Hello everyone, I'm new to the forum and I'm french so, sorry for the mistake! I'm doing a presentation on an internship in which I reduced an alkane to an alkyne. To do this, we used paladium on carbon with hydrogen gas. Unfortunately, I can't find anything on the internet about this... I "imagined" this shema on the assumption that it was the same as for alkenes, but it doesn't really seem professional. If anyone can help me, I'd really appreciate it. 

Thanks !

2 hours ago, Ertox said:

Hello everyone, I'm new to the forum and I'm french so, sorry for the mistake! I'm doing a presentation on an internship in which I reduced an alkane to an alkyne. To do this, we used paladium on carbon with hydrogen gas. Unfortunately, I can't find anything on the internet about this... I "imagined" this shema on the assumption that it was the same as for alkenes, but it doesn't really seem professional. If anyone can help me, I'd really appreciate it. 

Thanks !

I would expect it to be the same mechanism, just in 2 stages, with an alkene as the intermediate step. In both alkynes and alkenes you have π-bonds which can bind to the metal surface. Kinetically, I imagine it may be a bit faster for alkynes, as they can approach in any orientation and still bind to the surface.

There are descriptions of this on the internet. Here is one: https://www.masterorganicchemistry.com/2011/11/25/hydrogenation-alkenes-palladium-on-carbon-pdc/. This link suggests that alkynes are more readily reduced than alkenes.

The only respect in which I think the mechanism you have drawn may not be quite right is that, according to my understanding, the alkyne or alkene itself binds to the metal via its π-bonds, whereas you have shown the molecule staying above the H atoms, rather than binding to the surface itself before reacting.  (There is a diagram of the mechanism in the link.) 

 

 

Its the opposit

which I reduced an alkane to an alkyne.

You reduce the alkyne to the Alkane.

1 hour ago, exchemist said:

I would expect it to be the same mechanism, just in 2 stages, with an alkene as the intermediate step. In both alkynes and alkenes you have π-bonds which can bind to the metal surface. Kinetically, I imagine it may be a bit faster for alkynes, as they can approach in any orientation and still bind to the surface.

There are descriptions of this on the internet. Here is one: https://www.masterorganicchemistry.com/2011/11/25/hydrogenation-alkenes-palladium-on-carbon-pdc/. This link suggests that alkynes are more readily reduced than alkenes.

The only respect in which I think the mechanism you have drawn may not be quite right is that, according to my understanding, the alkyne or alkene itself binds to the metal via its π-bonds, whereas you have shown the molecule staying above the H atoms, rather than binding to the surface itself before reacting.  (There is a diagram of the mechanism in the link.) 

 

 

Thank you for your prompt reply it's much clearer now, I've redone the reaction shema. 
Have a nice day 
Sincerely

25 minutes ago, Ertox said:

Thank you for your prompt reply it's much clearer now, I've redone the reaction shema. 
Have a nice day 
Sincerely

Yes I think that looks better. 

Do you need to consider the stereochemistry of the hydrogenations?

 

 

19 hours ago, KJW said:

Do you need to consider the stereochemistry of the hydrogenations?

No, I don't think I'll be looking into that. I'll just point out that the addition is cys.