Sciencemadness Discussion Board

LAH Reduction

Cooper_Panda - 25-4-2022 at 17:42

Hello, I am going to perform a reduction of phthalic acid with LAH. For reference, I was going to follow the attached procedure. Specifically, the "normal" addition (starting on page 4 of the PDF).

I would really like to not use an overhead mixer, is this really necessary? Does dissolving LAH in THF create viscosity?

The way the set-up is sketched in Figure 1 doesn't show any obvious way for the nitrogen to escape, should it just leak out of the gaps in the rest of the glassware?

Finally, the procedure calls for a reflux in the set up shown in Figure 1, but the condenser is cocked to the side and there's positive pressure from the nitrogen (especially as shown, with no clear escape for the gas), it just seems like I would have a hard time getting a reflux going.


Thanks for any insight!

Attachment: LAH Reduction.pdf (721kB)
This file has been downloaded 369 times

SWIM - 25-4-2022 at 19:08

I never had trouble stirring LAH in THF with a magnetic spinbar.

Edit:

I didn't use shields either though, so obviously they've got higher standards for safety.

That diagram shows the bubbler and gas inlet right next to each other. So it looks like the gas can get out, but doesn't go through the reaction flask...WTF?

[Edited on 26-4-2022 by SWIM]

Tsjerk - 25-4-2022 at 22:12

The bubbler as well as the rest of the apparatus is schematically drawn. A bubbler like that is closed and only used to visualize the amount of nitrogen being introduced, no nitrogen can escape there. The two drawings are not complete, as stated they are closed and would pop some part somewhere, at least the first will. You should add a bubbler or drying tube to the first, or just only a bubbler on the outlet and no bubbler on the inlet.

The second might be fine if that contraption on the right is a distillation bridge.

[Edited on 26-4-2022 by Tsjerk]

Cooper_Panda - 26-4-2022 at 07:29

Ok, I'm thinking to ditch the overhead mixer and move the condenser into the middle neck of the flask so the nitrogen flows right up the center along with any solvent to be condensed.

Like what is shown here (forgive the scale): https://ibb.co/J5TQt1n

I settled on a pressure equalized addition funnel so there wouldn't be any weirdness when the stopcock is opened, but would it be better to go with an un-equalized sep funnel or dropper?

Dr.Bob - 26-4-2022 at 17:54

You only want the nitrogen at about 1 psi or less over 1 atmosphere, so that it barely pressurizes the flask. That will not cause things to pop apart, a simple ballow filled with N2 works well, or a line connected to a bubbler to allow excess pressure to escape.

An equilized addition funnel is key, as any other thing will be either open to the air, allowing air and water in (easy to start a fire this way), or will not drip well due to the unbalanced pressure.

magnetic stirring should work for small reactions, especially if done dilute enough. When you quench the reaction, it may get hard to stir for a while, there are multiple ways to quench lah reactions, the Fieser is common, but NaSO4, potassium sodium tartrate, and a small amount of ethyl acetate in THF before either are among them. You just add any of these VERY slowly at first, I have blown the addition funnel right off the flask getting in a hurry, as the quanch will generate hydrogen which will pressurize the flask.

zed - 26-4-2022 at 22:08

Ummm. Blew an addition funnel out of. a flask?

I had a buddy that blew apart a building performing that reaction. The reaction got out of hand, it could not be controlled, and the reaction flask vigorously vented hydrogen and ether vapor into the room, wherein it subsequently detonated.

How did my buddy avoid being killed? When it became obvious that something was very wrong.... He made a bee-line for the nearest exit. When the building blew, he was outside pondering whether it was safe to reenter.

You can't think of a better way, to make your desired product?

Lionel Spanner - 27-4-2022 at 03:50

Quote: Originally posted by zed  
Ummm. Blew an addition funnel out of. a flask?

I had a buddy that blew apart a building performing that reaction. The reaction got out of hand, it could not be controlled, and the reaction flask vigorously vented hydrogen and ether vapor into the room, wherein it subsequently detonated.

How did my buddy avoid being killed? When it became obvious that something was very wrong.... He made a bee-line for the nearest exit. When the building blew, he was outside pondering whether it was safe to reenter.

You can't think of a better way, to make your desired product?

Personally I'd use borane, partly because it's less hazardous in terms of fire, and also because I can't easily get hold of lithal or its precursors.

wg48temp9 - 27-4-2022 at 04:24

If you are using a pressure equalizing addition funnel you should attach the inert gas input to the top of the funnel.

A 1psi pressure on the inert gas is probably way too high unless you have a some thing to limit the flow. A pressure of 1psi is about a head of 27in of water. You probably only need a fraction of an inch of head plus the pressure to operate the bubbler.

crow6283 - 27-4-2022 at 09:06

Uh look it’s really simple, you cover one part or your apparatus with a septum and use a luerlock needle upside down in it to give a place for the gas to escape.

CharlieA - 27-4-2022 at 13:38

Quote: Originally posted by Dr.Bob  
You only want the nitrogen at about 1 psi or less over 1 atmosphere,


Note: Dr. Bob says a pressure of 1 psi or less over 1 atmosphere. So 1 psi is 0.07 atmoshere, LESS THAN ONE TENTH ATMOSPHERE ABOVE ATMOSPHERIC PRESSURE.. Even without venting to the atmosphere, ducking for cover should not be necessary. :D

Cooper_Panda - 28-4-2022 at 21:09

I'd feel more comfortable just passing gas through the apparatus rather than pressurizing anything. I want to just evacuate any air and allow a steady stream of gas to flow in and out.

The addition funnel would be sealed, and the gas would pass through the RBF up into the condenser, which would condense the THF out of the gas stream back into the RBF. That way there's no pressure build up at any point.

SWIM - 28-4-2022 at 21:24

Wish I'd had a good source for advice like here when I did my first LAH reductions.

Mine were scandalous and it's probably pretty lucky I didn't have a disaster.
Getting advice back in the 1990s was a whole different deal.

(and unlike you young whippersnappers I had to walk 5 miles in the snow to get to and from my lab. And it was uphill both ways.)



grump.jpeg - 25kB

[Edited on 29-4-2022 by SWIM]

karlos³ - 29-4-2022 at 13:53

I'd prefer Red-Al over LAH any day, if it wouldn't be so expensive.
It is so pleasant to work with in comparison, no suspended shit, no inane volume of solvent, etc.

Attached is a picture of an LAH failure(not mine).... go for red-al if you can get it, it is so much more fun to use this instead!

lah workup fail.jpg - 96kB

Cooper_Panda - 1-5-2022 at 07:59

It seemed like the cross flow of nitrogen through the reaction flask and up the condenser as shown in my schematic (https://ibb.co/PtP2T7s) would be a problem, and it was! I did a dry run yesterday that didn't work. Basically I was just trying to reflux THF with this inert cross flow (I ended up using Argon). The hope was that the condenser would strip the THF from the gas stream and drip back to the reaction, but (as I was afraid) the argon just carried the THF right out of the condenser! Maybe this would work with a much colder jacket like acetone and dry ice rather than ice water (THF bp = 66C).


https://www.youtube.com/watch?v=42_LPoWVJ1Q&t=7s
This link seems the best to follow, as horizontal bubblers are not too expensive. Basically, after purging the apparatus, the inert gas is pumped through the top part of the T intersection of the horizontal bubbler and this keeps a slight positive pressure of inert gas at the top of the condenser. The solvent vapors can easily push their way up the condenser in a pure form that condenses on the condenser walls. Any excess solvent vapors are carried off with the inert gas at the T intersection. If there is any loss of pressure in the condenser, the inert gas will fill the void. https://www.ctechglass.com/ctech-bubbler-horizontal-2440-inn...


https://www.youtube.com/watch?v=ni8fzSLCd1I
This link shows the use of a different kind of bubbler with a similar function, but it doesn't have a T intersection

https://www.youtube.com/watch?v=_wmx65zQk58
Finally, I NileRed shows a ginard reaction with just a drying tube in the top of the condenser so I guess there's that option?

SWIM - 1-5-2022 at 09:20

That first link shows a bubbler that resembles the one in the PDF schematic you posted at the top. (the one that confused me, but now I get what you mean)

If the cost isn't a problem one of those would give you a setup pretty much like theirs.

I've done other LAH reductions (okay, nitropropenes) with just a drying tube and hose from the tube to a regular oil bubbler to limit backward diffusion (like wearing both a belt and suspenders), but no inert gas.

Yields weren't great but it seemed to be okay.

My THF wasn't very dry (boiled quite a bit on adding the LAH) so I always assumed that was most of my problem. It was re-bottled from big containers by a local supplier and I'm not sure if they were keeping it dry enough. (Lab-Pro in Sunnyvale CA)

I was working with like 2 liter flasks so scale might have made a difference.

Curious about your inert gas feed as I haven't used those. I've got the 2-stage valve thingy, but still haven't bought a tank as I hate the Idea of having high pressure gas around the house.

Are you able to get a really slow bubble rate or is it hard to adjust down to really low levels?





chemrox - 3-5-2022 at 15:35

I wonder if I could mail some to Mara Lago?

crow6283 - 3-5-2022 at 17:52

SWIM typically you use a 2 stage regulator, and then instead of going directly to the system (mineral oil bubbler first to have a visual cue on how much gas is going through) you go first to a needle valve with a hose barb. This allows very fine tuning of the flow of gas.

Although I should add this is often unnecessary as a lot of people work in home foods and a the gas line and valve in their hood which is the same principle, gives better control. Effectively it is a 3rd stage to your regulator.

If you’re concerned about gas cylinders in general, well I wouldn’t be. Nitrogen and Argon are both pretty safe. Of course you want to follow proper safety precautions. I’m sure there’s lots of YouTube videos that show how to hook up and properly use an Argon tank for various welding processes - the theory is the same.

[Edited on 4-5-2022 by crow6283]

SWIM - 3-5-2022 at 18:20

Thanks, Crow6283

Dr.Bob - 7-5-2022 at 10:01

You can do LAH reactions on a large scale, but must be very careful, patient, and use sense. I have done many on 10-20 g scales, sadly (they are a pain to setup and workup in all honesty), and while they are tough, they do often work, but yields are often low or confounded by the workup and some overreduction or side reactions.

Just try purge flask first with inert gas, then add LAH to a dry flask, purge again, and then slowly add THF or solvent. It may get hot, but hard to catch fire in N2. Then once LAH soln is ready, you can slowly add substrate. Quench must be done slowly, and carefully as well, the first bit of water will be vigerous. I often add EtAc to kill any remaining LAH before starting the quench.

zed - 8-5-2022 at 00:20

Now, chemrox! You know perfectly well, that mailing LiALH4... is against postal regulations!

Cooper_Panda - 13-5-2022 at 06:18

I got my horizontal bubbler in the mail last week and I'm itching to do this reaction, but plan on working outside just in case and it's been non stop rain since I've been ready. Any day now I'll have a sunny day to do my reaction outside with. I have 0.22mol of tert-butyl isopthalic acid (t-BIPA) to reduce. My first reaction I'm going to use 0.05mol of it and 0.1mol of LAH (1:1 mol for each carboxylic acid moiety) which is about 4g LAH. I may scale up to react the remaining 0.17mol t-BIPA with 0.34mol (12g) LAH or just repeat the smaller scale a couple times.