I have read that LAH thermally dissociates when dissolved in ethyl ether. I'm wondering what this means in terms of actual behavior.
Help appreciated.DJF90 - 22-4-2015 at 03:20
As with dissolution in THF, there is a large enthalpy of solvation to take into account.
As a related aside, I'm kinda surprised that purification can be effected by soxhlet extraction with ether...
[Edited on 22-4-2015 by DJF90]soma - 22-4-2015 at 12:45
Thanks.
Yes - it seems the impurities aren't soluble in ether and can be filtered out. Actually, I was thinking of dissolving it and then pouring off the
supernatant, taking a small aliquot, evaporating off the ether in a water bath (at 70C), and weighing, to get an idea of the activity of the LAH
without having to try to measure the amount of hydrogen generated.
I'm not sure how long to give it to dissolve though.DJF90 - 22-4-2015 at 23:28
Quote:
As a related aside, I'm kinda surprised that purification can be effected by soxhlet extraction with ether...
What I meant was, given the large enthalpy of solvation, I'm suprised that it is a safe method of purification. Normally when preparing LAH solutions,
you charge LAH to the vessel and charge the required volume of solvent down the condenser as fast as practicable. Even then it may end up refluxing
due to the exotherm (THF as solvent) so I'm cautious about having a small stream of ether dripping onto solid LAH in a soxhlet extractor. I'd be
interested to hear from members here who have experience of this procedure.
...taking a small aliquot, evaporating off the ether in a water bath (at 70C), and weighing, to get an idea of the activity of the LAH without having
to try to measure the amount of hydrogen generated.
That seems like a fairly long winded approach when you can just titrate an aliquot. Supposedly iodine can be used, but personally I'd opt for
salicylaldehyde phenylhydrazone (see: http://www.sas.upenn.edu/~marisa/documents/gtitrate.pdf)soma - 24-4-2015 at 13:27
I'm going to be dissolving and pouring it off anyway to purify it so it won't be that much extra to evap off a small amount and weigh it.
