Hydrogenation is a chemical reaction between hydrogen gas and another compound or element in the presence of a catalyst such as nickel, palladium or platinum. It involves the addition of hydrogen atoms to reduce double or triple bonds. There are three types of reductions that differ in how hydrogen is added. The reaction rate depends on factors like temperature, pressure, catalyst surface area, time, and the ratio of hydrogen to the compound being hydrogenated. In general, higher temperatures and pressures increase the reaction rate.

2. Hydrogenation
 Hydrogenation – to treat with hydrogen – is a chemical
reaction between molecular hydrogen (H2) and another
compound or element, usually in the presence of
a catalyst such as nickel, palladium or platinum.
 The process is commonly employed to reduce or saturate
organic compounds.
 Hydrogenation typically constitutes the addition of pairs
of hydrogen atoms to a molecule, often an alkene.
 Catalysts are required for the reaction
 non-catalytic hydrogenation takes place only at very high
temperatures.
 Hydrogenation reduces double and triple bonds
in hydrocarbons

3. Hydrogenation
 Hydrogenation reaction is basically a reduction
reaction
 Various reduction reaction takes place. For example
reduction of alkenes, reduction of alkynes.
 In hydrogenation H2 is added as reducing agents in
one of three ways.

4. 4
• There are three types of reductions differing in how H2 is
added.
• The simplest reducing agent is H2. Reductions using H2 are
carried out with a metal catalyst.
• A second way is to add two protons and two electrons to a
substrate—that is, H2 = 2H+ + 2e-.
Reductions of this sort use alkali metals as a source of
electrons, and liquid ammonia as a source of protons.
These are called dissolving metal reductions.
Reducing Agents

5. 5
• The third way to add H2 is to add hydride (H¯) and a proton
(H+).
• The most common hydride reducing agents contain a hydrogen
atom bonded to boron or aluminum. Simple examples include
sodium borohydride (NaBH4) and lithium aluminum hydride
(LiAlH4).
• NaBH4 and LiAlH4 deliver H¯ to the substrate, and then a
proton is added from H2O or an alcohol.

6. 6
• Reduction takes place by addition of H2 as a reducing agent.
• The addition of H2 occurs only in the presence of a metal catalyst, and
thus it is called catalytic hydrogenation.
• The catalyst consists of a metal—usually Pd, Pt, or Ni, adsorbed onto a
finely divided inert solid, such as charcoal.
• H2 adds in
a syn fashion.
Reduction of Alkenes—Catalytic Hydrogenation

7. 7
• The Ho of hydrogenation, also known as the heat of hydrogenation,
can be used as a measure of the relative stability of two different alkenes
that are hydrogenated to form the same alkane.
• When hydrogenation of two alkenes gives the same alkane, the more
stable alkene has the smaller heat of hydrogenation.

8. Mechanism of hydrogenation of alkenes

9. 9
• The mechanism explains two facts about hydrogenation:

10. 10
• There are three different ways in which H2 can add to the triple bond:
Reduction of Alkynes

11. 11
Alkane formation:
Reduction of an Alkyne to an Alkane

12. 12
• Palladium metal is too reactive to allow hydrogenation of an
alkyne to stop after one equivalent of H2 adds.
• To stop at a cis alkene, a less active Pd catalyst is used—Pd
adsorbed onto CaCO3 with added lead(II) acetate and quinoline.
This is called Lindlar’s catalyst.
• Compared to Pd metal, the Lindlar catalyst is deactivated or
“poisoned”.
• With the Lindlar catalyst, one equivalent of H2 adds to an alkyne
to form the cis product. The cis alkene product is unreactive to
further reduction.
Reduction of an Alkyne to a Cis Alkene

13. 13
• Reduction of an alkyne to a cis alkene is a stereoselective reaction,
because only one stereoisomer is formed.

14. 14
• In a dissolving metal reduction (such as Na in NH3), the elements
of H2 are added in an anti fashion to form a trans alkene.
Reduction of an Alkyne to a Trans Alkene

15. 15

16. 16
Summary of Alkyne Reductions
Figure 12.5
Summary: Three methods to
reduce a triple bond

17. Thermodynamics and kinetics of
hydrogenation
Factors affect the hydrogenation reaction are;
• Temperature
• Pressure
• Catalyst surface
• Time
• Ratio of hydrogen to substance being hydrogenated

18. Temperature effect
 For the most part, the temperature for hydrogenation
reactions is usually below 400°C, except in reactions where
pyrolytic decomposition occurs concurrently with the
hydrogenation reactions
 Temperature is one of the most important variables
affecting a reaction
 hydrogenation reaction can be reversed by increasing
temperature.
 So hydrogenation reaction necessary occurs at low
temperature, where the reaction is satisfactory
 Catalyst affect the speed and course of reaction, while
temperature affects the equilibrium, speed, path or course
of reaction.

19. Temperature effect
 increasing temperature adversely affects the equilibrium
position,
 so that the maximum ultimate yield is decreased;
 but it affects favorably the speed of a reaction, so that in a
given time a greater quantity of product can be obtained.
 Fortunately in recent years knowledge of catalysis is
extended so satisfactory reactions are possible at lower
temperature. Where more satisfactory equilibirium
position is prevailed.
 In some cases increasing temperature has adverse effect on
catalyst,
 so the catalyst activity decreases and resultant rate of
reaction decreases.
 This type of case is known sintering of catalyst

20. Temperature effect
 In general, the noble-metal catalysts, such as platinum
or palladium, are used from room temperatures to
150°C
 catalysts of the nickel and copper type, from 150-
250°C
 various combinations of metals and metal oxides, from
250-400°C.

21. Pressure effect
 Pressure, like temperature, can affect the rate of
reaction.
 The rate of reaction is generally increased by
increasing pressure, because a gas phase is usually
present, and increased pressure gives increased
concentration.
 Pressure increases the equilibrium yield in a
hydrogenation reaction when there is a decrease in the
volume of the reaction as it proceeds.
 This is the simple application of the mass-action law,
or Le Chatelier's principle.

22. Pressure effect
 In general, however, increased pressure will result in
an increased reaction rate.
 Thus, Brochet observed that phenol is hydrogenated
very slowly at 150°C at atmospheric pressure using a
nickel catalyst but that at 15 atm at the same
temperature the reaction was complete and rapid.
 Armstrong reported hydrogenation of acetone to
isopropyl alcohol with identical batches of a copper
chromite catalyst and observed the following as shown
in table.

23. Pressure effect
EFFECT OF PRESSURE IN THE HYDROGENATION OF ACETONE
Pressure(atm) Conversion%
35 17
148 70
212 95

24. Catalyst surface
 For the most part, hydrogenation catalysts are solids
consisting of metals and metal oxides
 . The hydrogenation is effected at the surface of the
catalyst; so a highly extended surface is essential.
 Taking a piece of bar nickel or copper and subdividing
it mechanically to pass, say, a 50-mesh sieve would not
produce an active nickel or copper catalyst.
 Usually, the preparation of a catalyst is associated with
some chemical reaction whereby a highly extended,
porous, and honeycombed surface is produced so that
the density of the surface metal is far less than that of
the bulk metal.

25. Catalyst surface
 Certain surface atoms may become so removed from other
adjoining ones that they may approach a gasified state, at
conditions far removed from the normal vaporization of the
metal.
 These surface atoms, having varying degrees of unsaturation
compared with the bulk metal or metal oxide, will strongly
adsorb other substances with which they may come in
contact, and active catalysts usually have high absorptive
powers.
 Although absorption is closely related to the successful
performance of a catalyst
 Thus speed of a hydrogenation will depend on the type and
amount of active surface available. Increasing the ratio of
catalyst to the substance undergoing hydrogenation usually
increases the speed of the hydrogenation

26. Time
 The time necessary for a hydrogenation reaction may
vary from a few seconds to several hours, depending
on the materials being hydrogenated, the catalyst, the
temperature, and the pressure.
 In general, the more reactive the compound, the faster
the hydrogenation reaction.
 Thus, simple aldehydes are hydrogenated very readily,
whereas the reduction of aromatic rings to saturated
cyclic compounds is a or of esters to alcohols is a
slower reaction.

27. Ratio of Hydrogen to the
Substances Being Hydrogenated
 The ratio of hydrogen to the substance being
hydrogenated is conveniently expressed in terms of
partial pressures.
 It frequently happens that the speed or path of a
certain hydrogenation can be affected by the
proportion of hydrogen to the substance
 it has been found that ethyl lactate and malonate are
reduced to the corresponding alcohols in good yields
in a flow system at pressures of about 1,300 psig, where
practically the entire pressure is hydrogen and the
partial pressure of the esters is only a few centimeters.

28. Ratio of Hydrogen to the
Substances Being Hydrogenated
 In the examples previously cited, the higher total
pressure was lessened, and a higher ratio of hydrogen
pressure to the partial pressure of the substance being
hydrogenated was employed.

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