[limonade]

If I use H2 and a lindlar catalyst, can I reduce the benzene ring to a cyclohexane or do I need to have some sort of special conditions to do so?

[discodermolide]

Nishimura's catalyst, will do this for you. It's rhodium on Pt oxide.

[limonade]

Thanks for your reply. I meant to say H2 and Pd/C, not lindlar. Sorry. x/

[Archer]

Nishimura's catalyst, will do this for you. It's rhodium on Pt oxide.

I think you need relatively high pressure to achieve this too (i.e. a baloon of H₂ is not sufficient). I could be mistaken on this as I don't have my books with me so hopefully discodermolide or someone will confirm or refute this.

[discodermolide]

Yes, you need some pressure on the system. I can't remember how much but it should be easy to find out.

[Archer]

Thought so, I never had a high pressure hydrogenator for this so I used Birch reduction followed by hydrogenation. This may not always work with every substrate.

[curiouscat]

If I use H2 and a lindlar catalyst, can I reduce the benzene ring to a cyclohexane or do I need to have some sort of special conditions to do so?

Isn't Lindlar for selective -yne to -ene?

Here you are doing a complete hydrogenation, right?

[curiouscat]

Thought so, I never had a high pressure hydrogenator for this so I used Birch reduction followed by hydrogenation. This may not always work with every substrate.

Out of curiosity (coming from an engineer), when you say high pressure, how much do you mean in a hydrogenation context?

[discodermolide]

Something above that which a Paar can take.
Although with a high enough catalyst loading you may get away with a Paar. Reaction time won't matter here.
I've got conditions somewhere I'll try and find them for you.


I stand corrected by my own work!! But it does depend on the aromatic ring you are hydrogenating.
To a solution of 42.9g(0.17mol) of 22 in 500mL of methanol was added 5 g of Rh₂O₃/PtO2 (46% Rh, 20.15% Pt; Nishimura's catalyst)and the suspension hydrogenated at room temperature and normal pressure for 8.5 h. The catalyst was removed and the filtrate evaporated to dryness. The residue was stirred with diethyl ether for 1h and the solid collected by filtration and dried to give 43.7 g (100%) of 24 identical to that obtained by the reactions of Scheme 1.

This was to obtain a cyclohexylmethyl compound from a benzyl compound.
The reference for the catalyst is

Nishimura, S. Hydrogenation and Hydrogenolysis. V. Rhodium-
Platinum Oxide as a Catalyst for the Hydrogenation of Organic
Compounds. Bull. Chem. Soc. Japn. 1961,34, 32-36.

[Archer]

Out of curiosity (coming from an engineer), when you say high pressure, how much do you mean in a hydrogenation context?

Anything over 1 Bar is high pressure for bench scale. This is far easier on an engineering scale but lab scale requires specialised equipment, particularly for heterogeneous catalysts.

The issue is introducing the Hydrogen into the reactor.

[curiouscat]

Out of curiosity (coming from an engineer), when you say high pressure, how much do you mean in a hydrogenation context?

Anything over 1 Bar is high pressure for bench scale. This is far easier on an engineering scale but lab scale requires specialised equipment, particularly for heterogeneous catalysts.

The issue is introducing the Hydrogen into the reactor.

What P is a conventional H2 cylinder at? I remember using a Parr Hydrogenator in the lab. ~3 Litres or so. Was a pleasure to work with.

[discodermolide]

They are about 2500-3000 psi.
http://www.alspecialtygases.com/Prd_high-pressure_steel.aspx

[Archer]

I remember using a Parr Hydrogenator in the lab. ~3 Litres or so. Was a pleasure to work with.

I did not have the luxury of a hydrogenator. Always wanted one but it was too expensive for us to buy.

[Corribus]

I did hydrogenation of pyrene once with Raney nickel and hydrogen gas, using a high pressure steel reactor.  It worked pretty well, but setting it up scared the hell out of me.

[curiouscat]

I did hydrogenation of pyrene once with Raney nickel and hydrogen gas, using a high pressure steel reactor.  It worked pretty well, but setting it up scared the hell out of me.

I only had had the typical O'Chem experience behind me when in Grad School I got assigned to run a hydrogenation rig for 3 months.

Just the word "hydrogenation" made me uneasy as a novice. The thickness of the Parr and having to tighten a flange with 6 thick bolts accentuated the unease.

In hindsight though, my hydrogenation turned out to be pretty tame. It was Ruthenium catalyzed and only a few bars of pressure.