Friday, February 15, 2008

Catalytic Hydrogenation - now fire free

In my experience you should not do synthetic organic chemistry if:
(A) You are drunk (or the following day when you have a hang over)
(B) You are in a hurry
Recently, I violated Commandment B and decided to put a large scale catalytic hydrogenation on really, really fast. Bad idea!
Now hopefully most of you realise that the addition of palladium on charcoal (or charcoal alone) to organic solvents can result in instant bonfire. Unfortunately the chemists hand is often right above the flask as the catalyst gets added and some very nasty burns can result. Even worse the whole thing may take off and you'll frantically be looking for a fire extinguisher (Always know where the fire extinguisher is. You will need it one day).
There is an easy way to avoid this problem. If you first remove all atmospheric air inside your flask there will be no fire. In other words what you have to do is flush the flask with nitrogen or argon. I normally hook my flask up in such a way that I can evacuate it with a pump and fill it with nitrogen. I've shown one of my recent setups on the picture. You can obviously connect things in many different ways depending on what equipment you have available.
I normally evacuate the flask and fill it with nitrogen at least three times. Only then do I proceed to add my catalyst. After addition I repeat the evacuate the flask procedure but this time I flush the flask with hydrogen and at the end I attach a big fat balloon with hydrogen gas and stir the reaction mixture vigorously.
The other day when I was in a big hurry I decided to skip the flushing with nitrogen step (for the first time ever) and as a consequence I had my first Pd on charcoal fire. I was very lucky and didn't burn myself because the weighing paper was shielding my hand. The drama was quickly over as I had my lab book handy and sealed the flask with it. The hydrogenation worked fine and I made it in time to the pub. However, next time I think I'll skip the first pint with the boys and flush that flask as I normally do. D!


Ψ*Ψ said...

I'd agree to those conditions, and also add illness. I can't do anything synthetic if I feel like ick. inopportune sneeze can be terrible.
How'd the lab book fare after that?

milkshake said...

Pd-C ignites methanol without hydrogen.I much prefer to hydrogenate in EtOAc or water

As for the conditions under which not to perform preparative chemistry: "Don't make drugs when you are high"

Anonymous said...

Large scale hydrogenations long ago: put 200 odd grams in Parr bottle. Add catalyst from ancient jar (free from subbasement morgue). Pour in ethanol. Connect to homebrew hydrogen buret system. Start stirring.

Works really well. One day, no more prehistoric catalyst remains. PI passes stone, buys new jar of catalyst.

Put 200 odd grams in Parr bottle. Add catalyst from new jar. Pour in ethanol. Leap backwards with Olympic-worthy distance and altitude as fireball bellows up from bottle. Run to bathroom, pull chunks of pyrolyzed hair from side of head.

Guess what? Noble metals not so noble.

Paul said...

I remember hearing of a similar story at a UK university. The student in question panicked, and reached for a water-bottle to squirt at the still developing fire. However, the squeeze he grabbed was ether...

wepplo said...

I would bet that a lot (most?) Parr hydrogenations were done without an inert gas and without a fire. I believe the rules were different. Add catalyst to the bottle. Cool in ice, add ice cold solvent, ethanol, all at once. Because the bottle contains no vapor and by adding the solvent all at once in an effort to drown the catalyst, little or no vapor could form during that part of the operation. I can't say no one had any fires doing it that way, but I never heard of any. Filtering the catalyst at the end, different story.

Andrew said...

Slurry (5% Pd/C) in toluene first before addition to methanol. Works a treat, never bursts into flames.

I always keep bits of broken up fire-proof mat in my hood to cover funnel/flasks or whatever else might burst into flames.

Thomas said...

So what kind of balloon do you use? I have been trying to use a reg. party balloon but it keeps emptying. My school is to poor to get the fancy aldrich balloons... and I pretty much geto rig everything in the lab anyhow... Any advice on how to geto rig a sweet hydrogenation system for some lindlar reductions?

milkshake said...

Aldrich baloons cost about $14 for whole bag of 11-inch sized baloons, about 100 of them. A frikkin septa costs more than a baloon.

Hydrogen leaks out because of its great leakibility (the lowest viscosity of all gases). When you use Aldrich 11 inch baloons make sure you double them (like with double condoms - but with a pencil, the eraser-end and two baloons). The idea is that if one ruptures during overnight hydrogenation the stuff does not catch on fire. Incidentally it also leaks a lot less.

labguy said...

Like milkshake says, the trick is to double up the balloon. Even normal party balloons have never burst in my case but don't overfill and always watch out. Pd plus atmospheric oxygen and hydrogen equals water and a big flame. I seen a coworker light himself up like a leaky stove pipe and had some nice patches where his hair used to be...ouch! and you know what, he was in a hurry and didn't evacuate his flask beforehand and back flush with Inert gas.

Daniel Sejer said...

Nice one! I was pretty lucky not to set myself on fire. I agree that double balloons from Aldrich are the way to go. Cheap and good quality. Even party balloons do the trick. Definitely always double or triple balloons. I have had the outer balloon burst very often. Alternatively, you can use a breathing bag. They are really thick and durable. Great for hydrogenations. On the picture you can just make out the bottom of the breathing bag with a B24 tap adaptor attached to it. D!

stephane said...

I always make a suspension of Pd/C in dichloromethane and add it to my substrate solution. Degazed or not, it will never catch fire.
For the filtration part, I always added some celite to my solution. Then you can filtrate to dryness.

Daniel Sejer said...

I realise that some chemists will soak their catalyst in a solvent prior to addition. Obviously a lot is down to personal preference but I do find it overly complicated. If there is any pump/vacuum system in your hood it will take you less than a minute to degas the flask and then you just tip your catalyst in. D!

kirrasoul said...

Forgive what may seem to be a silly question but when I look at the image of your setup I don't really see how it works.

While I can see how the flushing with nitrogen works I can't figure out how you get the catalyst in there without opening the flask to the atmosphere?

Could you please explain.

Daniel Sejer said...

That is a perfectly reasonable question. I do open the system to the atmosphere, tip in the catalyst, seal the system again and repeat the nitrogen flushing before switching to hydrogen. D!

kirrasoul said...

Cheers for that. Soon will be my first hydrogenation in a couple of years and I thought it would be a good idea to brush up. I am not confident in asking my lab mates since they seem to have a small fire everytime they do that reaction.

Anonymous said...

I did a small scale hydrogenation with palladium oxide and methanol. My reaction flask was covered with Argon, but still I could see the sparks, fartunately it didn't catch hydrogen was there..of course.. can anybody tell me what is the theory behind the sparks?

thank you.

Xing said...

For my hydrogenations with Pd/C i never had any fires happen, and neither the other members of our workgroup, as far as I know.
However, if you filter the catalyst off using a filter paper you should never just put the filter paper in the normal trash bag. We had at least two trash fires started that way :p
I keep a bottle filled partly with water for the Pd/c and Pt remains of my reactions.

Anonymous said...

You can now do catalytic hydrogenation risk free - pre packed catalyst cartridges so no addtition/filtration, hydrogen generated from H20 so no cylinders, compound in and reduced compund out - check this

Daniel Sejer said...

H-Cube = great and incredibly expensive. But if I had the money I would get one. Until that happens I (and most other academic researchers) are stuck with the balloon method. D!

Anonymous said...

I haven't run a hydrogenation in awhile but my solution to fires is to dissolve/slurry the substrate in the solvent. Then simply sparge the solution with a nitrogen line. It degasses the solution and expels all oxygen from the flask. Dry catalyst can be poured in (remember the nitrogen must be sparging and not just blowing around above the solution or it will blow the dry catalyst out of the flask)