[rita87]

Hi everybody,
Does anyone know a safe procedure for hydrogenation of phenylacetylene to styrene?As H2 is highly flammable I am not sure how to handle it.

Thank you

[gfunk]

What kind of scale are you talking about?  Generally, H₂ comes in a tank and you hook it up to a hydrogenator apparatus.  Provided you don't exceed the shaker flask's pressure rating, it should be fairly safe.

[nj_bartel]

There's dissolving metal reduction if you really don't want to mess with hydrogen; sodium amide as a byproduct.

[rita87]

Thanks,
I was reading in an article that low pressure apparatus should be used. do you know why low pressure is required?

[Squirmy]

"low pressure" is a relative thing. In the context of hydrogenation, I would say they're calling for atmospheric pressure or just slightly above.

For partial hydrogenation with Lindlar's catalyst, I believe you need a way of monitoring the hydrogen uptake. The styrene is not as reactive as the starting material, but eventually it will react, too.

[rita87]

Yes the pressure should be around 1 atm , do you know if there is any reason for that. what would happen if the pressure was higher or lower.

For partial hydrogenation with Lindlar's catalyst, I believe you need a way of monitoring the hydrogen uptake. The styrene is not as reactive as the starting material, but eventually it will react, too.

That's right, but I couldn't think of any, do you have any suggestions.

Thank you,

[Squirmy]

I've never done it myself, but you need something like the apparatus shown in here:

http://stoltz.caltech.edu/classes/chem5/10.pdf

[rita87]

thank you very much for the link, but do you know the difference between this atmospheric pressure hydrogenation apparatus and the low pressure one shown here?
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv9p0589

 I am using this article http://orgsynth.org/orgsyn/prep.asp?prep=cv5p0880 and it says low pressure apparatus but the pressure should be kept just above 1 atm. so don't know which apparatus is the correct one to use.

Thanks

[Squirmy]

Disclaimer: I've never run one of these reactions, so I don't REALLY know. But, I'll throw this in, anyway.

That reaction is run at 4 atm, which compared to 100 atm is still considered "low" pressure. However, 4 atm is much higher than 1 atm and I think over-reduction would be a problem for the Ph-acetylene -> styrene hydrogenation. Higher pressures are used to increase reactivity (with high enough pressure you can hydrogenate benzene). So, if the reaction already goes at 1 atm, it should go faster at higher pressure. Unfortunately, that would probably be faster than you like and all the way on to ethylbenzene.

I suppose you *could* use that setup at a pressure closer to 1 atm, but the burette setup seems more practical if you're trying to run the reaction at 1 atm. The outside atmosphere does the heavy lifting with pressure control, and with 22.4 L/mol and a burette, precision should be excellent.

[rita87]

Hi,
I went through all the posts again, but honestly I still don't know why pressure is so important in this reaction. I read somewhere that hydroen is a high pressure gas, can someone explain what is meant by that, please. and why the "hydrogen is admitted to a pressure slightly above 1 atm" according to http://orgsynth.org/orgsyn/prep.asp?prep=cv5p0880

[kiwi]

I have done one of these before, and you're overthinking it. The H2 is just let in from the cylinder to fill the burette, its pressure is irrelevant as the 1 atm of pressure pushing back onto the gas in the gas burette will ensure the final pressure is at 1 atm. "Slightly over 1 atm" means "we opened the cylinder tap until the gas gently entered the gas burette, pushing the water out." Not sure what is meant by "hydrogen is a high pressure gas", where did you get that from? In practice the uptake of the second equivalent of H2  is much slower than the first, so by monitoring the change in water level vs change in time you can spot the completion time. If your Lindlars proves to be a little bit too reactive you can lob in some more quinoline to quieten it down. I assume you are aware that phenylacetylene is ~15x the price of styrene (at Sigma-Aldrich at least, you could get it cheaper if you tried), even without including the cost of the Pd and H2?