Hydrogenation- definition, catalytic hydrogenation, homogeneous and heterogeneous catalytic hydrogenation, mechanism of catalytic hydrogenation, advantages and disadvantages of catalytic hydrogenation, applications of catalytic hydrogenation
2. HYDROGENTAION
1. Catalytic Hydrogenation
2. Homogenous and heterogeneous catalytic
hydrogenation
3. Mechanism of catalytic hydrogenation
4. Advantage and disadvantages
5. Applications
3. HYDROGENATION
Hydrogenation- it is the catalyzed addition of molecular hydrogen.
Example:- hydrogenation using nickel as catalyst,
CH3-CH=CH2 + H2 →CH3-CH2-CH3
propene propane
Hydrogenation is a chemical reaction between molecular hydrogen
and another compound or element, usually in presence of a
catalyst such as nickel, palladium or platinum. The process is
commonly employed to reduce or saturate organic compound.
Non catalytic hydrogenation takes place only at high temperatures.
It has three components:- 1) the unsaturated substrate, 2) the
hydrogen or hydrogen source, 3) a catalyst. The reduction reaction
is carried out at different temperature and pressure depending
upon the substrate and the activity of the catalyst.
Reduction :- the removal of oxygen or addition of hydrogen or gain
of electron.
4. 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 alkenes can be saturated in very high yield by
treatment with hydrogen and a metal catalyst.
WHAT IS THE NEED OF CATALYST?
The reduction of alkenes with molecular hydrogen does
not occur at room temperature without a catalyst but
often takes 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.
6. HETEROGENEOUS CATALYTIC HYDROGENATION
Palladium- Active form of palladium is obtained from
palladium chloride. It is reduced in presence of charcoal
or any other solid support on which the metal is
deposited in a very finely divided state.
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.
7. HOMOGENEOUS CATALYTIC
HYDROGENATION
There are several disadvantages of heterogeneous catalytic
hydrogenation:-
1. Lack of selectivity if more than one unsaturated center is
present.
2. Double bond migration may occur.
3. Several groups undergo an easy hydrogenolysis may occur.
Many of these difficulties could be overcome by
homogenous catalytic hydrogenation in which the metal
gets replaced by a soluble complex of rhodium or
ruthenium and conduct the reduction in homogenous
solution.
Many soluble catalyst have been used and the more
effective include the ones derived from rhodium and
ruthenium.
8. HOMOGENEOUS CATALYTIC
HYDROGENATION
The rhodium complex used is (Ph3P)3RhCl called as
Wilkinson’s catalyst [tris(triphenylphosphine)chlororhodium].
Wilkinson’s catalyst has an advantage that where olefins and
acetylenes get reduced, other common group like C=O, C≡N,
NO2 remain unaffected. Thus selective reduction can be
carried out.
Monsanto acetic acid process:- Monsanto developed the
rhodium catalyzed process for the carbonylation of methanol
to produce acetic acid.
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.
Hydroformylation:- The reaction of an alkene with carbon
monoxide and hydrogen, catalyzed by cobalt or rhodium salts
to form an aldehyde.
9. MECHANISM OF CATLYTIC
HYDROGENATION
STEP 1- Hydrogen molecules react with the metal atoms at the
catalyst surface. The relatively strong H-H σ bond is broken and
replaced with two weak metal-H bonds.
STEP 2- The π bond of the alkenes interacts with the metal catalyst,
weakening its bond. A hydrogen atom is transferred from the
catalyst surface to one of the carbons of the double bond.
STEP 3- The π bond of alkene interacts with the metal catalyst. A
second hydrogen atom is transferred from the catalyst surface
forming the alkane.
STEP 4-The alkane is released from the catalyst’s surface allowing
the catalyst to accept additional hydrogen and alkene molecules.
14. CATALYTIC HYDROGENATION
Advantages :-
Relatively high specificity.
Relatively low reaction temperature.
Generally far more selective for a single product.
Far more active
Disadvantages :-
Far more difficult to achieve product / catalyst
separation.
