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Catalytic Hydrogenation Author: Sukumar Honkote Department of Chemistry, IIT Bombay 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 (C 2 H 4 ) using platinum catalyst.

Learning objectives: After interacting with this learning object, the user will be able to:  explain the process of catalytic hydrogenation of ethene

Definitions of the components: 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

Process: 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

 The adjacent diagram shows the ease of hydrogenation reaction with the compounds given and their products. Table/Facts:

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 Lets call this figure H-Ethene. This is when ethene is rotated nearly parallel to the horizontal plane. Diagrams:

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 Diagrams

This is the surface of the catalyst. This is called the H Atom. It is connected to the catalyst (as shown in the animation figure). This is Hydrogen molecule. Lets call figures like this in pair of H atoms attached to the surface as Adsorbed H Atoms.

Master Layout Surface of the catalyst Hydrogen atom Hydrogen molecule Ethene molecule Ethane molecule Ethene molecule rotated to be parallel to catalyst surface

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

Step 1: Reaction Hydrogenation of Ethene Description of the actionAudio Narration 1) Show the formula image from left to right. Ethene reacts with hydrogen in the presence of a finely divided platinum catalyst at a temperature of about 150°C. Ethane is produced. Let us understand the reaction mechanism at the molecular level. EtheneEthane

Description of the actionAudio Narration Display the entire master layout. All the molecules are moving continuously. The catalyst used is platinum. In this slide only animation part is shown. When hydrogen (2 yellow spheres) moves down and hit the grey surface, it gets stuck to the surface (check animation done in this slide) Step 2: Adsorption of Hydrogen

Description of the actionAudio Narration Display the entire master layout. All the molecules are moving continuously. Hydrogen molecule gets chemisorbed onto the platinum surface and the H-H bond is broken. In this slide only animation part is shown. Once Hydrogen spheres are stuck show following 2 things at same time- a)Brown lines forming (fade in) b) Yellow bond between hydrogen becomes thin and disappears. Step 3: Formation and breaking of bonds

Description of the actionAudio Narration Display the entire master layout and as above. All the molecules are moving continuously. The ethene molecule will align itself parallel to the catalyst surface The arrow indicates the movement of ethene molecule from position 1 to 2. When it is coming down it rotates to come to position 2 Step 4: 1 2

Description of the actionAudio Narration Display the entire master layout and as above. All the molecules are moving continuously. A bond between the adsorbed hydrogen atoms and ethene starts to form. Only the animation part is shown in this slide. Show the following two things at the same time 1)Brown lines from H to C 2)Show the green line becoming thin (since one bond is broken) Step 5.1: Formation and breaking of bonds

Description of the actionAudio Narration Display the entire master layout and as above. All the molecules are moving continuously. The hydrogen atoms of ethene are pushed upwards to provide conformally stable intermediate. Only animation part is shown in this slide. At the same time show the Hydrogen (yellow spheres) attached to carbon (green spheres) move upwards as shown in figure above. Step 5.2: Formation and breaking of bonds

Description of the actionAudio Narration Display the entire master layout and as above. All the molecules are moving continuously. Only animation part is shown in this slide. The brown lines from catalyst to hydrogen disappear. Step 5.3: Formation and breaking of bonds

Description of the actionAudio Narration Display the entire master layout and as above. All the molecules are moving continuously. Thus Ethene is reduced by breaking its double bond to form the saturated ethane. Steps 5.1 to 5.3 should happen at the same time and then show step 5.4 Rearrange the Hydrogen atoms (yellow spheres) so that the molecule looks like as shown above.C-C-H angle should be 109°. (rotate 1 st carbon from left in previous figure by ) Step 5.4: Formation of ethane

 Hydrogenation is the chemical reaction that results from the addition of hydrogen (H 2 ).  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 Lindlar’s catalyst (Pt + BaSO 4 ) we get only a cis alkene.  Vanaspati ghee is obtained by hydrogenating vegetable oil with nickel catalyst Summary:

1. Hydrogenation is an example of __________ a) Physisorption b) Chemisorption ‏ 2. Which of these compounds cannot get hydrogenated? a) Hexeneb) Cyclohexanec) Cyclopentene d) 1- Pentyne 3. Which is the correct parameter to measure the amount of catalyst for a hydrogenation reaction? a. Surface area/ gramb. Moles 4. Which of these compounds is easier to hydrogenate? a. aromatic compoundb. ketonec. aldehyde Questionnaire

5. What is the major product on hydrogenating the following product using Lindlar’s catalyst (Pt + BaSO 4 ) ? (the answer is a) a) b) c) d) e) Questionnaire (contd..)