Carboxylic acid nomenclature and properties | Organic chemistry | Khan Academy


Voiceover: Let’s look at the nomenclature and physical properties
of carboxylic acids. We’ll start with nomenclature. If we wanted to name this carboxylic acid, it’s the simplest one possible. There’s one carbon; a one
carbon carboxylic acid. If we had a one carbon alkane,
we would call that methane. To name a carboxylic acid,
you drop the E ending and add oic acid. This one would be methanoic acid. Let me go ahead and write this out. Methanoic acid. You can see we dropped the E
and added oic and then acid. Methanoic acid is the IUPAC
name for this molecule. The common name for this is formic acid, which is what you’ll hear more often. Formic acid. The name comes from the Latin word for ant because formic acid is found in ant venom. There are lots of really
cool carboxylic acids with interesting common names,
such as this example, here. Let’s look at this one. A two carbon carboxylic acid. Using IUPAC nomenclature,
that would be ethanoic acid. It’d be ethane, drop the E, add oic acid. Ethanoic acid. The common name for this is acetic acid. Once again, that’s the one
that you will hear more often. Acetic acid. This name comes in the
Latin word for vinegar because vinegar is just a dilute solution of acetic acid in water. Let’s name this one down here. You’ll want to find the
longest carbon chain that includes the carbon
of your carboxylic acid. That’s going to make this carbon over here carbon number one. Two, three, four, five. A five carbon carboxylic acid. That would be pentanoic acid. Go ahead and write pentanoic acid here. Pentanoic acid. Then we have a bromine on carbon four. It’d be 4 bromo. The full IUPAC name would
be 4-Bromo pentanoic acid. Let’s look at this one. Three carbons. This would be carbon
number one, two, and three. A three carbon carboxylic
acid would be propenoic acid. Since we have a double bond present, we need to chance the A to an E. It’d be propenoic acid. Let me go ahead and write this out. It’d be propen. I’ve got the E here. Propenoic acid, like that. We can just go ahead and put in a 2 here to indicate the start of the double bond. 2-propenoic acid. We don’t have to worry about
the stereo chemistry of the double bonds since that’s
a monosubstituted double bond. For long carboxylic acid, you would have to think about these stereo chemistry. Let’s do some more examples. Let’s look at this molecule
over here on the bottom left. We can see there’s a benzene ring attached to a carboxylic acid. We’ve seen this before. We called it benzoic acid. Let’s go ahead and start
that as our parent name. We have benzoic acid right here. If we’re going to name it as benzoic acid, that gives the carbon attached
to the carboxylic acid carbon number one. Then we want to give our substituent the lowest number possible. Of course, this is going
to carbon number 2. We have [88] OH of carbon 2. It’s two 2 hydroxybenzoic acid. 2 hydroxybenzoic acid. Benzoic acid is actually a
common name, but it’s again, it’s used so frequently
in organic chemistry, it’s been incorporated
into IUPAC nomenclature. If we wanted to name this
another way, we could say it’s 2-hydroxy or orthohydroxy. We see there’s a benzene ring. 2 hydroxybenzene. Then we have our carboxylic acid. 2 hydroxybenzene carboxylic acid is another IUPAC name for this molecule. You don’t see people
name it that way usually just because it’s so
long and it’s much easier just to say benzoic acid. The common name for this
molecule is salicylic acid. Let me go ahead and write that. Salicylic acid is famous
because it’s a precursor to Aspirin and wintergreen. The name for salicylic acid
comes from the Latin word for willow tree because
you can get this compound from the bark of the willow tree. The Greeks knew about this. It would reduce fevers and decrease pain. That’s, of course, why salicylic acid was transformed into Aspirin. Let’s look at this one
over here on the right. Instead of having a benzene ring, we have a cyclohexane ring. This would be a clohexane. Now we have our carboxylic acid over here. We could say cyclohexane
carboxylic acid for this one. Cyclohexane, almost
running out of room, here. Carboxylic acid, like that. Instead of saying benzene carboxylic acid, it’s cyclohexane carboxylic acid. What do you do if you
have two carboxylic acids in the same molecule? That’s what we have here. We can see there are two carbons present. We go ahead and start by writing ethane. We have two carboxylic acids, so we’re going to use di in here. Ethandi, and now oic. Ethanedioic acid would be the
IUPAC name for this molecule. The common name for this
molecule is oxalic acid. Oxalic acid, like that. Let’s look at properties
of carboxylic acid. We’ll start with boiling points. Let’s compare these two molecules in terms of their boiling points. Over here on the left,
we have acetic acid, which has a boiling point of approximately 118 degrees Celsius. Over here on the right, we have ethanol. The boiling point of ethanol is approximately 78 degrees Celsius. Acetic acid has a higher boiling point. We can think about why by
thinking about two molecules of acetic acid interacting
and intermolecular forces that are present. Let’s go ahead and draw a
molecule of acetic acid. I’m going to go ahead and draw it. There’s my carbonyl. Then we have an oxygen and a hydrogen and on this side, a methyl group. There are opportunities for
a hydrogen bonding, right? There could be a hydrogen bond right here and a hydrogen bond right here. Remember, oxygen is more
electro negative than hydrogen. Oxygen gets a partial negative. The hydrogen gets a partial positive. Then this oxygen over here
is also partially negative. You have these opposite
charges attracting. This partially negatively
charged oxygen attracted to this partially positively
charged hydrogen, here. This is your hydrogen bond. The strongest intermolecular force. The same thing down here. We could think about two
hydrogen bonds forming for two molecules of acetic acid. Over here on the right, we have ethanol. Let’s go ahead and draw in
another molecule of ethanol. Here we have our second
molecule of ethanol. We can see there could be
a hydrogen bond right here. Once again, we have our
partially negative oxygen and partially positive hydrogen, partial negative oxygen like that. We have one hydrogen bond. There’s more opportunities
for hydrogen bonding in acetic acid than in ethanol. Because there are more
opportunities for hydrogen bonding, there’s stronger forces holding
these two molecules together so it takes more energy
to pull them apart. That’s the reason why it
has a higher boiling point. Let’s think about solubility in water. Let’s stick with thinking
about acetic acid right here. We know acetic acid is soluble in water because vinegar in acetic acid in water. If we don’t have a lot of carbon, acetic acid only has two carbons, this molecule is polar
enough to dissolve in water. We could show acetic acid
interacting with water. Let me go ahead and draw a
water molecule right here. I’ll draw a water molecule over here. There’s, of course, some
hydrogen boding that can go on. You can think about a
hydrogen bond right here and hydrogen bond right here. Acetic acid is soluble in water. Water is a polar molecule. Acetic acid is polar enough
to dissolve in water. However, as you increase the number of carbons in your R group, as you increase the number of carbons, you get more carbons and hydrogens. You get more of a non-polar character. The more non-polar you make this molecule, you decrease its solubility in water. Once you get past somewhere
around five or six carbons, you decrease the solubility dramatically. That’s just a little bit
into the physical properties and the nomenclature of carboxylic acids.

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