Molecule of the Day

So, yesterday was Sodium Dodecyl Sulfate, or SDS. Today is the carboxylic acid analogue, or lauric acid. This is just a twelve-carbon, aliphatic, unsaturated carboxylic acid. One big difference between it and SDS is that it can form an insoluble complex with divalent metal ions like magnesium and calcium. This is, of course, soap scum. Follow that link! If you are one of those unfortunate souls who has soft water, you can make your own soap scum at home.

The structure:

SDS, for a refresher:

These regular carboxylic acids can form micelles, solvate hydrophobic ligands in water, and pretty much do everything SDS does. They’re less soluble, though, and form the aforementioned scum.

Why would we want the scummy soap when we can have the good stuff? Well, two reasons: historical and financial.

Remember when Brad Pitt went on his crazy rant about soap being made from the ashes of the first heroes? You probably weren’t paying attention, because he went on his share of crazy Chuck Pahalniuk asides in that movie, but I was. Because it was about chemistry, you see:

Lye — the crucial ingredient. Ancient peoples found their clothes got cleaner if they washed them at a certain spot in the river. Why? Because, human sacrifices were once made on the hills above this river. Year after year, bodies burnt. Rain feel. Water seeped through the wood ashes to become lye. The lye combined with the melted fat of the bodies, till a thick white soapy discharge crept into the river.

Skip a paragraph if you’re terribly squeamish. It turns out Chuck Pahalniuk took some license with this story, and it is probably apocryphal (scroll down to the “history” section). The chemistry is not necessarily bad, however. It is plausible that someone being cremated before the advent of modern techniques might warm up just enough to melt their lipids and have some flow away before they really got burning. After they started burning really hot, they’d be reduced to ash, which is mostly inorganic oxides and carbonates, both of which are good bases. I present to you the hero soap synthesis:

Hero Soap Synthesis

Although the story might be a little off, this is actually analagous to the production of soap today. At some point we figured out that when you treat fat with base, you get soap. This is the “historical” part of the reason. Since heroes are in short supply, beef tallow is often used as a source of lipids. We have lots of fat laying around from animal rendering, et cetera. As you can see below, the preparation scheme is EXTREMELY simple - it’s no wonder that we figured out how to make this before we got the hang of other detergents!

Nonhero Soap Synthesis

The precursor compound above is called a “triacylglycerol,” “triglyceride,” “neutral lipid,” or “fatty acid triester,” along with many other names. In the case of our laurate, the “R” is the remaining 11 carbons (one is part of the carboxylic acid. Note that you get glycerol out, too. In practice, you see R ranging from 11 to 15 carbons, but more or less isn’t at all unheard of. In butter, there is the inimitable vomit-smelling butyric acid. Normally locked up in these esters in butter, as butter turns, the esters hydrolyze, releasing the rancid-smelling butyric acid. Interestingly, the Wikipedia article claims it occurs in Parmesan cheese, which I didn’t know, but it no doubt has something to do with that it-smells-like-feet-but-I-love-it thing it has going. Here is probably the same paradoxical dilution thing we saw with indole before.

These fatty acids have more uses than just soap. They are also used in modeling clay and napalm. The term napalm, it is said, comes from NAphthalene and PALMitic acid - palmitic acid is just the C16 fatty acid. This is one of the few things certain dangerous cookbooks for bored teenagers looking for new ways to set things alight got right.

Note also that you get glycerol out of the reaction. It is lucky that’s so useful too.

Performing an analagous reaction in alcohol instead of water is called an alcoholysis. This the the synthesis route for Biodiesel, which we’ll tackle another time.

Saponification still is the least involved way to make a detergent, and you can do it at home. If you are lazy but still easily excited, you can make some a lot more quickly, as long as you don’t care about using it.

Just get some lye (plumbing stuff aisle at the grocery store) and oil of living origin (i.e., any vegetable or animal oil. Motor oil et cetera aren’t triglycerides and will just sit there, stubbornly refusing to react.) and leave them in a loosely capped container for awhile. Heat will speed things up, but be careful (be careful if you’re doing it the right way, too, but I’m assuming the dump-some-lye-in-a-jar-with-oil types aren’t exactly direction readers. It’s okay, me either.) Just remember that you didn’t use a stoichiometric amount of base, so the “soap” could burn you.

Enjoy. See you tomorrow.