Molecule of the Day

Long before we invented plastics of our own, we had to make do with what nature provided. The most abundant organic compound by mass on earth is lignin, which is a bit of a nightmare. This is what makes up a good chunk of wood. I was taught that the evolutionary idea behind it was if the polymer is random and stable enough, enzymes will have a hard time evolving to eat it. This makes good sense. Trees have a hard time moving around and tend to stay put for long periods of time, so it serves them well to have mechanisms to prevent being eaten. Lacking teeth and all. Lignin is hard to make much use of, but we get better at it all the time.

The second most abundant organic compound by mass is cellulose. This is a bit more regular in structure. It is actually very close in structure to that of starch (your body’s own glucose polymer). The major difference is that one bond (the one in the upper right in the image below) points out of the plane of the screen instead of in (from this point of view). That means your enzymes can’t work on it. That’s why you can’t digest cotton (cellulose), paper (cellulose mostly, lignin’s not the best stuff for paper), etc.

At a surprisingly early time (1832), we worked out that you could nitrate cellulose - that is, react it with nitric acid. This gave something a lot like plastic. We didn’t have plastic, so we started doing all sorts of plastic-like things with it. “Nitro” doesn’t always mean “boom,” but here, it is the case. Nitrocellulose burns eagerly and readily.
This sucked - we had plastic, but it had a nasty habit of burning up. This is where the archetypal image of a flaming movie projector comes from - early film stocks were nitrocellulose. This is why so many of them are degrading - over time, NC breaks down into cellulose and nitric acid, and the acid continues to hydrolyze the film in a self-catalyzed reaction. Refrigerating old film helps slow this down. We lost countless old movies to this process - this is the reaction people are talking about when they talk about old films degrading over time. Despite all this, nitrocellulose still finds some minor uses today.

Marginally better was cellulose acetate. This was “safety film” (it had the same hydrolysis problem, though - it turned into acetic acid and cellulose over time, which wasn’t much better). It still finds use in clothes (this is “acetate” on a fabric label), and its absorbancy causes it to wind up in all sorts of disparate areas (tampons and cigarette filters).

Since cellulose acetate reacts with acid, this can actually cause “acetate” fabric to disintegrate. Most normal people will never encounter this in their everyday life. This causes great levity when said normal people reach college age and take a chemistry class, carelessly splashing some (very dilute) acid on their shirt and mistaking it for drops of sink water). The reaction is relatively slow (hours), causing one not to notice until home from lab a few hours later. The shirt actually crumbles like ash. Going through all my chem courses in college, I got pretty lucky and only lost 4 or 5 shirts this way. Ironically, the vast majority were in my last year, when I should have known better…

Later came Bakelite (one of my favorites), PVC, rayon, and all the modern plastics. It’s a testament to just how much cellulose we have and how cheap it is that you still find its esters in clothes today.