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Drying Alcohol Using Magnesium and Molecular Sieves


Warning: Alcohols and magnesium metal are highly flammable. Fire safety protocols must be in place. Greetings fellow nerds. In this video we’ll dry some organic solvents using magnesium metal and molecular sieves. Now before we must first explain what we mean by drying solvents. We don’t mean drying as in evaporating them. When chemists say they want a dry solvent what they really mean is they want a water-free solvent or anhydrous solvent. All solvents, even those that don’t dissolve in water, absorb tiny amounts of moisture from the air. This is particularly pronounced with highly polar solvents like alcohols, and still occurs to a lesser extent with non-polar solvents like oil and toluene. If your chemistry is sensitive to water, you need to remove it from your solvent. Now i know that saying we’re drying a solvent sounds a bit strange, but we chemists are weird like that. Anyway, there are lots of different techniques to dry different classes of solvents. You’ve already seen me use fractional distillation to remove water from dilute hydrobromic acid and concentrate it. We’ll go over two methods for alcohols this video, and we’ll cover other methods for other solvents in later videos as we need them. The first method for alcohol is chemical drying with magnesium. We start with 5g of magnesium metal turnings in an erlenmeyer flask. These can be purchased online or made out of magnesium firestarters. You can even use magnesium ribbon if that’s cheaper for you to get. Now we add 300mL of methanol. I’m doing methanol for demonstration but this method can be used for any alcohol. Now i should mention that before beginning any sort of drying process, you should start with the driest solvent you can get by simpler means. Solvents should be fractionally distilled to remove as much water as possible. Chemical drying is effective, but expensive if you have excessive amounts of water and need to add even more chemicals to compensate. Garbage in garbage out as they say. Now on top of the flask we set up a condenser as a reflux column. We don’t want any of our alcohol boiling away. On top of the condenser i’m going to put some aluminum foil to limit how much air flows back into the flask. For extra sensitive work you would use an oil bubbler setup or a drying tube. Now all we do is wait. Methanol is very reactive with magnesium and you can see bubbles forming. What’s happening is the magnesium is reacting with water to produce magnesium oxide and hydrogen, and also reacting with methanol to form magnesium methoxide and hydrogen. The magnesium methoxide reacts with any additional water in solution to make magnesium oxide and methanol again. The idea is that overall we’re consuming all the water by converting it to magnesium oxide. Now this reaction is very exothermic and the methanol will eventually boil. So we needed the reflux column earlier to prevent losses. Eventually all the magnesium will be consumed and the bubbling will stop. For me this took about four hours. If you’re impatient you can heat up the solution to get it started sooner. For larger alcohols like ethanol or isopropanol, heating is likely necessary as they’re much less reactive and might not even start at all. Now occasionally the magnesium might be initially covered in too much magnesium oxide making the reaction difficult to start. In that case drop in half a gram of iodine crystals or a few drops of chloroform to etch the surface and start up the reaction. As said before magnesium drying is mostly for alcohols. Do not use it for chlorinated solvents like chloroform or for acids. The reactions in those cases do not dry the solvents and in some cases can lead to dangerous thermal runaways. Solvents like ethers don’t react with magnesium at all and cannot be dried that way. Okay our reaction is done. If you have large amounts of solvent you usually put it on a hotplate and reflux for a few more hours to ensure all the water is destroyed. Anyway, we have here a mixture of magnesium oxide and magnesium methoxide in what we hope is dry methanol. Now those substances can be detrimental to our chemistry so to isolate just the methanol, we rearrange our reflux condenser setup into a distillation setup. Now we just distill off the methanol. You’ll notice that this particular distillation setup is a sealed system. Exposure to air would allow moisture to get back in. While air may be unavoidable in later steps, you should try and reduce air exposure as much as you can. In professional setups, the exhaust tube at the right of the screen would be connected to a nitrogen line or a drying tube. Mine just exhausts out the fume hood but you get the idea. Okay we’re done distillation. You can see here the pure and dry methanol solvent ready for use. It may still contain very minute traces of water but for the vast majority of sensitive experiments this is not an issue. Back over here we have the residue of magnesium oxide and magnesium methoxide. Now if the methanol was already very dry then this should mostly be magnesium methoxide. You can actually add new wet methanol directly to this and use up its remaining drying power if you want. Just reflux for a few hours to ensure the water is destroyed before distilling another batch. Okay so that was chemical drying with magnesium. Now we’re going to do absorption drying with molecular sieves. Now these are molecular sieves. I’m using the 3A type which means it has pores that are 3 angstroms in size. This stuff might seem exotic but it’s pretty easy to buy off ebay and other online stores and relatively unrestricted. Anyway these things are awesome. They’re a type of zeolite or aluminosilicate clay that’s been engineered to have pores of a regular size. Molecules smaller than those pores, like water, and wriggle in and stick, while bigger molecules cannot fit and just pass right by. Overall, this can be used to dry solvents. What’s amazing is that these can actually get solvents even drier than most other methods including chemical. I didn’t believe that last point either but apparently there is a journal article written about it that i’ve linked in the video description. But before we can start, we need to dry the molecular sieves themselves first. They’ve been sitting on air since they were made in the factory and thus have absorbed water. Get an amount of molecular sieves that corresponds to about 10%-20% by weight of the solvent you want to dry. Now we need to bake them. Put them on a heat source and raise the temperature to whatever the manufacturer specified as the regeneration temperature. I’m using a hotplate set to 250 degrees celsius which is the temperature that this particular brand of sieves regenerates at. A hotplate is not the best way to bake them and most labs i know use vacuum ovens or oil baths with the drying flask connected to a high vacuum system. A strong vacuum helps to remove the water. Now in a professional lab a highly sensitive vacuum gauge can tell you when the sieves have stopped releasing water. Since i don’t have one i’ll just leave these sieves to dry on heating overnight. Here we are and the sieves have been heated overnight and cooled back down to room temperature. We now add our alcohol to the sieves, in this case ethanol. We seal the flask and let it sit for another day. The longer the solvent is over the sieves the better, the absorption of water is a rather slow process. Here are the next day. We now pour the dried alcohol into a storage container over freshly dried sieves to ensure that the alcohol remains dry. There might be some sieve dust suspended in the solvent. This is not a problem for most reactions but if you want you can filter or distill the dried solvent again just before use. As for these used sieves, we can actually dry them again and reuse them. Just be careful that the lingering organic solvents on them don’t start a fire. You might want to let them evaporate dry first for a few days before heating them again. To minimize cross contamination, reuse sieves for the same solvents they were used with before. And that’s all there is to it. While this was a slower process that took a couple of days for best results, it’s much easier to actually execute. Nonetheless, use good solvents and try to fractionally distill or otherwise dry those solvents with simpler approaches before using molecular sieves as the final step. They work great, but they don’t perform miracles. I’ll be needing dry solvents in future organic synthesis reactions. Thanks for watching. If you would like to support the continued production of science videos like this one, please support the channel on patreon. Links are in the video description.

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