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organicchemist25
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[*] posted on 16-11-2020 at 09:33
LAH reduction question(s)


I’ve been wanting to do an LAH reduction on a styrene for quite sometime. I have brand new, unopened LAH and freshly bought, unopened, stabilized anhydrous THF.

One question is since the THF is stabilized, do I need to distill it and get use the THF without the stabilizer, or is it ok to use with it? I have 500g of Na in kerosene if I should even go further in drying it, then distill it? I do believe it will work on this cyclic ether?

Also, I am going to do a small run first. Maybe a gram or two. I know to add small amounts slowly. Does it have to be under an inert atmosphere? I do have a small cylinder on Nitrogen. Shouldn’t the gas be dry in the cylinder, or should I dry the gas anyways?

On small amounts can one get by without an inert gas if humidity is low?



[Edited on 16-11-2020 by organicchemist25]
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ArbuzToWoda
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[*] posted on 16-11-2020 at 09:40


Is this topic still under construction or...


[Edited on 16-11-2020 by ArbuzToWoda]
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[*] posted on 16-11-2020 at 09:45


Quote: Originally posted by ArbuzToWoda  
Is this topic still under construction or...


[Edited on 16-11-2020 by ArbuzToWoda]


I hit submit on accident before I had posted the questions. I edited it and resubmitted. I apologize for that.

Thanks
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[*] posted on 16-11-2020 at 10:11


Here are your answers:
1. It's okay to use the stabilized product. The butylated hydroxytoluene doesn't interfere in such amounts.
2. If the product is anhydrous, why worry with drying it even more? You can warm it up and store it under freshly cut sodium if you're unsure. The bottled gases are usually drier than the atmospherical air, so if you've got the option to use it, do. The nice thing is that you don't need to keep flushing the whole apparatus, but just flush it once or twice and stopper the flask. In case of any hydrogen evolution, be sure to vent it often though! We wouldn't like to have an explosion there.
3. You can get through, it may influence your yield though (especially if the reaction is ran for 24h or even more and isn't stoppered).
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[*] posted on 16-11-2020 at 10:17


2. anhydrous is a sale category, not a guarantee, they are "anhydrous", and sometimes are actually not totally.
It can't hurt to dry and distill it again.
Each time you open it, it will become somewhat moist.
I second the sodium, also, benzophenone so you know from looking at it already.
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[*] posted on 16-11-2020 at 12:31


The LAH will remove any water still left in the THF, if you use a slight excess of LAH, it should destroy any water just fine. Unlike some reactions, trace water will not hurt an LAH reaction, if you have a slight excess. However, they are easy reactions to set fire to, so please do it in this order, unless you have a good reason not to.

Purge the rbf with Nitrogen or other dry gas, you can always use some dryrite to dry it, but most compressed N2 is fine. Mostly, you want to remove the oxygen from the flask, as that is bad when combined with LAH and an ether. The LAH and ether or THF will often release some heat when combined, so best not to be in oxygen or air.

Once the flask is dry and inert, put the LAH needed in the flask and purge it again, carefully, as LAH is easily blown about.

Once you are confident that there is no oxygen present, you can transfer the THF or ether into the flask via a syringe or cannulae safely. It may bubble a bit at first, due to both the LAH reacting with any traces of water, and also the LAH complexing the ether as it dissolves. But if there is no oxygen this cannot start a fire. Many people chill the reaction flask now in ice water, making sure not to crack the rbf while stirring, as that will be exciting.

Once the LAH is in solution or suspension (depending on the concentration) the next step is to dissolve the substrate into the THF and SLOWLY drip it into the LAH, at least at first until you see how reactive it is. In some cases you can add it quickly, but if added too fast, the whole thing can erupt like a volcano and shoot fire out the top. I have seen it done by someone else, so I know it can happen.

If you try to add LAH powder to the THF or worse yet, ether, it can create a dust cloud that will ignite readily in air and cause a fire, this is the WRONG way to do this. I have seen this also, shortly after telling someone not to do that, and they did it anyway and burned their entire hood. DON'T do this, unless you know exactly what you are doing, and are well trained in fire control and away from people.

Hope I am not being too bossy, but I have seen the results of bad LAH reactions, they must be done carefully, and are best done by adding substrate to a solution of LAH. There are likely some exceptions to that rule, but I don't know any off hand. The temp needed to keep the reaction under control depends on the substrate and scale/concentration. For acids, you might have to heat it to get a reaction, for ketones, you likely need to cool it to keep if from overheating. I am setting up a ketone reduction currently, which is iced to keep it slow, but the substrate must be added slowly so as to not blow it up from overheating the solvent.


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[*] posted on 16-11-2020 at 12:45


Quote: Originally posted by Dr.Bob  
If you try to add LAH powder to the THF or worse yet, ether, it can create a dust cloud that will ignite readily in air and cause a fire, this is the WRONG way to do this. I have seen this also, shortly after telling someone not to do that, and they did it anyway and burned their entire hood. DON'T do this, unless you know exactly what you are doing, and are well trained in fire control and away from people.


Are you sure your location is correct? I could swear we have students in common, but they'd be unlikely to travel that far.




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Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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[*] posted on 16-11-2020 at 12:53


Quote: Originally posted by Dr.Bob  
The LAH will remove any water still left in the THF, if you use a slight excess of LAH, it should destroy any water just fine. Unlike some reactions, trace water will not hurt an LAH reaction, if you have a slight excess. However, they are easy reactions to set fire to, so please do it in this order, unless you have a good reason not to.

Purge the rbf with Nitrogen or other dry gas, you can always use some dryrite to dry it, but most compressed N2 is fine. Mostly, you want to remove the oxygen from the flask, as that is bad when combined with LAH and an ether. The LAH and ether or THF will often release some heat when combined, so best not to be in oxygen or air.

Once the flask is dry and inert, put the LAH needed in the flask and purge it again, carefully, as LAH is easily blown about.

Once you are confident that there is no oxygen present, you can transfer the THF or ether into the flask via a syringe or cannulae safely. It may bubble a bit at first, due to both the LAH reacting with any traces of water, and also the LAH complexing the ether as it dissolves. But if there is no oxygen this cannot start a fire. Many people chill the reaction flask now in ice water, making sure not to crack the rbf while stirring, as that will be exciting.

Once the LAH is in solution or suspension (depending on the concentration) the next step is to dissolve the substrate into the THF and SLOWLY drip it into the LAH, at least at first until you see how reactive it is. In some cases you can add it quickly, but if added too fast, the whole thing can erupt like a volcano and shoot fire out the top. I have seen it done by someone else, so I know it can happen.

If you try to add LAH powder to the THF or worse yet, ether, it can create a dust cloud that will ignite readily in air and cause a fire, this is the WRONG way to do this. I have seen this also, shortly after telling someone not to do that, and they did it anyway and burned their entire hood. DON'T do this, unless you know exactly what you are doing, and are well trained in fire control and away from people.

Hope I am not being too bossy, but I have seen the results of bad LAH reactions, they must be done carefully, and are best done by adding substrate to a solution of LAH. There are likely some exceptions to that rule, but I don't know any off hand. The temp needed to keep the reaction under control depends on the substrate and scale/concentration. For acids, you might have to heat it to get a reaction, for ketones, you likely need to cool it to keep if from overheating. I am setting up a ketone reduction currently, which is iced to keep it slow, but the substrate must be added slowly so as to not blow it up from overheating the solvent.




No not at all! This is the type of descriptive information I’ve wanted from first hand experiences. I know it’s dangerous. I just wanted clarity on a few details. Important ones. I’m ready. I really appreciate your great advice
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[*] posted on 16-11-2020 at 12:55


Quote: Originally posted by karlos³  
2. anhydrous is a sale category, not a guarantee, they are "anhydrous", and sometimes are actually not totally.
It can't hurt to dry and distill it again.
Each time you open it, it will become somewhat moist.
I second the sodium, also, benzophenone so you know from looking at it already.


That was a thought of mine. I’m just going to put some Na in it for at least 24 hours and distill what I intend on using.
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[*] posted on 16-11-2020 at 13:06


Quote: Originally posted by ArbuzToWoda  
Here are your answers:
1. It's okay to use the stabilized product. The butylated hydroxytoluene doesn't interfere in such amounts.
2. If the product is anhydrous, why worry with drying it even more? You can warm it up and store it under freshly cut sodium if you're unsure. The bottled gases are usually drier than the atmospherical air, so if you've got the option to use it, do. The nice thing is that you don't need to keep flushing the whole apparatus, but just flush it once or twice and stopper the flask. In case of any hydrogen evolution, be sure to vent it often though! We wouldn't like to have an explosion there.
3. You can get through, it may influence your yield though (especially if the reaction is ran for 24h or even more and isn't stoppered).


I am going to set up for reflux with at least a 36” Liebig condenser. I wasn’t ever going to seal the vessel. Am I missing something? I was intending to reflux for 24 hours. I was just going to reflux open ended at the top of the condenser. I would definitely want to help hold on to best yield as possible.

So, will leaving the condenser open, but refluxing for 24 hours affect yield?

After a point in the reaction does it become “safer” after all the substrate has been added and “calmed” down? Clearly I can’t watch it for 24 hours. Just wondering how long constant supervision is required. I’m thinking I will be able to determine when addition is over, slowed down and just needs to be refluxed.

Thank you for all your help with this!
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[*] posted on 16-11-2020 at 14:47


Quote: Originally posted by Dr.Bob  
The LAH will remove any water still left in the THF, if you use a slight excess of LAH, it should destroy any water just fine. Unlike some reactions, trace water will not hurt an LAH reaction, if you have a slight excess. However, they are easy reactions to set fire to, so please do it in this order, unless you have a good reason not to.

Purge the rbf with Nitrogen or other dry gas, you can always use some dryrite to dry it, but most compressed N2 is fine. Mostly, you want to remove the oxygen from the flask, as that is bad when combined with LAH and an ether. The LAH and ether or THF will often release some heat when combined, so best not to be in oxygen or air.

Once the flask is dry and inert, put the LAH needed in the flask and purge it again, carefully, as LAH is easily blown about.

Once you are confident that there is no oxygen present, you can transfer the THF or ether into the flask via a syringe or cannulae safely. It may bubble a bit at first, due to both the LAH reacting with any traces of water, and also the LAH complexing the ether as it dissolves. But if there is no oxygen this cannot start a fire. Many people chill the reaction flask now in ice water, making sure not to crack the rbf while stirring, as that will be exciting.

Once the LAH is in solution or suspension (depending on the concentration) the next step is to dissolve the substrate into the THF and SLOWLY drip it into the LAH, at least at first until you see how reactive it is. In some cases you can add it quickly, but if added too fast, the whole thing can erupt like a volcano and shoot fire out the top. I have seen it done by someone else, so I know it can happen.

If you try to add LAH powder to the THF or worse yet, ether, it can create a dust cloud that will ignite readily in air and cause a fire, this is the WRONG way to do this. I have seen this also, shortly after telling someone not to do that, and they did it anyway and burned their entire hood. DON'T do this, unless you know exactly what you are doing, and are well trained in fire control and away from people.

Hope I am not being too bossy, but I have seen the results of bad LAH reactions, they must be done carefully, and are best done by adding substrate to a solution of LAH. There are likely some exceptions to that rule, but I don't know any off hand. The temp needed to keep the reaction under control depends on the substrate and scale/concentration. For acids, you might have to heat it to get a reaction, for ketones, you likely need to cool it to keep if from overheating. I am setting up a ketone reduction currently, which is iced to keep it slow, but the substrate must be added slowly so as to not blow it up from overheating the solvent.




Thank you for all your knowledge and info! I’ve learned quite a few things through the years off of your posts.

Just so I’m clear you say that a little water is ok? I’ve thought this whole time any water would cause it to spark/react and ignite the THF, or anything think else that was flammable in its presence.

I read one time something about it’s the quenching that scared a particular chemist. Is 91% IPA ok for the IPA step for quenching, or should I order 99.9% IPA or dry mine further with appropriate measures?

Sorry so many questions. I just want to know what all proper protocol is. I was pretty disappointed when my university didn’t include an LAH reduction in any lab experiments. Only test questions on on it.

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[*] posted on 16-11-2020 at 17:33


When I say a trace of water is OK, I mean that it will destroy an equivalent amount of LAH, but the product of water plus LAH will not interfere with the reaction. There are many reactions where any trace of water will keep the reaction from occuring at all. Same for air, traces are OK, a lot can cause a fire.

If the LAH is in a flask filled with nitrogen, trace water will react with it to release hydrogen and heat, but without oxygen, they will harmlessly dissapate. If LAH and water react near solvent and oxygen you can start a fire, but very few things burn in nitrogen (Mg for one).

You typically do not need to reflux LAH for 24 hours, even for acids, and I would never leave a reaction like that refluxing unattended, you can heat it for a whille, let it sit overnight, and then heat it more the next day if not complete, but I don't leave them refluxing, way too easy to boil dry if the water stops or something goes wrong. But I don't know your substrate.

My favorite way to quench LAH is with ethyl acetate or acetone first, which just reduce to traces of alcohols, and then I usually do the Fisher workup, just google it, but it well known process of adding water and NaOh soln. in a very specific manner.

LAH is a great chemical, but it is quite more hazardous compared to NaBH4, so most places use that. The fires I have seen were almost all from coworkers over the last 30 years, only one or two while teaching, best was a Grignard reaction, the student set the variac to 30 volts and then plugged the heating mantle directly into the eletric outlet, and the reaction was glowing red in moments and shooting flamming ether out the top. Very exciting. I have put out more fires than I like to think about, most caused by sheer stupidity.
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[*] posted on 17-11-2020 at 06:39


If you were to reflux it for longer periods of time and was worried about the moisture, there's a great way to prevent it from coming in. A drying tube with join fitted to your reflux condenser filled with anhydrous CaCl2.

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[*] posted on 17-11-2020 at 08:29


Quote: Originally posted by ArbuzToWoda  
If you were to reflux it for longer periods of time and was worried about the moisture, there's a great way to prevent it from coming in. A drying tube with join fitted to your reflux condenser filled with anhydrous CaCl2.


I do have 2 drying tubes. One U-shaped and one linear straight one with fresh CaCl2 I’ll make sure to use that.
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