One of the most famous tricks in chemistry is “The Rope Trick.” First discovered in 1959 by the chemist who went on to invent kevlar, this lets chemists pour two liquids into a glass . . . and then pull out a seemingly endless nylon rope from between the still-separated-liquids.
Here’s how this trick goes. (If you don’t like long names, I suggest you skip this paragraph.) First you take 1,6-diaminohexane and dissolve it in water. This molecule is a corrosive six-carbon chain with a -NH2 group at both ends. Then you mix sebacoyl chloride with heptane and get decanedioyl dichloride. This is a ten-carbon chain with -COCl group at both ends.
When you combine these, they should look like unmixed salad dressing, one solution sitting placidly on top of the other. Then you take tweezers or a wand and reach down to right where the solutions touch each other. Pull up. You’ll get a thin cord of rope. Wind it around a nail, a stick, or a bobbin. Turn. Just keep turning. You’ll get a seemingly endless stretch of rope. This isnylon 6, 10, named for the number of carbon atoms in each of its components.
What, you might ask, the hell?
Look down at the equation. In the top line you’ll see 1,6-diaminohexane with its NH2, also known as the amino group, at both ends. There’s the plus sign. Then there’s the decanedioyl dichloride with its -COCl, or acid chloride, group at both ends. At the interface where the fluids are touching, the amino and acid chloride groups combine.
Which brings us to the second line. At the right we have HCl, or hydrochloric acid, which is why you wind the string of nylon around a rod and you wash it before touching it with your fingers. On the left we have a very long string of atoms, which still has a NH2 (amino group) at one end and a -COCl (acid chloride) group at the other. It can just keep combining.
nH2 N(CH2 ) 6 NH2 + nClOC(CH2 ) 8 COCl →
H2 N[(CH2 ) 6 NHCO(CH2 ) 8]nCOCl + nHCl
As the chemist takes away the solid formed at the border between the two liquids, they create a new border between liquids, where more “rope” forms. This little demonstration has been called “The Rope Trick” ever since chemist Stephanie Kwolek used that as a title for her paper on the discovery in 1959. It’s become a staple of chemistry classrooms. Now that we know what’s going on – who wants to see it?
Here we have it with some explanation.
And here’s a close-up.