Dry ice and acetone

In my organic chemistry class there are several reactions that I’ve been told are generally conducted at $\pu{−78^\circ C}$, which is apparently also the temperature of dry ice in acetone. This is all well and good, but I don’t really get what that actually means. Do you put the dry ice in the acetone and then toss in your reactants? Do you add the dry ice and acetone and it makes the surrounding air colder? Something else?


I’m just not sure how this is actually accomplished, and if you do add the reactants to the dry ice/acetone mix then how do you avoid reacting with either of those things?




The mixture of dry ice and acetone is used *outside* your reaction vessel to keep the vessel cool just as a water bath is sometimes used to provide uniform heat to a reaction vessel. The only role is to provide a uniformly cool environment for the reaction vessel.


Acetone isn't the only choice here and isn't essential (anything still liquid at the temperature of dry ice will do). Acetone is cheap and (relatively) harmless which is why it is commonly used. 


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You put broken up Dry Ice in in an evaporating dish in a bigger evaporating dish lined with glass wool, the whole atop a magnetic stirrer base, then carefully add acetone. Fizz. Add and clamp your flask with stir bar under dynamic inert gas into the slush. Go for it, solvent(s) and reactants. Ace glassware has problems with star cracks.


Don't try this in a styrofoam bucket. Note that acetone vapor is wildly flammable. Cheesecloth atop NOT touching the slush for more insulation and to keep condensing moisture out. Never plan to fill a flask more than half-full, because the universe hates you.


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Above two excellent answers have addressed your concern brilliantly. Thus, for yours and some other readers' benefit, I'll address why acetone/dry ice bath maintain at $\pu{-78 ^\circ C}$ temperature, which make it so popular in organic chemistry research laboratories worldwide.


Look at the phase diagram of dry ice (solid $\ce{CO2}$) below (from [Wikipedia](https://en.wikipedia.org/wiki/Dry_ice)):


[![Dry Ice Phase Diagram](https://i.stack.imgur.com/VhAvB.png)](https://i.stack.imgur.com/VhAvB.png)


It clearly shows the $\ce{CO2}$ sublimation point at $\pu{1 atm}$ is $\pu{-78.5 ^\circ C}$. That means either sublimation or deposition (the reverse of sublimation) occurs at $\pu{-78.5 ^\circ C}$ ($\pu{194.65 K}$) at $\pu{1 atm}$ pressure. Thus, you may use any liquid with its melting point below $\pu{-78.5 ^\circ C}$ with dry ice, so the heat transfer occurs between dry ice and liquid such a way that the minimum equilibrium temperature of the mixture will be at $\pu{-78.5 ^\circ C}$ and maintain at that temperature as long as you supply enough dry ice. The solvent of choice could be acetone, methanol, ethanol, 2-propanol, hexane, and pentane, all of which have melting points below $\pu{-78.5 ^\circ C}$ (*e.g.*, melting point of acetone is $\pu{-94 ^\circ C}$). 


Additionally, if you need to maintain temperatures above $\pu{-78 ^\circ C}$, you can use a solvent with a freezing point exactly the required temperature, which is obviously above $\pu{-78 ^\circ C}$. For example, if you need to perform a chemical reaction at $\pu{-60 ^\circ C}$ instead of $\pu{-78 ^\circ C}$, you should choose diisopropyl ether, which freezes at $\pu{-60 ^\circ C}$ with dry ice. However, you must add dry ice slowly to avoid freezing the entire mixture, because when temperature reached $\pu{-60 ^\circ C}$, the solvent starts to freeze. Thus, keep adding dry ice as needed, just to maintain the solid-liquid slush.


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