H
H
HYDROGENATION OF ETHYLENE

5. H H C C
A
B
X
Y
H H
MECHANISM OF CATALYTIC HYDROGENATION
5
Figure 1. Approch to the catalyst surface

6. H
C C
A
B
X
Y
H H H
MECHANISM OF CATALYTIC HYDROGENATION
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Figure 1. Adsorption of H2 Molecules to the catalyst surface

7. H
H H H
C
C
A
B
X
Y
MECHANISM OF CATALYTIC HYDROGENATION
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Figure 3. Adsorption of alkene molecule to the catalyst surface

8. H H
H
C
C
A
B
X
Y
H
MECHANISM OF CATALYTIC HYDROGENATION
8
Figure 4. Transfer of hydrogen atoms to carbon atom of the alkene

9. H H
H
C
C
A
B
X
Y
H
MECHANISM OF CATALYTIC HYDROGENATION
9
Figure 5. Release of product from the catalyst surface

10. Ni, Pd and Pt metals are common catalysts
Order of selectivity of various alkenes. Strained alkenes react faster.
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Exocyclic alkenes react faster than endocyclic alkenes.

11. There is syn addition to the most accessible face.
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Steric approach also control the Product formation.

12. Some functional group interact to the surface of the catalyst and enforce
the addition to that surface. This is known as haptophilicity.
12

13. 13
Homogenous catalytic reduction of alkenes:
Wilkinson’s Catalyst

14. 14
• The rates of hydrogenations decrease with increase in the alkyl
group substitution on double bond mirroring their relative
binding affinities to the metal center.
• Less substituted and sterically less hindered double bonds are
selectively hydrogenated.
• Exocyclic double bonds are selectively hydrogenated over
endocyclic double bonds.
• Cis alkenes are reduced rapidly than trans alkenes.
• Isolated double bonds are rapidly hydrogenated over
conjugated dienes. These hydrogenations involve stereospecific
syn hydrometallation of the multiple bond followed by
stereospecific reductive elimination.
• Hence the hydrogenation of olefins or alkynes result in syn
addition products.

15. HOMOGENOUS VS HETEROGENOUS CATALYSIS
S.No. Homogenous Heterogenous
1. Selectivity Almost non selective Stereoselective and
stereospecific
2. By products By products of
isomerization, double
bond migration and
hydrogenolysis are
observed
Relatively less by
products are obtained
3. Ease of removal the catalyst is
easily removed from the
reaction mixture by
filtration.
Removed by
chromatography
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16. ACKNOWLEDGEMENTS TO:
1. DIRECTOR UGC-HRDC KUMAUN UNIVERSITY, NAINITAL
2. PRINCIPAL, KNIPSS, SULTANPUR
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Thank you