3
$\begingroup$

I was planning on doing some small scale hydrogenation practicals at atmospheric pressure (probably from Vogel) and ran into a couple of questions. I will be using the "balloon method" (described in Procter's Advanced Practical Organic Chemistry book) as I consider it an easier technique for small scale reactions than constructing a Brown hydrogenation apparatus. Note that I'm working in an amateur environment (i.e. my home lab which does not have a fumehood). Now for the questions:

  • What rule of thumb can be used to determine the amount of catalyst (in these cases $\ce{Pd/C}$) needed per mole of hydrogen used (I suppose these two variables are depending on each other)?
  • There are different types of $\ce{Pd/C}$ out there: 3%, 5% or 10% $\ce{Pd}$ by weight, with or without a variable amount of added $\ce{Pb}$, different solvents (mostly I see is water) and also there are different bases available, not just palladium on carbon. I understand that $\ce{Pb}$ is added to poison the catalyst such as with Lindlar catalyst, but if total reduction is required then why would this be necessary? Also how would I decide between 3%, 5% or 10% $\ce{Pd}$ by weight?
  • What is an effective method for generating the hydrogen to fill the balloon with? I suppose that normally a pressurized hydrogen tank would be used for this, but I find this overkill for the reaction I'm planning. Furthermore, the use of pressurized hydrogen tanks is not quite without danger and I'm not trained to handle them safely.
  • Why would it be required to flush the reaction mixture with an inert gas after the hydrogenation has completed? I understand that this is necessary because the dissolved hydrogen gas can ignite and possibly explode when, while filtering the catalyst, the catalyst spontaneously ignites (e.g. when the catalyst is not suspended enough and direct contact with the atmosphere is possible). However, why is an inert gas advised? What is wrong with ordinary air?
  • What is the function of Celite when filtering the catalyst?
  • How many times can the $\ce{Pd/C}$ catalyst be reused? How would you spot whether or not your catalyst has become too poisoned to be successfully used in hydrogenation?
$\endgroup$
  • 1
    $\begingroup$ The answer to some (but not all!) of your questions are probably in this video. It's a pretty cool demo of "home lab" hydrogenation. youtube.com/watch?v=oqdDWA9-DSY $\endgroup$ – Curt F. Mar 4 '15 at 19:13
2
$\begingroup$

I've adddressed a part of your question here.

As far as the catalyst is concerned, 3-5% Pd/C is fine.

What is an effective method for generating the hydrogen to fill the balloon with? I suppose that normally a pressurized hydrogen tank would be used for this, but I find this overkill for the reaction I'm planning. Furthermore, the use of pressurized hydrogen tanks is not quite without danger and I'm not trained to handle them safely.

In the lab, I only used pressurized tanks. While commercial hydrogen generators are available - they are mostly used to generate hydrogen gas as a carrier gas for GC/MS - these things cost a fortune and really would be overkill.

Actually, pressurized tanks are pretty safe if you follow some rules:

  • Obviously, you can't attach the pressure regulator and operate the whole thing unless the steel cap over the main valve is removed. Don't start anything unless the tank is secured to the lab bench or the wall with a clamp, a chain or another sturdy contraption that prevents the tank from falling over! You want hydrogenation, not a rocket ;)

  • Get familiar with the pressure regulator.

  • When you're done, remove the regulator again and put the cap over the main valve. Keep the regulator at a dry and clean place.
  • No grease on the threads!

When operated with a bit of common sense, a small pressurized tank is the most convenient option.

What is the function of Celite when filtering the catalyst?

When you filter off the catalyst, you'll typically use a suction filter with a base of sintered glass (fritted glass). Palladium on charcoal is a very fine powder that will clog the fritted glass. A pad of Celite in the suction will prevent this. Celite is cheap, suction filters are not!

$\endgroup$
  • $\begingroup$ Thanks for your answer! Could you perhaps also address the other points? I cannot really accept it as an answer to the question if not the entire question is addressed. $\endgroup$ – Jori Mar 6 '15 at 12:45
1
$\begingroup$

Make sure you do not have any flames, anything which makes sparks or other sources of ignition near you when you do hydrogenation.

I have done plenty of hydrogenations at atmospheric preassure, I tend to use a gas burrete to follow the reaction. It is possible to see the change of rate when you consume all of the vinyl groups and then start to react a trisubsistuted alkene.

I would suggest that you consider using the nickel cataylst which is formed from nickel chloride and sodium borohydride, I have found it is a cheap and nice cataylst. It also is less pyrophoric than many other cataylsts when it has been exposed to hydrogen and is then exposed to air.

I have found that the easy way to do it is to take a 50 ml burette and turn it upside down, stuff a bung in it with a hole connected to a rubber tube which goes to a separation funnel. Put copper sulfate solution in the funnel. The blue colour makes it easy to read it more quickly and stops things growing in the water. The other end of the burette needs to be connected to the reaction flask.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.