| Palladium on carbon (Pd/C) frequently ignites when it first comes in contact with methanol (and to a lesser extent,any flammable organic solvent) and as such represents a significant safetyrisk. These procedures are recommended whenever Pd/C is used in conjunction with hydrogen gas.
Hydrogenation using a balloon:
1) Evacuate the reaction vessel(this should have at least two openings) and backfill with an inert gas(nitrogen or argon). 2) Weigh out the desired amount ofPd/C and transfer into the reaction flask under an inert atmosphere. 3) Add a small amount of ethylacetate, dichloromethane, or toluene to the reaction flask, making sure to washdown any Pd/C stuck to the flask walls. All the Pd/C should be submerged at this point. 4) Carefully add methanol bycreating a stream down the side of the flask wall. 5) Add the reaction substrateeither as a solution or neat. 6) Begin stirring the reactionmixture and then evacuate the flask just until the solvent begins to bubble,then carefully backfill with inert gas. 7) Repeat step 6 twice more. 8) Attach a balloon of hydrogen toyour flask with an adapter that allows the balloon to be closed off from thereaction flask. 9) With the hydrogen balloonclosed off, evacuate the flask until the solvent begins to bubble, and thenopen the balloon to the flask. 10) Repeat step 9 twice more. Hydrogenation using a bomb reactor:
1) purge the bomb with inert gas, then follow steps 2-5 from theprocedure above (“hydrogenation using a balloon”). 2) Seal the bomb under an inert atmosphere and then evacuate it using amild vacuum source. Close the bomb offfrom the vacuum source and take it to the hydrogen source. 3) Connect the bomb directly to the hydrogen tank regulator usingthreaded fittings and fill the bomb with hydrogen up to the desired pressure(use the gauge on the bomb to determine the reaction pressure). 4) Close the bomb off from the hydrogen source so that it is nowsealed. Close off the hydrogen cylinderusing the regulator on the hydrogen tank, and disconnect your bomb from theregulator.
Your reaction isnow ready to be left to its own devices. When you are ready to work it up, use the following procedure. Workup:
1) For balloon reactions:
Detach the hydrogen balloon from the flaskand fill with inert atmosphere.For bomb reactions:
Connect a vent line to the bomb and thenrun the vent line to the back of the fume hood. Then slowly and carefully ventthe reaction mixture to atmospheric pressure.
2) Filter the reaction mixture through a bed ofCelite (or similar filter aid) 3) Taking care not to let thefilter cake filter to dryness, wash with the desired solvent (typically the same solvent used inthe reaction) 4) Disconnect the filter from the receivingflask, and then add several mL of water to the filter. 5) Discard the slurried Pd/C and filter aid in adedicated waste jar that contains water.
NOTE: If you must add catalyst to solution, prepare a slurry of Pd/C indichloromethane or toluene (using steps 1-3 above) and then add this slurry toyour reaction mixture. However, thisshould not be seen as a preferred method. |
