Aniline is a very useful building block in organic chemistry, primarily as a precursor to many dyes. It is a colorless liquid when pure, but commercial and homemade samples will often appear yellow, or even black, due to its tendency to form highly colored polymers. Although the impurities will not interfere with some reactions, it is still preferable to have fresh, clean aniline, especially if you are going to use it to make dyes.
The easiest way to separate aniline from the polymer gunk is by vacuum distillation. I tested out my trusty-rusty rotary vane vacuum pump with a simple vacuum gauge, and as you can see, it is able to pull down to 740 mmHg below atmospheric pressure, or about 20 mmHg. And yeah, I know that’s terrible and barely qualifies as a vacuum, but it’s doing its best, okay? As it turns out, this is a totally acceptable pressure for distilling aniline. Using the Clausius-Clapeyron equation, we can see that lowering the pressure to 20 mmHg will lower the boiling point of aniline from 184ºC to 72ºC, which is a much more comfortable temperature for a distillation.
The aniline that I have is technical grade, and several years old, so it is a very dark red color. I measure out 25 mLs of it and add that to a 50 mL round bottom flask with some boiling stones. Next, I set up a vacuum distillation apparatus. Usually I’m not a big advocate of greasing glass joints, but in this case it is necessary to achieve a good vacuum. Adding an aluminum foil skirt around the flask will help it retain heat. I also placed a cold water bath around the receiving flask, which probably isn’t necessary, but is never a bad idea. Finally, I turn on the vacuum pump.
As soon as the vacuum is pulled, the liquid in the receiving flask begins to bubble, even though it’s still at room temperature. This is due to dissolved gasses and water being pulled off. After a couple minutes, the bubbling slows down a lot. Some water condensate fogs up the inside of the apparatus, but there isn’t enough to actually form drops. As heating continues, a refractive vapor front climbs into the still head, and as it reaches the thermometer, the temperature shown quickly climbs, leveling off at 73ºC as distillation begins in earnest. Everything continues nicely for the next several minutes… and then it bumps.
I let the apparatus and my frustration cool down for a couple hours, and then I returned to the lab to salvage the aniline in both flasks. I transferred the aniline from each flask back to the graduated cylinder and topped it up to 25 mL again with a little more from the bottle. I then poured that into a 100 mL round bottom flask. Since I know now that this aniline contains a significant amount of water, I also added a few spatulas full of sodium hydroxide as a drying agent, and allowed it to rest overnight.
The next day, I attempted the distillation again out of the larger flask. Click here to see a time lapse video of the distillation in progress! The bumping was just contained enough to yield a clean product, though a tiny bit of yellow did get ejected into the receiving flask just before the end of the run. Because of this, the distillate was colored a pale yellow color instead of being completely colorless, but it’s still a huge improvement over what I started with.
Oddly enough, on the second distillation run, the aniline distilled at 51ºC, more than 20ºC lower than the previous day. After the distillation was finished, I checked my vacuum pump with the pressure gauge. It turns out that it was just having a better day!