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Catalytic Hydrogenation
It is the reduction of compounds by hydrogen
adsorbed onto the surface of metals?. This
Learning Object shows the mechanism of catalytic
hydrogenation of ethene (C2H4) using platinum
catalyst.
Author Sukumar Honkote Department
of Chemistry, IIT Bombay
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Learning objectives

• After interacting with this learning object, the
  user will be able to
• explain the process of catalytic hydrogenation
  of ethene

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Definitions of the components
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1) Catalyst a substance that causes or
accelerates a chemical reaction without
itself being affected. 2) Adsorption the
binding of molecules or particles to a
surface. 3) Adsorbate An adsorbed
substance. 4) Adsorbent An substance on which
chemicals adsorb. 5) Physisorption is a process
in which the electronic structure of the
atom or molecule is barely perturbed upon
adsorption. 6) Chemisorption To take up and
chemically bind (a substance) onto the
surface of the adsorbent.
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Process
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Hydrogenation is a reaction where a given
compound reacts with hydrogen. In this process,
metals like nickel or platinum are used as
adsorbents. Hydrogen molecule gets chemically
adsorbed (binds) onto the metal surface. The
bond between the two hydrogen atoms is broken
and a bond between the metal surface and
hydrogen atom forms. A compound (in this case
ethene) comes near the hydrogen atoms adsorbed
to the metal. The bond between metal surface
and hydrogen atom is broken and a bond between
ethene and hydrogen is created. Thus ethene
reacts with hydrogen to form ethane.
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Table/Facts

• The adjacent diagram shows the ease of
  hydrogenation reaction with the compounds given
  and their products.

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Diagrams
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This is Ethene molecule. All the C-C-H angles are
120. All the C and the 4 H are in the same
plane. There is a double bond between the C
atoms.
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Lets call this figure H-Ethene. This is when
ethene is rotated nearly parallel to the
horizontal plane.
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Diagrams
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This is Ethane molecule. All the bond angles are
109. When viewed along the C-C bond, the HCH
bonds appear to be at 120 with each
other.
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For the animator
1) All molecules are 3D. 2) The molecules should
be randomly moving and rotating. 3) The
molecules can also leave the animation area. 4)
The animation should have 2-3 molecules of ethene
reacting with the adsorbed H Atoms at
different points on the catalyst
surface. In this IDD the animation has been shown
for only 1 molecule at a time
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Summary
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• Hydrogenation is the chemical reaction that
  results from the
• addition of hydrogen (H2).
• Hydrogenation is a process highly dependent on
  temperature and
• pressure.
• Catalysts are required for the reaction to be
  useful non-catalytic hydrogenation takes place
  only at very high temperatures.
• Process
• Hydrogen molecule gets chemically adsorbed
  (binds) onto
• the metal surface. The bond between the two
  hydrogen
• atoms is broken and a bond between the metal
  surface and
• hydrogen atom forms.
• A compound (in this case ethene) comes near the
  hydrogen
• atoms adsorbed to the metal. The bond between
  metal
• surface and hydrogen atom is broken and a bond
  between
• ethene and hydrogen is created.
• Thus ethene reacts with hydrogen to form ethane.
• On hydrogenating an alkyne with Lindlars
  catalyst (Pt BaSO4)
• we get only a cis alkene.
• Vanaspati ghee is obtained by hydrogenating
  vegetable oil with
• nickel catalyst.

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Questionnaire
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1. Hydrogenation is an example of __________
a) Physisorption b) Chemisorption ? 2. Which
of these compounds cannot get hydrogenated? a)
Hexene b) Cyclohexane c) Cyclopentene d) 1-
Pentyne 3. Which is the correct parameter to
measure the amount of catalyst for a
hydrogenation reaction? a. Surface area/ gram b.
Moles 4. Which of these compounds is easier to
hydrogenate? a. aromatic compound b. ketone c.
aldehyde
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Questionnaire (contd..)?
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5. What is the major product on hydrogenating the
following product using Lindlars catalyst (Pt
BaSO4) ? (the answer is a)? a)
b)? c)

d)? e)
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