Transcription
Warning: Acetic anhydride and sulfur chloride are both corrosive liquids that produce irritating fumes. Proper safety equipment must be implemented. These chemicals might also be restricted in your area, so be sure to check your local laws before attempting this procedure. This reaction also produces sulfur dioxide gas, which is highly irritating to the respiratory system. Be sure to work outside or in a well-ventilated area.
Acetic anhydride is an extraordinarily useful chemical with a wide range of chemical and industrial applications. Unfortunately, it is fairly difficult to buy online since it is considered a restricted substance in certain countries. In the United States, however, it is only a List II precursor, similar to acetone or hydrochloric acid, so owning and making it shouldn't get me into any serious legal trouble.
Acetic anhydride can be synthesized in a number of ways, but most of the pathways to it require inaccessible reagents like acetyl chloride or dangerous setups like a Kjeldahl lamp. The pathway I'll be sharing, however, is highly accessible and not excessively risky, requiring only vinegar, baking soda, and a healthy dose of nasty sulfur chlorides.
To start, I filled a beaker with about 300 mL of 5% acetic acid, otherwise known as common white vinegar. In total, we will need 400 mL, but my beaker could only fit 300 mL at a time. Next, 28 grams of sodium bicarbonate was weighed out on a scale. This was gradually added to the vinegar with strong stirring to prevent excessive foaming. Unlike the kids in grade school, we want a calm and controlled reaction instead of a runaway science fair volcano. It took several minutes to add all the baking soda, but once the addition was done, the solution was brought to a boil to remove the water. At some point, when enough liquid had evaporated, the last 100 mL helping of vinegar was mixed in. Then, once the volume was reduced enough, the solution was transferred to a smaller beaker where the rest of the water was boiled off. As the liquid began to crystallize, the stir bar was removed, and the boiling was allowed to continue. After some time, the solution turned into a gooey paste, which was cooked further until it turned solid and powdery. This solid is sodium acetate, and it is what we are after for this reaction.
Now you may be tempted to just stop here since everything appears dry, but that isn't really the best idea. Some water is likely still present, which could inhibit our main reaction later on. To fully dry the sodium acetate, it is best to completely melt it. Fortunately, this compound has a fairly low melting point, so I was able to liquify everything with just my torch and hot plate. Keep in mind that sodium acetate will decompose at 200°C vigorously, though, so if you try this yourself, don't rush things too much. As soon as my sample was molten, the heating was stopped, and the liquid sodium acetate was manually mixed with a metal spatula. This action helps prevent it from solidifying into one solid cake, which would have been a massive pain to deal with. After only a few minutes of mixing and cooling, I was left with a beaker full of nice, anhydrous sodium acetate. This was allowed to fully cool before proceeding.
And now for the main reaction. First, 10 mL of sulfur monochloride was measured out. I made the sample in a previous video, and you can clearly see that storing it in a bottle with a plastic lid was a bad call on my part. Originally, my sample was mostly sulfur dichloride, but as it sat in storage for the past year, the extra chlorine seems to have reacted with the plastic lid, leaving me with amber-yellow sulfur monochloride. Fortunately, this is exactly what the reaction calls for. So, to an ice-cooled 100 mL round-bottom flask, I poured in my entire sample of anhydrous sodium acetate, followed by the 10 mL sample of ice-cold sulfur monochloride. The addition of sulfur monochloride to sodium acetate is fairly exothermic, so it's best to keep everything cool and combine the reagents slowly. Once all the sulfur monochloride was added, the resulting paste was mixed thoroughly with a stir rod, and a reflux condenser was connected to the top of the flask. Then, the ice bath was swapped out for a boiling water bath, and the main reaction was allowed to commence. During this reaction, a sort of heat-driven double displacement takes place, which converts the sulfur chloride into sulfur acetate, otherwise known as diacetyl sulfide. This compound acts as an unstable intermediate, which quickly decomposes into acetic anhydride, sulfur dioxide, and elemental sulfur. The boiling water bath keeps the temperature high enough for the reaction but not high enough to melt the produced sulfur onto our glassware.
Now, as the reaction progressed, a couple of things happened. For one, a pale liquid could be seen refluxing in the flask, which was a good sign that acetic anhydride was being produced. The reflux rate was pretty slow, though, since acetic anhydride can't reach its boiling point in a hot water bath. The other thing I noticed was a strong, all-too-familiar smell: the rich sulfurous odor of organosulfur compounds. Apparently, some kind of side reaction was happening, which produced a volatile chemical that smelled a lot like ethanethiol, the stinky additive used in natural gas. Interestingly, the smell behaved a lot like thioacetone. When I was close to the setup, it just smelled thick and sulfurous, but further away, it took on a more accurate rotting garlic smell. Whatever this chemical was, we know the sulfur chlorides were entirely to blame for its existence because sulfur chlorides are disgusting, evil, and most importantly, the only source of sulfur in this reaction. But I digress. Anyways, after refluxing for 2 hours, the condenser and hot water bath were removed, and a short-path distillation was set up. In theory, it would have been best to perform this distillation under vacuum, but I wasn't in the mood to mess up my vacuum pump with acidic vapors and stinky smells. So, I cranked up my hot plate, wrapped the reaction vessel in aluminum foil, and began distilling off my precious product into an ice-cooled receiving flask. Much to my delight, a decent amount of liquid actually came off, and best of all, it wasn't yellow enough to be sulfur monochloride. This first distillation worked out great, but the end product still smelled accurately and garlicky from the organosulfur compounds tainting it. So, to clean it up, I set up another simple distillation and added a small spoonful of potassium permanganate to the acetic anhydride to destroy the sulfurous contaminants. This time, I made sure to isolate the separate fractions during the distillation. The fraction that came over below 130°C contained most of the remaining organosulfur compounds along with other contaminants like acetic acid. The higher boiling fraction, on the other hand, held most of the acetic anhydride, and if you try this synthesis yourself, this is the fraction you want to keep. In the end, I had roughly 10 mL of product in total.
So now let's check out a few of its properties. For one, acetic anhydride is supposed to be moderately flammable, so let's pour some onto a watch glass and light it up. As expected, the acetic anhydride is fairly reluctant to ignite, due in part to its high boiling point, but eventually, I did manage to get a decent fire going. As you can see, the flame itself is somewhat dim, and it has a very nice orangish-red color. Acetic anhydride actually has a better oxygen balance than a lot of common fuels, so it tends to burn very cleanly without producing any soot or residue.
All right, now let's check out its reaction with water. I've added a few drops of universal indicator to my water to help spice things up a little, and as you can see, the solution turns highly acidic as soon as the anhydride is added. This is to be expected since the anhydride reacts with water to produce acetic acid. Something I did not expect was the apparently poor solubility of acetic anhydride in water. I kind of figured it would react violently and exothermically, but much like the sulfur chloride used to make it, it just beaded up and sank to the bottom of the flask. It took some stirring, but eventually, I was able to dissolve everything. And now I figured it was only suiting to neutralize that produced acetic acid in the same way we did at the beginning of the video with good old-fashioned baking soda.
You may remember that I started this video talking about how useful acetic anhydride is, so why on Earth did I just waste everything I made by burning it and turning it back into the starting material? Well, if this were my only sample of acetic anhydride, it would be a different story, but as you can see, I already have a whole bottle full. This is actually provided to me by this channel's newest sponsor, Backyard Science 2000, who runs an eBay store which I've linked to below. He sells a ton of different chemicals that are normally quite hard for home chemists to get, including phosphorus pentoxide, iodine, acetonitrile, and even collectible pieces like these radium watch dials. If you can think of it, he can probably sell it. He actually sent me a whole box of goodies to use in my video, which includes this sample of selenium that'll be coming in handy quite soon. If you do home chemistry and need specialty reagents, definitely go check him out.
All right, guys, there you have it: making acetic anhydride at home from vinegar and baking soda and a very stinky helping of sulfur chlorides. Thank you very much for watching. I had a great time making this video, and I hope you all learned something in the process. If you like what you see, consider subscribing to my channel. Trust me, I have a ton of amazing videos planned out or in production that you do not want to miss. And if you'd like to help support the creation of science videos like this, consider donating or becoming my patron on Patreon. The links, as always, are down below. A special thanks goes out to all the Lab Coats patrons; this channel truly wouldn't be where it is today without them. Stay safe everyone, and I'll catch you next time. Lab coats out.