Carboxylic Acids: Naming + Properties INSTRUCTOR: All right. If you've been following this series, you can already name alkenes, alkanes, alkynes, alcohols, aldehydes, and ketones. Next in line, we're going to be doing carboxylic acids. Carboxylic acids are the easiest to identify, because there's a COOH at the very end of the molecule. Because you have the double-bonded O and the single-bonded OH, a carboxylic acid group can only appear at the end of molecules. Also, because of that, we don't need to show a number to say where it is. We know that this is at carbon 1 all the time. So let's take a look at how we name these. How many carbons do we have in the chain? Looks to me like we have 6. You know that the prefix for that is hex. It's all single bonds, so we have an, as opposed to ene or yne. And because we have a double-bonded OH or a COOH group at the very end, it's a carboxylic acid, and we add oic acid to the end of the name. This is hexanoic acid, six carbons, all single bonds, COOH at the end. Beauty. Let's see what else we got here. Carbon 1, 2, 3, 4, 5, 6, 7, 8, so that means we have an oct. It looks to me like we have a double bond starting at carbon 4. So I'm just going to call this oct-4-ene, and again, it's going to be oic acid at the end, because it's a COOH. If you're worried about the stereochemistry here, I've drawn this in a trans configuration. So that means I'm going to call it 4E. You could just as easily call it trans. And here, we have a COOH at both ends, and let me show you how we deal with that. It's still 6 carbons, and they're all single bonded together, so hexan. But if we have 2 carboxylic acid groups-- and you can't have more than 2 because there's 1 for each end-- we add the E back in and call it a dioc acid. hexanedioic acid is a 6-carbon chain of all single bonds with a COOH at both ends of the molecule. Pretty sweet. Oh, yep. No matter what other substituents you have, you'll always name a COOH group as an oic acid. I don't care if there is like a double-bonded O here, you're going to name that as a substituent. Actually, if it's there, we're going to number 1, 2, 3, 4, 5, and 6. That's on carbon 2, and we would have ended up calling it 2-oxo, just because a double-bonded O as a substituent is an oxo. The point is, I don't care what the functional groups are on it. Just name it is the acid, and then stick to functional groups on then. Methyls, chloros, benzyls, I don't care. Last thing I want to point out to you are the properties of carboxylic acids. Now, clearly, because we have a bunch of oxygens at one end of the molecule, it's going to be very polar. In addition, the OH bonds mean we're allowed to hydrogen bond the molecules together. This gives them higher melting and boiling points than even you might expect, because again, the molecules are sticking together. Back to polarity, the shorter the chain, the higher the polarity. That makes sense, because for extraordinarily long molecules, sure, this end is polar, but by the time you get to the end, those carbons don't even recognize that there's a polar end to the molecule. So shorter ones are more polar. Again, the hydrogen bonding gives us high boiling and melting points, but the other thing to point out is that these things form dimers. This is kind of cool. This happens even in the gas phase. So here is one carboxylic acid, here is another carboxylic acid, and they end up hydrogen bonding together in a pair to become one. It's beautiful. And so that means that the molecular mass of this molecule is actually double what you'd expect, and that means the boiling and melting points are even higher. And the last thing I want to point out, if it's not already obvious, is that these compounds are acidic. The H can easily come off, the electron pair goes to the oxygen, and you end up with something called a carboxylate ion which is a COO with a minus charge. I've drawn this here as its resonance, or like a hybrid structure. Because once that hydrogen comes off, the double bond could be between these O's or these O's, and it kind of flips back and forth. It doesn't flip back and forth, but I mean, the actual structure doesn't have a full double bond in it. Anyways, that's all I wanted to say. Good luck naming carboxylic acids. Take a look at my next video for esters and other oxygen stuff.