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New Tricks With Glassware

leycolumnCourtesy of Steve Ley’s group, here’s a lab trick I’d never come across before. They were trying to purify a nasty mixture of closely related isomers, and found that the best chromatographic separation came from a long, long, run in ether/hexane. I’ve been in that situation myself, but it’s hard to have the patience to run a large column for such a long time, and it’s even harder to evaporate down the ridiculous amounts of solvent that you generate. (Even experienced organic chemists tend to underestimate how long that last part can take).
Ley’s group hit on an interesting solution. They loaded the crude material from a 42-gram reaction onto silica gel, and hooked a water-cooled condenser up to the top of the column. Under the condenser was a one-liter flask of 1:1 ether/pentane, heated to reflux. Those two solvents form an azeotropic mixture (about 1:1) that happens to match up well with the solvent brew needed for the column. This way, fresh solvent was continuously dripping down through the column, which was rigged to elute back into the flask of boiling solvent.
Chemists will recognize this as a variation of the Soxhlet extraction, and a rather ingenious one. To switch fractions, you turn off the heat, pour out the 1-liter flask, and charge it up with fresh pentane and ether. The solvents are so low-boiling that the material coming off the column doesn’t decompose while it’s cooking around in there in between. With one kilo of silica gel, they ran the column at about 80 mL per minute, and cut fractions about every 7 hours. (Told you it was a slow column!). After five days of this, they’d separated out their isomers. That took them out to 19 fractions, which seemed to be enough, but it turned out that washing the column with acetone furnished a pretty good amount of the final (most polar) component (which was presumably coming out very dilute by that point).
They used about 17 liters of solvent, which is a fair amount of rota-vapping, but is nothing compared to the 590 liters that would have been used under normal column conditions. (No one would have been able to put up with that). This idea will probably always have limited application – there are only so many solvents (or solvent mixtures) that can be used, for one thing. And in many cases people will grit their teeth and turn to large-scale HPLC when it’s available. (That’ll use more solvent than this, but less than an old-fashioned column, in most cases). But if someone had thought of this technique back in, say, 1955, it would have been everywhere.
And it could still be especially useful in academic labs, where labor is cheaper than solvent, and worth considering elsewhere. I’m always glad to see something new constructed out of the sort of equipment that’s in the drawers of every lab bench.

15 comments on “New Tricks With Glassware”

  1. Sleepless in SSF says:

    This seems sort of amusing to someone who normally works with 75 micron columns with 500 nl/min flow rates and peak widths of 10 seconds FWHM. I know there’s nothing really funny about it, but since I rarely use (or even see) prep LC the contrast is startling.

  2. George D. McCallion says:

    I actually read this article. From an innovative standpoint, it is very good. But now come the hard realities:
    1. The use of ether (diethyl or DIPE) on a large scale is NOT SAFE WHATSOEVER.
    2. Would this ‘process’ be truly applicable on-say-a targeted 100gm scale of FINAL product?
    3. Vale lettered ‘l’ should be a switch valve.
    4. Heating, cooling, then re-heating the system is not efficient.
    Don’t get Me wrong; I like the innovation here. I have applied similar thinking on many scale-up projects. I have no doubt that alterations to these diagrams (en route to scale-up) will be forthcoming.
    I applaude the authors for this publication!

  3. fat old man says:

    This technique bears some resemblance to simulated moving bed chromatography: solvent recycling, close Rf’s. SMBC is used on very large scale to even separate enantiomers. Not to take away from a brilliant solution using standard lab equipment.

  4. Nick K says:

    What, haven’t they heard of gradient elutions?! I wonder if they tested the eluant for peroxides before evaporating it – I bet there was a lot after all that exposure of the warm ether to oxygen on silica.

  5. JC says:

    Ether/pentane fractions will evaporate overnight in the hood.

  6. tapper says:

    evaporation of solvents in the hood is illegal with the liability of huge fines

  7. TNC says:

    Even experienced organic chemists tend to underestimate how long that last part can take.
    Word. No matter what kind of golliwog pump is connected to your rotavap, it never pulls fast enough to make you happy.

  8. TWYI says:

    Having read this paper, one still has to TLC the eluent coming off the column pretty frequently – then once your ‘top spot’ is off take it out of the flask and restart the cycle?

  9. provocateur says:

    there MUST MUST be a better way….

  10. TWYI says:

    I agree.
    As far as I can see, this is basically no different to attaching a resevoir of solvent and refilling it with the solvent that comes out of the bottom of the column…. and when you do this you can TLC the fractions coming off for peace of mind. Instead of leaving it to boil away.
    Also, as Derek says it is totally dependant on the pretty limited solvent mixture.
    No surprises Steve Ley is not the lead author IMO.

  11. Spiro says:

    I have done many flash chromatographies on a 100 g scale, with large amounts of silica, and from my experience, this apparatus is a toy.
    Let’s be realistic: the Rf differences here are so big that any “creative” method would have given a succesful separation. If you use a solvent system that is not polar enough (in the present case 1:1 ether/pentane), you can make any claim about the amount of solvent needed (supposedly 590L).
    I do not want to seem pretentious, but just present my perspective: 42g on 1kg of silica, with the present Rf difference, I typically elute it with 1L-fractions with a polarity “doubling” every 10 fractions, and after 40 fractions you should be done. Elution speed = 15/20 L/h. 10 fractions max to evaporate (thanks again to the gradient). A 6h-job including evaporations.

  12. Tot. Syn. says:

    I bow to your columing skills, but you’ve still used 40 litres of solvent – no small amount! It might be fast, but sometimes the amount of solvent to be disposed of is a concern.

  13. Lucas says:

    What, haven’t they heard of gradient elutions?!
    That depends a lot on the solvent system and the molecules being separated. A gradient can improve your resolution drastically, or it can utterly destroy it. I’ve run a few columns where the slightest change in eluant polarity sent all my peaks whomping off the column in one giant mess, or worse, done wonky things with elution order. Not fun.
    I agree with Spiro that a creative choice of solvent system and an appropriate gradient might still have done a better job (or maybe not – I haven’t read the paper), but I still have to applaud the ingenuity of the technique.

  14. Nick K says:

    Given the relatively good TLC separation shown in the Ley paper I don’t know why they didn’t try a gradient. Maybe they did and it failed, but in that case they should have reported it. I’ve almost never experienced the problems you relate with gradient elution.

  15. fleetwoodmac says:

    I think some of you guys are a bit quick to critisize. Maybe if you actually read the paper instead of just glancing at the abstract you would see that the TLC separation shown is only acheived after running the plate 15 times. If this is good separation I’d hate to see what you call bad. I’d also be willing to bet that they tried several other methods before arriving at this one. Seems to be a nice solution to a difficult problem.

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