You may have heard about the mishap at the University of Missouri in which a mixture of hydrogen and oxygen gases ignited, doing a great deal of damage to the lab, as well as injuring 4 people. Fortunately no one was killed. A report with photos was put up on Derek Lowe’s blog.
Hydrogen and oxygen mixtures are obviously dangerous in the right (or rather wrong) proportion but most of the danger in hydrogenation type reactions comes from the active catalysts used in such reactions. That they absorb hydrogen gas is bad enough but even in the absence of hydrogen, they can spark spontaneously. I would warrant that many chemists who have filtered their palladium on carbon catalyst through celite and let it dry out too long will have seen some sparks flying. Stronger catalysts – dry 10% palladium on carbon for example – will do it even as you try and weigh it out, which is obviously an issue when you are about to add highly flammable organic solvents to it. Adding hydrogen to the mix as well, sometimes at high pressure and temperature as well, makes hydrogenation procedures an occasionally daunting prospect. It is not a coincidence that many places have dedicated areas for hydrogenation reactions. Some have entirely separate buildings.
Correctly done, these reactions can be routinely run safely. A lot of problems come from bad habits; people run a reaction using only a few milligrams of catalyst with no problem (or no problem they observed) and then when they come to do a similar reaction on a larger scale, the throw it all in and hope for the best approach begins to fall down.
Common bad habits include not using a blanket of nitrogen (or argon, which is even better) while handling the catalyst. It should also be weighed out first, not added to the solvent, as a spark as it is being added will obviously have bad consequences, whereas adding the solvent to the catalyst under nitrogen is much safer.
Methanol is a commonly used solvent in hydrogenation but it has a couple of problems. It is very flammable but worse it burns with an almost invisible flame, so the chemist may not even realize they have a situation. It is also not the best solvent for the purpose, as it has relatively poor hydrogen solubility; other higher alcohols and esters are significantly better. This is one where I think people have just always done it that way, so people read other people’s procedures and copy it, continuing the circle. They do work, so no one sees a reason to change it.
The catalyst is at the heart of this whole reaction but there are a variety of catalysts available and the wet versions are much safer. That might mean a lower activity or could give problems in the reaction, but I have never noticed any loss of activity in using wet catalyst. The things I make are usually not too water sensitive, so that is rarely an issue.
The introduction of hydrogen to the system presents a new hazard, but there are a number of transfer reagents that mean no need for a balloon, such as ammonium formate and hydrazine monohydrate. I have had a great deal of success with such methods, but with difficult substrates they are unsuitable: something that might require high pressure hydrogenation will likely prove recalcitrant.
High pressure reactions are an area where it is hard to find a simpler safer alternative. The key here is to make sure you know how to use the equipment, don’t over-pressurize and don’t use damaged equipment. I have seen in my time glass used for the reaction chamber but metal ones seem to becoming more the standard. We do have glass ones for our Parr bottles here. Soemthing overly scratched up or chipped should be discarded and shielding should be in place while the reaction is pressurized.
There is one alternative for all these reactions, that can use high temperature and high pressure. The flow reactor H-cube uses sealed up catalyst so you never come into contact and generates its hydrogen internally (so no cylinder either). It is an effective piece of equipment and as long as you are not the one paying for it, you will be enamored of it if you try it. It is not a cheap option though, unfortunately (although perhaps in time they will become more affordable) and until they are, we will have to do some of our hydrogenations the old fashioned way.