LAH ?

[Midi]

Can someone scientifically explain what is happening when LAH is introduced to (dry) THF?

7.5g LAH in total was added to 300ml of THF, inside a 1L flask.

The first 3g was added in small incriments, about 250mg at a time. Each addition caused a considerable amount of bubbling, and what appears to be hydrogen evolving from the addition. This became a pain in the ass as I was holding a buchner filter in which the 3g was weighed out on. Trying to dodge a suspecting explosion and holding 3g of this fine grey dust was comical, had I been recorded.

Now, after the last of the initial 3g was added, I noticed the amount of fizzing had steadily decreased, in contrast to the amount of LAH I was adding. By the time I was adding the last of the 7.5g, I was able to dump in a gram all at once without shitting my pants.

What casued this fizzing?? I have read UTFSE that some people believe the fizzing isn't from h2o being present in the solvent, but something else.

The THF was dried X2 with a considerable amount of anhydrous magnesium sulfate. I doubt much water was in there if any.

Why did the addition of LAH fizz so hardcore at first, then not at the end?

The addition of LAH reminded me of NaBH4, just with a little more spice. Great stuff!!  

[Midi]

At what point is the rxn fairly safe? If whatever you decided to reduce, was redunk, then a reflux was to take place for say, 20 hours, would you be able to walk away for 6 hours, or does this type of rxn need constant attention?

[Xicori]

your THF wasnt absolutely dry....

so the LAH reacted with the water in your THF envolving H2-gas.

until all your H2O was consumed the bubbling stopped, btw. decreased

[Midi]

Thats what I suspected.  

So if the first 2g was ate up by the h2o, then in all actuality there was only 5.5g added that is actually active = 20% reduction in yield.    ah well.    I guess the THF should have been dried 2 more times. It was hard to tell wether it was wet. It was pretty clear, and 800ml had been dried in about 800g of agent.

[Rhodium]

The fizzing heard when adding LAH to even rigorously dried THF is from boiling THF due to localized heating brought on by the heat of solvation/complexation for LAH in THF.

See:

Post 227588 (missing)

(terbium: "Re: LAH and THF", Chemistry Discourse)

Post 227621 (missing)

(Lilienthal: "Re: LAH and THF", Chemistry Discourse)