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Nomenclature and properties of esters

Video transcript
Lecturer: Esters are another kind of carboxylic acid derivative. If we started with a generic carboxylic acid on the left we can turn that into an ester over here on the right. Since we're not concerned with things like mechanisms and reactions in this video we don't need to worry about details, like if this oxygen is really this oxygen. It depends on how you're making the ester. All we're concerned about is how to name the ester. Let's think about how to approach it. The first thing you would do is look at this R-prime group, the one on this oxygen. You can name this R-prime group as an alkyl group. Then you're going to look over here and think about the carboxylic acid over here on the left to name this portion of your ester. Let's go ahead and do an example. If we're going to name this ester down here, once again we look at the R-prime group and name that as an alkyl group. That's a two carbon alkyl group. Here's one carbon and here's the second carbon. A two carbon alkyl group would be ethyl. Let's go ahead and write that down. We would have ethyl to start with. Next we think about this portion of the ester. We can think about that as coming from this carboxylic acid. We know this carboxylic acid is called acetic acid. Let's go ahead and write that. This is acetic acid here. To finish naming off our ester we're going to drop the ending for our acetic acid. We're going to drop - Let me go ahead and change colors here. We're going to drop the -ic part and the acid, and then we're going to add -ate instead. We would have this portion plus -ate. Let's go ahead and write that and let's go ahead and use red for that. We would have this portion, we drop the -ic and the acid, and we add -ate. The name of this ester would be ethyl acetate. Instead of ethyl acetate if we had named this using IUPAC nomenclature, if we had called this ethanoic acid, which most people would not do that, but if we called it ethanoic acid we could do the same thing. We could go ahead and drop the -ic and then the acid part and add -ate. Once again we would still have the ethyl portion, this would still be ethyl, then when we're naming the rest of it we would take this portion and add -ate. It would be ethanoate. Another name for this would be a ethyl ethanoate. Two different ways to name this ester, although you are most likely going to see this form, ethyl acetate is what everyone says. Let's name another ester. Once again our goal is to name the ester over here on the right. We start with the R-prime group. The R-prime group is the one that's attached to our oxygen. Here's our R-prime group attached to this oxygen. Notice the symmetry, we have another one over here as well. We have two ethyl groups this time, so we're going to write diethyl to start naming our ester. To finish naming our ester let's think about the carboxylic acid portion. If we're thinking about this portion right here, this was derived, you can think about this as being derived from this dicarboxylic acid over here. The IUPAC name would be propanedioic acid, the common name would be malonic acid. This would be malonic acid, so let's go ahead and write that. Malonic acid. To finish the name of our ester, remember we're going to drop the -ic and the acid. We drop all of this and add -ate. This would be malonate, so I'll go ahead and write that in here. The name of this molecule, this ester, would be diethyl malonate, I'll write that. Let's do another one, let's look at another ester. A very famous ester over here on the right. You might recognize this as being wintergreen. Let's go ahead and name it, let's find the ester portion of our molecule. The ester portion of our molecule is over here. We start by naming the R-prime group. Here's our R-prime group on this oxygen. That's one carbon, so that's a methyl group. Let's go ahead and write that, we have methyl. Then we think about the carboxylic acid, the corresponding carboxylic acid would be this portion, and we see it over here. We know this is salicylic acid, let's go ahead and write that. This would be salicylic acid, which we talked about in earlier videos. Once again, If you want to finish the name of your ester you would have to drop your -ic and your acid ending and add -ate, so salicylate, let's go ahead and write that. This would be methyl salicylate as the name for this ester, which is, of course, oil of wintergreen. One thing that esters have is a lot of esters have really nice smells to them, it's one of the properties of esters. Wintergreen is one of the best esters, and there are many, many, many more that you can make, using something like a Fischer esterification reaction. Let's look at one more ester to name. Let's look at how to name this one over here on the right. Once again, we first start with our alkyl group coming off of this oxygen, and once again we have a methyl group. Let's go ahead and write methyl here. When we're thinking about our carboxylic acid portion, this portion right here, I would draw the carboxylic acid over here on the left. If we remember how to name that, we have a cyclohexane ring. This part would be cyclohexane. Then we have a carboxylic acid coming off of that. Cyclohexanecarboxylic acid, kind of long here. Once again we're going to drop our ending. We're going to drop the -ic and the acid and add -ate. Cyclohexanecarboxylate would be the name. Hopefully we'll have enough room over here. Methyl cyclohexanecarboxylate, let's see if I can squeeze this in. Methyl cyclohexanecarboxylate, would be the name for our ester. Finally let's look at physical properties of esters. Here I have some different molecules. Let's look at our ester here. If we were to name this ester we would have an alkyl group, that's a methyl group, so we have methyl. Then we think about the carboxylic acid portion. What kind of a carboxylic acid would this be? A two carbon carboxylic acid, that would be acetic acid. We drop the ending and add -ate, that would be acetate. We could call this ester methyl acetate. Let's compare methyl acetate to some similar-sized molecules. Over here on the left we have 2-methylbutane. If we just go ahead and number really fast, we see this is 2-methylbutane. Then over here on the right we have a four carbon alcohol. One, two, three, four. This would be 2-butanol. Let's compare methyl acetate to these other molecules in terms of boiling points. Let's start over here with 2-methylbutane. When we think about boiling points we can think about our intermolecular forces. The only intermolecular forces present between these two non-polar [unintelligible] same molecule, 2-ethylbutane is a non-polar molecule, so the only forces present are London dispersion forces, which we know are the weakest. It's the easiest to pull these two molecules apart. The boiling point of 2-methylbutane is approximately 28 degrees Celsius. We think about methyl acetate, we have some polarity here. This oxygen is partial negative, this carbon right here is a partial positive. Something like methyl acetate, a small ester is moderately polar so we have a little bit of polarity here. Same thing for this molecule of methyl acetate. The attractive intermolecular force between these two molecules would be dipole dipole. Dipole dipole we know is a stronger intermolecular force than London dispersion, so the boiling point of methyl acetate should be higher than the boiling point of 2-methylbutane. The boiling point turns out to be approximately 57 degrees Celsius. It takes more energy, more heat to pull apart molecules of methyl acetate because dipole dipole is a stronger attractive force. Finally 2-butanol, the boiling point for 2-butanol is about 99 degrees Celsius because of the hydrogen bonding that's present. Hydrogen bonding right here, the strongest intermolecular force, so it takes increased energy to pull apart molecules of 2-butanol. Methyl acetate in terms of boiling point is somewhat in between that for an alkane of a similar size and that of an alcohol of a similar size because of its polarity. In terms of solubility in water, let's draw out water here. If we have water, we know that water is of course a polar molecule. Partial negative on the oxygen, partial positive on the hydrogen. We have a polar water molecule, so there could be some hydrogen bonding right here. Between this oxygen on methyl acetate and water there could be some hydrogen bonding. We have a polar molecule and a polar molecule so methyl acetate is soluble in water. But of course as you increase the number of carbons that you have for your ester - Let's think about this ester. Here we have two carbons and then one carbon. As we increase the number of carbons we increase the non-polar character of our ester, and we would therefore decrease the solubility of our ester in water. Although methyl acetate turns out to be soluble in water, if you go up to something like ethyl acetate you decrease the solubility. Ethyl acetate can actually be used to do something like extract a non-polar molecule like caffeine from water. Of course as you add more and more carbons you would decrease the polarity, make it even more non-polar, and decrease the the solubility in water even further.