Cauldron
“The solution was stirred overnight.” — Why?

If you have a reaction going at a fixed temperature (especially room temperature), nothing remains to be added and all reagents and products are either dissolved or soluble, then what is the point of stirring?

This is not a rhetorical question. I really want to know the answer! Dispersions,1 suspensions,2 emulsions,3 partly dissolved starting materials, precipitating products and heterogeneous catalysts – I can understand why these need stirring. But true solutions?

Does not the Brownian motion of particles ensure a perfect mixture, especially in the absence of a temperature gradient, regardless?

I do stir all room temperature reactions, even if everything appears dissolved. Bad habit? Enlighten me, please. (By the way, did you know that Albert Einstein solved the tea leaf paradox in 1926?)

 

1,2,3 I will probably never learn the difference between these.

 

11 Responses to Stirring the pot – Why?

  1. Simon says:

    If you are expecting a precipitate and you don’t want big crystals that adhere to the side of the flask…? But generally I agree – my postdoc adviser forbade the stirring of solutions and so when I did it, it was out of devilment.

  2. Ben says:

    I don’t know the answer to this, but the first step of my total synthesis is a Keck allylation which is left in the freezer for 3 days without stirring and works fine. So Brownian motion seems to work, even at -20C

    • pit says:

      @Ben: for the Keck allylation how do you dry the molecular sieve. In my case I left the molecular sieve in the oven at 110 degres 5 days and seems that’s not good. The solution becomes orange-red but slow conversion. What s the key for this reaction. Thanks for the response.

      • Ben says:

        @pit, sorry about the slow response, only just seen your comment! I use mol sieve powder (looks like flour – and gets everywhere like flour too!) I tend to leave a 50ml beaker in the oven constantly and top it up when it runs low, but after 24 hrs it should be good to go. Red solution is good – I assume that’s the Ti-BINOL complex. I don’t know about speed of conversion, I tend to leave them for 3-4 days depending on how organised I am at making it to demand and it usually goes in ~80% yield although there always is some aldehyde left, someone in our group used to leave her Keck’s for 6 days and still saw aldehyde left!

  3. Pamplemousse says:

    Because even with perfect solubility stratification can still occur? I would think that the act of dissolving a reagent in the solvent can potentially lead to denser solvent and uneven distribution.

    Also, a reaction might not appear to be heterogeneous but still be viscous, or forming colloids or whatever. It’s far better to uselessly stir than to not stir when it is required.

    Why not do a simple experiment, running two identical reactions overnight, one with stirring and one without. If you stop it prior to completion HPLC or NMR should be able to give you the % completion.

  4. Alex says:

    One of the reasons I can imagine is slow evaporation of the solution that leads to the deposition of stuff on the walls. If there is stirring it will redissolve it.

    Emulsion — liquid in liquid. Suspension — solid in liquid. Isn’t it?

  5. Anderson says:

    Isn’t mass transfer still important, even for reactions happening in solution? I would imagine that stirring would be used to control rate limitations.

    • Edward G. says:

      I’m afraid this is very important. I recall someone found his reaction was much slower without stirring, but I forgot the detail. For refluxing reaction, the stirring is more like a boiling chip I’m afraid.

  6. CatalysisAlex says:

    Dispersion – solid in gas?
    Emulsion – when you have some sort of micell formation
    Suspension – solid in liquid

    Regarding stirring – you reduce the risk for surface reactions if sensitive to e.g. air? Hm, or increase….
    Anyway, in small scale one probably won’t often notice a difference with(out) stirring. On larger scales, especially for fast reactions, you should notice the heat transfer – exo-, or endothermic reactions. Maybe more safety though than anything else?

  7. […] blog post to U. of Minn. page hosting the video. I also enjoyed Synthetic Remark’s post on stirring a reaction mixture. I’ve had many conversations with colleagues on that issue and while I don’t think […]

  8. CTchem says:

    no problem with Brownian motion…but i think with stirring, it just increase the rate of collision and therefore increase the rate of reaction….reaction occur faster…
    stirring also dissolved any solid formed from reaction to ensure or to unblock the rest material to complete the reaction…

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