This document summarizes catalytic hydrogenation, including: - Heterogenous catalytic hydrogenation occurs on metal surfaces like Ni, Pd, Pt. - Homogenous hydrogenation uses complexes like Wilkinson's catalyst in solution. - Rate of reduction decreases with increased substitution. Selective reduction of double bonds is possible. - Catalytic hydrogenation is stereospecific, giving syn-addition and racemic mixtures for cis-alkenes.

1. DEPARTMENT OF CHEMISTRY
TOPIC – CATALYTIC HYDROGENATION
Presented By
Thippeswamy B A
M.Sc. 2nd Semester
JSTP21CHE028
JSS SCIENCE AND TECHNOLOGY
UNIVERSITY
MYSURU

2. INDEX
 INTRODUCTION
 CLASSIFICATION
 HETEROGENOUS HYDROGENATION
 HOMOGENOUS HYDROGENATION
 RATE OF REDUCTION
 STEREOCHEMISTRY
 PROBLEMS
 REFERANCE

3. INTRODUCTION
 A reaction in which hydrogen molecules are added to an alkene
is called a hydrogenation, the reaction is also called reduction
reaction. Alkenes will undergo an addition reaction with
hydrogen in the presence of a catalyst to form alkanes. Catalytic
hydrogenation is the one of the most convenient method to
reduction of alkene. Catalytic hydrogenation is categorised into 2
types
1) Heterogenous Catalytic Hydrogenation.
2) Homogenous Catalytic Hydrogenation

4. HETEROGENOUS HYDROGENATION.
 Here the reaction will occurs on the surface of metal by absorption of hydrogen. Some
of the metals which or involved in the reaction such as Ni, Pd, and Pt.
 General Mechanism

5. Rate of reduction
 The case of reduction of an alkene decreases with degree of substitution that is some
times allows selective reduction of one double bond, if more than one double bond is
present with in the molecule.
 Degree of substitution is different then selective reduction takes place.

6.  Where as if degree of substitution is equal then completely reduced compound will be
only the product.

7. Stereochemistry
 Catalytic hydrogenation is a stereospecific reaction and gives syn-addition that means
cis -alkene gives racemic mixture.

8.  If the compound is meso, then it contains more than one assymetric centre or chiral
carbon and plane of symmetry present.

9.  But experimentally it has been found that the addition of hydrogen is not completely
syn-addition.

10.  To obtain selective reduction of C=C bond in the presence of other unsaturated groups
in the molecule, Palladium is the best catalyst.

11. Homogenous hydrogenation
 It is proceed by Wilkinson catalyst and Hydrogen. The use of complexes of Rhodium or
Ruthenium as catalysts enables hydrogenation to be carried out in homogenous
solution.
 [(PPh3)3RhCl] = Wilkinson catalyst is a well known organometallic compound.
 Chloridotris(triphenyl phosphine) Rhodium[I]

13.  It is also gives syn-addition.

14.  Generally Wilkinson catalyst is used for reduction of non -conjugated alkenes.
 Functional groups like ,Oxo, Nitro, halo, Cyano, azo etc are not reduced by Wilkinson
catalyst, so selective reduction of alkene may occur in the presence of these groups.

15.  Less substituted double bond are more rapidly reduced than more substituted double
bonds.

16. CSIR PROBLEMS
1) Reaction of Cyclohexyl benzyl ether with hydrogen in the presence of 10% Pd/C yields-
--------
(A) Cyclohexanol and toluene
(B) Cyclohexanol and benzyl alcohol
(C) Cyclohexane and benzyl alcohol
(D) Cyclohexane and toluene

17. 2) Catalytic hydrogenation of the following compound produces saturated
hydrocarbon(s). The number of stereoisomer(s) formed is
A)1 B)2 C)3 D)4

19. REFERENCE
 Organic Reaction Mechanism- V.K.Ahluwalia
 Principles of Organic Synthesis-R.O.C.Norman and J.M.Coxon