CycloRook
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How to deal with long processes involving heat.
I have a question. How does one deal with long reactions that involve heat. I don't have a specific reaction in mind but say for example I go to
school and I need to leave a reaction running for hours or I have a reaction that takes 8 hours and I need to be elsewhere.
What does one do ? Do you just leave everything and hope it doesn't burn down the house in the case of a hotplate stirrer or do you start the reaction
then stop midway through.
I know this is very subjective.
Thanks
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timescale
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Depends on a lot of factors, such as the thermodynamics of the reaction, the stability of your heating system, etc.
Personally I am okay with letting most refluxes run unattended. Worst case is probably that the hotplate gets stuck at max heat and burns the reaction
mix. Or the water pump breaks and the solvent evaporates. Neither of which I consider likely.
For example, I've heated mineral oil at 200C overnight for the Nurdrage alcohol-catalyzed sodium method. I consider this safe because it's a
nonvolatile solvent + slow reaction + very stable temperature. But it's a gamble to leave magnesium metal refluxing in methanol for even a few
minutes, since the reaction quickly goes into thermal runaway.
Consider the worst-case scenario, but also consider what is most likely to go wrong, and decide whether it's an acceptable risk.
Est-il une beauté aussi belle que le rêve ?
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BromicAcid
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You have to do risk mitigation. Evaluate what could go wrong, what the outcome would be, and how it can be avoided. Sometimes the outcome of such
thinking is that a reaction should be avoided. Other times though it just means you have to wire on water connections or that you should do an
addition in stages.
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Sulaiman
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A good start is to read the section "2.16 Precautions for unattended operations" Vogel 5th edition.
My copy does not allow cut-paste, sorry.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Fyndium
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You can always get a web accessed camera so you can monitor the stuff, and a remote switch to kill the power in case something goes wrong.
Interrupter could also be effective. If electronics fail, it will kill the power.
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Fantasma4500
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you have to be intelligent about it, if theres something i have going that i feel needs my potential attention and ill be gone for a bit ill lower the
heat a few nods so i wont have to start all over with heating it up when im back
sometimes im evaporating large amounts of liquids and i leave them to evaporate carefully overnight at a lower heat setting, then when i have my full
attention to pay to it ill knock heat up an extra bit
it can be an issue if your scaling up a reaction, im hearing this is especially dangerous with organic chemistry reactions
and of course if you have an increasing temperature in distillation and there might be oxygen-like species in the mixture youre distilling you kinda
want to, and yet kinda dont want to be around when pushing your luck with the temperature
set up for a bigger batch of the simple chemicals and be a bit extra safe.
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Fyndium
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One thing for first, if you are intending to leave any reactions unmanned, perform at least a couple of those reactions in _full scale_ to see how
they behave.
I would never, ever leave an untested reaction going without immediate babysitting, because things can escalate very quickly in organic chemistry when
you scale up or increase heat if there is something exothermic going. I have done a couple of reactions which I have experience on several times over,
and I'm confident leaving them overnight because I'm fully aware of the reaction kinetics, and the reagents I used are either from reputable supplier
or same batch. A bonus was these reactions were mostly water based or of very low flammability. Refluxing ether is not one of those things.
[Edited on 15-3-2022 by Fyndium]
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