Hydrogenation

Hydrogenation

1. Hydrogenation & reaction Mechanism thermodynamics and kinetics

2. Hydrogenation Hydrogenation  – to treat with hydrogen – is a  chemical reaction between molecular hydrogen (H 2 ) 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 H 2 is added. The simplest reducing agent is H 2 . Reductions using H 2 are carried out with a metal catalyst. A second way is to add two protons and two electrons to a substrate—that is, H 2 = 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 H 2 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 (NaBH 4 ) and lithium aluminum hydride (LiAlH 4 ) . NaBH 4 and LiAlH 4 deliver H¯ to the substrate, and then a proton is added from H 2 O or an alcohol.

6. 6 Reduction takes place by addition of H 2 as a reducing agent. The addition of H 2 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. H 2 adds in a syn fashion . Reduction of Alkenes—Catalytic Hydrogenation

7. 7 The H o 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 H 2 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 H 2 adds. To stop at a cis alkene , a less active Pd catalyst is used—Pd adsorbed onto CaCO 3 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 H 2 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 NH 3 ), the elements of H 2 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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