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I've never used Soxhlet apparatus before and I have troubles understanding how to properly perform the following step from my protocol:

To a gently refluxing suspension of 1 g Lithium aluminium hydride in 100 mL of diethyl ether, there was added 1.2 g of compound as a saturated diethyl ether solution with use of a Soxhlet extraction condenser which is modified allowing the continuous return of condensed solvent (through the thimble). After the addition was complete, the refluxing conditions were maintained."

Soo... How do I do it? I run RBF with condenser on top until reflux stabilizes and then I open it, add Soxhlet with compound and continue refluxing or I set up everything cold (RBF with ether + Soxlet with compound in ether + condenser) and then start refluxing?

Also - I'm not quite sure if I understand the thimble modification - can anyone shed some light on it for me?

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  • $\begingroup$ Puzzling indeed $\endgroup$ – Alchimista Jan 11 '18 at 22:05
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    $\begingroup$ Perhaps it is so. ... They place the 1.2 g of compound in the thimble. At beginning. The gently refluxing do the rest by dropping diethyl ether to the thimble. You get a kind of gentle addition of an almost saturated solution filtered by the thimble dropping down to the reaction flask. They basically innested a syphon at the bottom of the soxhlet .It seems a clever way to slowly add the compound without opening the systems with volatile solvents. This offer the way to keep a fixed volume of solvent all through the reaction.. But this means sacrificing a soxhlet and skill on molding glass. .. $\endgroup$ – Alchimista Jan 11 '18 at 22:23
  • $\begingroup$ Obviously if compound does not readily dissolve in diethyl ether. $\endgroup$ – Alchimista Jan 11 '18 at 22:26
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In my opinion the intention is to have a very slow addition of the reagent to the hydride. The compound is dissolved in ether, so a Soxhlet is of no use since it is for solids.

What they are possibly referring to is a liquid-liquid extraction apparatus, specifically the one for extracting with solvents lighter than water:

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The saturated solution of the compound is then placed in the thimble and when reflux starts, the solvent is condensed, falls into the tube and returns to the flask carrying over the compound. Normally the tube goes to the very bottom of the thimble. In fact, usually it is a loose tube with a wide opening on the top. How to know that the addition is complete is another story. If the compound is not colored, it will not be easy without opening the system. In any case, maintaining reflux for a long time will ensure that everything goes into the flask.

I suppose that for some reason it is difficult to perform a slow addition through a funnel in this case. It would be, however, better to use a syringe pump but not everybody has one of those.

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  • $\begingroup$ Yes but why to call it modify soxhlet? I really think of something as in my comments. In that case addition is completed when no solid remains in the paper thimble .surely not a clear description anyway.... $\endgroup$ – Alchimista Jan 12 '18 at 12:04
  • $\begingroup$ Modified in the sense that it is not an extraction. It is also said that a "saturated solution" is placed in the thimble, which is not posible in a Soxhlet with a bypassed siphon. Anyway, without some more information we can only guess. $\endgroup$ – Raoul Kessels Jan 12 '18 at 12:25
  • $\begingroup$ Understand. But I would have written liq liq extractor :) I thought they were forming a saturated solution drop by drop. But in your case, why not to use a dropping fumnel and just a bit more solvent. I just tried to help. $\endgroup$ – Alchimista Jan 12 '18 at 12:29
  • $\begingroup$ True. As I have said in the answer, I suppose that there might be some problem for using a funnel. This is many times the case when volumes are small. Increasing the volume for having a slower addition might lead to other problems. By using this setup the addition is slow and the volumes remain constant as you have pointed out already. $\endgroup$ – Raoul Kessels Jan 12 '18 at 12:39
  • $\begingroup$ Thank you for great responses so far! Indeed - the protocol is quite minimalistic, but it was writen by a proffesional. Which Im not :D The reflux conditions are supposed to last for 48 h, and the compound is yellow so I guess Ill notice the addition. Corect me if Im wrong, but wouldnt it be simpler to omit the thimble completly? Just fill the Soxhlet with saturated solution of compound, set everything and star slow reflux. Refluxed solvent would slowly dilute compound and add it dropwise to LAH. Am I not seeing something? $\endgroup$ – A.Fleming Jan 12 '18 at 18:30

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