*For more cool stories, pictures, and videos of chemistry demonstrations, click here*

This week’s lab involved taking the unknown carboxylic acid from experiments 1 (acid/base extraction) and 2 (purification by recrystallization) and converting it to a primary amide. The acid was refluxed in neat thionyl chloride and added dropwise to 30% ammonium hydroxide. Extract with dichloromethane and evaporate. The fumes from this lab are pretty nasty. The reagents are bad enough – the lab smelled like ammonia all day – but sulfur dioxide and HCl gas are liberated during the reaction. The “fume hoods” in the ugrad labs aren’t so hot, so the students attach a long-stem funnel (where do I sign up to be a glassware photographer?!) to the water aspirator and invert the funnel over the reaction mixture as it refluxes. This collects the fumes as an extra “mini fume hood” during the reaction.

Amides are really interesting compounds. I didn’t realize how much I had to say about them until I was preparing notes for the pre-lab handouts/lecture. Amide bonds are how our amino acids link together to form proteins, but that’s really the least exciting property. The lone pair on nitrogen can participate in resonance with the C=O double bond. This zwitterionic resonance structure is actually a fairly significant resonance contributor.

If you were to take an NMR of a dimethyl substituted amide like the one shown above, you might expect to see one singlet integrating to 6 protons corresponding to the two equivalent methyl groups. If this were an isopropyl ketone instead of a dimethyl amide, you’d be right, as the rotation about the single bond makes the methyl group equivalent. However, due to the significant double bond character of the C-N bond, there is restricted rotation about the C-N bond. The rotation is so restricted, that the bond rotates so slowly that the two methyl groups are actually not equivalent, and you see two singlets integrating to 3 protons each, one for each methyl group.

If you heat the sample, though (and many NMR’s have variable temperature capabilities), you begin to apply more energy to the system. The system can use this energy to rotate about the C-N bond. As you apply more and more heat, eventually the amide has enough energy to rotate about the C-N bond fast enough to make the methyl groups equivalent, and the two singlets coalesce into one singlet. You can take NMR spectra at all these various temperatures and plot them on a single paper to give the cool 3-D effect of the signals coalescing into one signal:

We did that experiment in my ugrad advanced organic class. It was one of the cooler experiments I did as an undergrad. (No, that’s not my VT NMR. I have no idea what that NMR is from)

All that’s really cool, but it has nothing to do with the demo for this week. The demo I did was the Nylon Rope Trick. If you add a diacid chloride (like adipoyl chloride) in an organic solvent to a diamine (like hexamethylenediamine) in water, you get a biphasic mixture. At the phase boundry, the diamine reacts with the diacid, where one side of the amine adds to one of the diacids to give an amide bond. With two reactive sites on both molecules, the amide grows and grows into one long chain of amide bonds. This polymer is nylon (click for larger).

This type of polymer, where the polymer alternates each monomer one unit at a time, is called an alternating copolymer. There are aparently several ways to make nylon. In theory you should just be able to add the diacid (dissolved in water) to the diamine (dissolved in organic solvent) and the diacid is supposed to act as both catalyst and reagent. That did not work in my hands. This page gives a really nice background as well as two other procedures for making nylon. It is significantly more common to take the diacid chloride in an organic solvent and add that to the diamine in water. Several pages (here, here, here) suggest dissolving the diamine in aqueous sodium carbonate or sodium hydroxide. That, too, never worked in my hands. You can also ring-open polymerize caprolactam. I didn’t have access to caprolactam, so I couldn’t try that way. You can also use either the 6-carbon diacid (adipic acid) or the 10-carbon diacid (sebacic acid). I only had access to adipic acid, so that was all I could use.

I must have tried this demo 5 or 6 times before I got it to work. For each try, I had to make the acid chloride from the diacid, so it was not trivial to practice this demo. As I said, just mixing the diacid and diamine didn’t work. Neither did preparing a soludion of the diamine in an aqueous base. It also appears diamines with carbon chains shorter than 5 carbon atoms are a no-go, too. I had the 3 carbon and 2 carbon diamines, but neither of them gave good results.

What did end up working for me was a solution of the diacid chloride in hexanes and a solution of the diamine in water. This worked once in practice before I tried it for real. When I did it in lab, I suspect the solutions were too dilute. I kept getting phase-boundry-sized disks of nylon, but no continuously growing string. I think having the two phases be more concentrated would have helped.

When I practiced, I was able to pull a string about 4 feet long or so. The picture is below (click for larger). There are two main ways of pulling the nylon from the solution, it seems. One is to catch it on a stirring rod or boiling stick, then wind the string around the stick as it forms. The other way is to grab the nylon with forceps and just keep pulling. You can get a surprising amount of nylon from a few grams of starting materials.

Video below