Hydrogenation in Organic Synthesis
Catalytic hydrogenation carries the risk of fire and/or explosion. Read these instructions carefully. If in doubt about any operation, then see your supervisor. Protective equipment must always be worn and the reaction must always be carried out in a fumehood.
Reactions with hydrogen both under low and high pressure are not unduly hazardous provided the user maintains the hydrogenation vessels in good condition and operates it carefully. It is the users responsibility to realise that hydrogen is highly flammable and that pressures and reaction rates must be carefully controlled at all times.
1. HANDLING OF HYDROGENATION CATALYSTS
A wide variety of catalysts can be used in hydrogenations at atmospheric pressure. All should be handled with care before, during, and after the reaction. All catalysts must be handled cautiously because of their highly reactive nature. Do not add dry catalyst to a bottle containing a flammable solution or vapour without a counterflow of inert gas. The vapour might ignite causing a fire.
Raney Nickel: This is usually stored under ethanol or methanol or under the solvent to be used in the hydrogenation. It is pyrophoric when dry and must be kept moist with the solvent at all times. In cases where large volumes of solvent are undesirable, a spoon is useful for introducing a thick suspension of Raney Nickel in solvent into the hydrogenation flask; otherwise transfer can employ a glass filter funnel and plenty of solvent to wash the catalyst suspension into the flask.
Platinum and Palladium: Modern preparations are extremely active, particularly platinum black, platinised charcoal (Pt/C) and palladised charcoal (Pd/C). When mixed with an organic solvent in air, these catalysts may cause the solvent to ignite.
Hence, place the organic substrate and solvent in a suitable flask (preferably long-necked). The flask should not be more than one-third full. Expel air with a gentle stream of N2 or Ar for about 1 min. Then add the catalyst against the counter current of inert gas; use solvent to rinse down the sides of the flask. The inert gas stream may then be turned off.
Alternatively, add the catalyst first and cover it immediately with the sample in solution.
other noble metal catalysts should be treated similarly.
If breakage or spills occur, flush the contaminated area immediately with large volumes of water and keep the area wet until all traces of catalyst have been removed.
2. ATMOSPHERIC PRESSURE WORK
In most chemistry departments a technique has been developed whereby a rubber balloon is used as the hydrogen reservoir. This is a potentially dangerous practice and the use of balloons for containing hydrogen must be specifically sanctioned by individual research supervisors. If this technique is used, you need to make sure that there are no heat sources in the fumehood and that other fumehood users are aware that hydrogen is being used.
3. HIGH PRESSURE HYDROGENATIONS
If you wish to use this facility you should speak to Jim Tweedie in the first instance.
Description: A steel bomb of total capacity of ca. 250 ml is available for reactions at high pressure, with or without increase in temperature in the special operations laboratory. Pressures up to 250 atmospheres, at a maximum temperature of 250 °C, are possible.
Technical personnel should be present when:
The bomb is being assembled and dismantled.
The bomb is being charged with H2 or when H2 is being released from the equipment.
It is the responsibility of the student to:
Load the bomb with the reactants, solvent, and catalyst.
Install a safety shield and make people aware that a high pressure reaction is taking place
Remove the products and spent catalyst.
Clean out the bomb thoroughly and dry it.
The student should periodically monitor the experiment to check the temperature and monitor pressure. Pressure readings are only approximate.
Acidic solvents and reactions producing acidic by-products should be strictly avoided. Pitting of the steel bomb can occur, leading to a weakening of the structure.
Pd and Pt catalysts are not generally suitable for hydrogenations at elevated temperatures owing to their sensitivity to poisoning.
The bomb is fitted with a bursting diaphragm, which is a safety device in the event of an excessive pressure build-up. If the diaphragm does burst, the contents of the bomb are sprayed into the room. Hence, reactions with toxic substances are not allowed. If in doubt, consult with your supervisor.
4. WORK-UP OF THE REACTION MIXTURE
Vacuum filtration through a paper filter can be dangerous if air is drawn through the filter in the presence of a catalyst. Hence always ensure that the catalyst is kept moist with solvent to prevent spontaneous ignition. Use either gravity filtration through a fluted filter paper or partial reduced pressure filtration at the water pump. In the latter case it is usually necessary to use a pad of Kieselguhr (i.e. Celite) on the Buchner funnel filter paper to entrain the very finely divided catalyst particles. Do not suck dry! Wash the catalyst with a little pure solvent.
The moist catalyst should be washed (use water) into a bottle for later recovery. If a catalyst recovery bottle is not available, and you do not intend to do further hydrogenations, then wash it down the sink with plenty of water.
Note: Separate bottles should be kept for the various noble metal catalysts (Pd, Pt, Rh, Ir, etc.). Contamination with Kieselguhr does not matter. Disposal is arranged as for other heavy metals.
Any problems with the high pressure equipment should be reported to Jim Tweedie.
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