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In many organic reactions that I have seen, running the reaction at $\mathrm{-78\ ^\circ C}$ seems to be quite a popular choice, but I've never seen any explanation for this. What makes this temperature such a popular choice?

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    $\begingroup$ That's easy one - sublimation of CO2 $\endgroup$ – Mithoron Feb 17 '16 at 23:03
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    $\begingroup$ A little like so many reactions being 'refluxed for 12-15 hours' => I put it on before I went home and shut it off in the morning. $\endgroup$ – long Feb 17 '16 at 23:56
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    $\begingroup$ Just like 'run for 72 hours' --> "I finally got it set up and running Friday afternoon, then didn't get around to shutting it off until after lunch on Monday." $\endgroup$ – hBy2Py Feb 18 '16 at 16:23
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    $\begingroup$ Relevant answer to a different question. $\endgroup$ – Jan Aug 17 '16 at 11:45
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Dry ice (solid carbon dioxide) sublimes at −78 °C. Dry ice and acetone are a common cold bath for chemical reactions. The melting point of acetone is -95 °C so the bath never gets cold enough to freeze the acetone. The bubbling of the carbon dioxide gas as the dry ice sublimes keeps the cold bath well stirred.

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    $\begingroup$ Could you elaborate a little? $\endgroup$ – Mithoron Feb 18 '16 at 16:43
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As MaxW said, dry ice (solid carbon dioxide) sublimes at -78°C and it can be used in acetone, which is a cheap non-toxic solvent.

The biggest problem with cooling baths is that temperature must be monitored and corrected if necessary. It implies that the actual temperature is not constant but oscillates around an average value, which is highly undesirable. Also, constantly correcting the temperature makes it impossible to stray from the bench. Cryostats (cold generating devices) are useful sometimes but they usually are a real pain to use.

By contrast, the dry ice/acetone bath stays at -78°C, constantly and possibly for hours. When you put ice (a lot) in water, the water temperature will remain 0°C as long as there is still ice (roughly) and the same stands for the dry ice/acetone; as long as there is some dry ice left, the temperature will remain -78°C.

That is why the -78°C is so popular among chemists for low temperature experiments.

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  • $\begingroup$ I don't think acetone is non-toxic. $\endgroup$ – Technetium Aug 17 '16 at 9:01
  • $\begingroup$ @Joel Though it is non-toxic. It is considered "harmful if swallowed" (who would do that?). Of course precautions must be taken but it is one of the safest solvents in the lab from a health point of view. $\endgroup$ – SteffX Aug 17 '16 at 10:30
  • $\begingroup$ I’ll partially agree with @Joel that nothing is non-toxic. But I’ll also have to say that acetone is a lot less toxic than many other things in the lab (e.g. methanol which one would use for $-95~\mathrm{^\circ C}$). $\endgroup$ – Jan Aug 17 '16 at 11:44

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