06-19-02 10:53
No 323011
Help!! Advice needed, SWIM is having problems with workup of a LAH reduction of 2,5-Dimethoxy Beta-Nitrostyrene.
From a new, sealed can of LAH, 20g was added to stirring dry THF in an ice bath. No fizzing was noticed throughout the addition. The THF was from a new bottle and dried overnight over MgSO4. 23g of 2,5-DMBNS (orange needles recrystalized from IPA) was dissolved in 350ml dry THF. The DMBNS solution was slowly added to the LAH/THF in a way such that the temperature never exceeded 15C. Mixture was refluxed w/stirring for 20h. Afterwards, mixture was grey.
25ml of 50:50 THF/H2O was added slowly to the rxn mixture, followed by ~10ml IPA, then 20ml 4M NaOH, then an additional 60ml H2O. There was a lot of foaming and heating of the solution. Flask was once again placed in an icewater bath. Stirring was continued for 1hour, and here's where SWIM is stuck!
SWIM expected the mixture to turn pasty white, then the Li/Al hydroxide salts would fall out as filterable granules. But this didn't happen! After 1 hour stirring, mixture was still grey. SWIM added ~10ml more IPA (still stirring of course) and no fizzing was noted, so SWIM assumed all LAH is destroyed. SWIM proceeded to slowly add ~50ml of ~20% NaOH. Solution was now light brown, but there was no pasty sludge or precipitate nicely settling out.
This was all done under light Argon flow by the way.
SWIM is letting the suspension settle out as much as possible, then SWIM will try to vac filter slowly as much as SWIM can. The mother liquor becoming visible during settling is deep red in color. What might SWIM do? If SWIM can get the mother liquor away from all suspension, is it safe to remove THF/H2O (under reduced pressure SWIM supposes) and proceed to either vac distill what is hopefully 2C-H, or take it through an acid/base extraction?
Just when SWIM thinks they know what they're doing, something unexplainable to SWIM happens! SWIM considered continuing to add NaOH solution and hope something happens, but they've added what seems to be far more than required in other bee's writeups. pH is ~9-10 right now.
Thank you, any prompt recommendations would bee appreciated.
SpicyBrown
(Stoni's sexual toy)
06-19-02 12:11
No 323022
It's dangerous to do that.
Have you tried adding more THF or other solvent, and maybe heating up the whole shebang while stirring?
Converting Al sludge into an easily filterable consistency is a bitch (same in the Al/Hg), but it CAN be done.
I'm not fat just horizontally disproportionate.
(Hive Bee)
06-19-02 14:11
No 323037
Swim had the same problem. He solved it by simply adding WAY to much NaOH. The 2CH i pretty stable, so a very high pH wont hurt it. Swim simply poured in 10% NaOH till it went white and precipated. Swim thinks he used X5-6 times the amount on rhodiums page. Maybe not the most correct thing to do, but IT WORKED.
Regards
Bandil
(Newbee)
06-19-02 15:35
No 323056
Add a 5-10% aq NaOH solution to the whole thing and stir until you see that two clear layers forms.
Approximately 180ml 5% NaOH soln/mol Al should be enough. The aluminum will then form a soluble sodium salt (sodium aluminate) which will stay in the water phase. Then simply separate tha layers and proceed as usual. You might want to extract the aqueous phase once with some solvent just to be sure to get all the product out from it.
(Moderator)
06-19-02 16:29
No 323069
You now have way too much water in it. Don't add more than the calculated maximum amount of water (about 32 ml in your case). You can add it as a conc. MgSO4 solution or as a NaOH solution (which may help to prevent the amine sticking to the inorganic salts during extraction).
Directly before complete neutralization of the hydrides the mixture becomes very thick, upon addition of a bit more water and heavy stirring it liquifies again. Stop then. The slurry stays dark gray all the time, only the washed filtercake may become somehow white on drying.
After vacuum filtration of the cake (Celite = diatomeous earth helps) you may want to wash the cake with ether after THF extraction to liberate some more amine from the salts.
(Hive Bee)
06-21-02 03:46
No 323549
Thank you all very much for your responses and help! SWIM eventually got the majority of the solution filtered, and after a seamless extraction currently has a yellow solution of 2CH in DCM awaiting further dreaming.. In the future, SWIM will know how to deal with the Al salts, whether it be a LAH or an Al/Hg reduction.
SpicyBrown
(Hive Bee)
06-22-02 13:02
No 323991
Why is it so bad with to much water? When swim dreamt of the reaction he added quite a bit, vacuum filtered it and removed the IPA/THF/water with vacuum and heat and then proceeded with the A/B extraction! And swim made mighty fine 2Cx ;)
Regards
Peter
(Moderator)
06-22-02 18:06
No 324037
I had the impression that it makes the sludge sludgier. It may not be such a big problem with THF, but with more lipophilic solvents it will be.
(Hive Bee)
06-26-02 04:09
No 325402
SWIM's problem during the workup was that it wouldn't vac filter. The filter eventually became so clogged, SWIM's vac gauge read 29 inches Hg and stuff in the filter flask that did come through began boiling at room temp! SWIM was using medium speed filter paper, this could have been the problem.. Was all SWIM had though.
SpicyBrown
(Stoni's sexual toy)
06-26-02 11:10
No 325570
MaDMAx, that's what he did!
I'm not fat just horizontally disproportionate.
(Hive Addict)
06-26-02 22:28
No 325740
Couldn't it be that his LAH - in spite of being `fresh'- wasn't so fresh as he thought it was? When I did my first LAH reduction, adding the nitrostyrene was an adventure in itself. Ditto for quenching the LAH: if you don't see any fizzing, it's almost guaranteed that your LAH wasn't up to spec.
(Hive Bee)
06-27-02 10:24
No 325985
Damn damn damn!
Well perhaps SWIM's LAH is the culprit here then. Thanks for the picture MaDMAx- Although SWIM's LAH wasn't one BIG chunk like that, it was a lot of smaller chunks of the exact same color. @$#%&*!!!!!
Well, time to try again.. Perhaps a small scale run w/a shot at the Zn/HCl rdxn is in order.
Thanks again everybody-
SpicyBrown
(Moderator)
06-27-02 15:52
No 326044
Lithium aluminum hydride usually isn't bright white but light gray.
(Moderator)
06-27-02 19:53
No 326112
Large industrial scale quantities of LAH are normally shipped as chunks, which can be crushed by wrapping them in aluminum foil and tapping lightly with a rubber hammer, in a fume hood to reduce the risk of LAH dust inhalation or hydrogen gas ignition. On the other hand, if it's partially hydrated (as that chunk appears to be), it's useless as a reagent
(Hive Addict)
06-27-02 22:43
No 326211
>and very reactive!
LAH dust sucks. Hurts your throat & all.
I've once destroyed .5 gr LAH by setting it afire, and that was quite spectacular. After that, I was glad that I never dropped a 500 ml 3-necked flask filled with LAH-saturated THF.
(Chief Bee)
07-02-02 23:47
No 328087
I once tipped over a reflux setup, crashing a 1000ml RB flask with 500ml ether and 20g LAH into a stainless sink. I saw it all in slow-motion, the flask slowly drifting in mid-air towards the inevitable.
I don't know how it is possible, but nothing happened. I guess the karma accountants decided that it wasn't time to terminate me yet (this was just before Strike gave me the Hive).
The whole desk and sink was frizzling and popping by decomposing LAH meeting water and moist air, becoming even more violent as the solvent evaporated, filling the whole room with explosive ether vapor and exposing more and more dissolved LAH. If there had been one single spark present I had been blown to smithereens. When the ether was mostly gone, I quickly flooded the area with a bucket of water, immediately covering all the LAH dust with water, in my reasoning to keep it from being able to heat up and ignite in the air.
This is not the proper procedure for the destruction of LAH! I was very lucky not to get hurt.
(Moderator)
07-03-02 02:03
No 328137
Actually, Herbert Brown, the inventor of LAH, states that one can decompose the excess LAH from a reduction by carefully adding the reaction solution to ice water. Massive amounts of hydrogen are generated by this method, the only method that doesn't generate large amounts of hydrogen gas is using ethyl acetate
Aqueous acid will decompose the lithium aluminum alcoholates, allowing recovery of alcohols
(Hive Addict)
07-03-02 12:31
No 328279
Personal I cool down my reaction, then ,drop by drop, ad plain wather with good stirring, when the reaction goes from gray to white I filter it throug 2cm MgSO4 plased into a buchner. I now have my amine disolved into dry ether.
redy for destilation or just gassing in the ether.
(Newbee)
07-03-02 14:27
No 328298
One time, in the winter, I added 25 gm. LAH to a flask full of supposedly DRY ether. The flask immediately started getting hot in my hands! (The lab assistant had put wet ether in the flask by mistake) Having to make a split second decision, I ran out of the lab and plunged the flask in a snow bank outside. Sure got my heart racing!
Yellium, if your throat is getting burned, you're not being careful! ALWAYS use at least a mask, and preferably a fume hood too. The MSDS says: "Potential Health Effects - Inhalation:
Extremely destructive to tissues of the mucous membranes and upper respiratory tract. Symptoms may include burning sensation, coughing, wheezing, laryngitis, shortness of breath, headache, nausea and vomiting."