Cyclohexadiene (Just uphill from benzene)

Integral to organic chemistry is hydrogenation - sometimes you need to make a functional group something a little less oxidized. For example, an azide or a nitrile can become an amine.

Often, we just use hydrogen for this! Metals like palladium can work some real magic doing the heavy lifting here. Process chemists hate hydrogen gas, though, and rightly so - everyone knows about its tendency to explode. It's worse, though - most hydrocarbons have a flammable range of about 1-8% in air - any more, and there's just not enough oxygen to support combustion. Any less, and you don't get enough heat to keep the reaction going. Hydrogen, however, starts a little higher, about 4%, and is still flammable in air up to a whopping 75%! The stuff's dangerous.

Fortunately, palladium can rip a hydrogen off of just about anything and move it to something else, as long as it's energetically "downhill." When you're not using hydrogen, it's called transfer hydrogenation. Cyclohexadiene, if it gives up a couple hydrogens, becomes benzene, downhill by nearly 6 kcals!

Often, this will suffice for a hydrogenation. Something a bench chemist runs at work will likely use hydrogen on the first go (although we use transfer hydrogenation at the bench sometimes!) because it works well. Unfortunately, you have to run it at high pressure in a vessel unsettlingly named a "bomb," and the process guys tend not to like bombs. Transfer hydrogenation will get a close look.

This is not to say that it's never used. In fact, using dihydrogen for hydrogenation at the process scale is often unavoidable (you don't want benzene in your cooking oil, or drugs!). Your Crisco and trans fats are products of a gaseous hydrogenation process.

Please correct me on any of this, process chemists, it's a big day when I work at 10 gram scales.