Chemistry - Why is −78 °C a magic temperature for organic reactions?

Solution 1:

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.

Typically, though, the temperature in the flask with an ongoing reaction is at least about 5 °C higher than the one in the cooling bath. (Except if using a thermocouple, working at a scale of 10...25 mL and less often implies that the thermometer is in cooling bath, and not in the reaction mixture. Thus, this temperature difference isn't recorded.)

Solution 2:

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.