Carboxlic Acid Introduction Carboxlic Acid Introduction
Carboxlic Acid Introduction
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- Let's see if we can learn thing or two
- about carboxylic acids.
- They have the general form of the carbonyl group, just like
- we've seen in aldehydes and ketones, and they will be part
- of a longer carbon chain.
- But instead of having a hydrogen here, as the case
- with an aldehyde, or having another carbon chain here,
- with the case of a ketone, we have an OH group.
- And you're probably saying, hey, why is
- this called an acid?
- It must be called an acid because it is an acid, and
- you'd be right.
- And the reason why it is an acid, and why it is more
- acidic that just something with an OH group, it's
- actually a good bit more acidic than alcohols, is
- because once this thing loses its hydrogen-- remember,
- depending on what type of acid you want to think about, acid
- in the Lewis sense could be an electron taker, and this
- oxygen can take the electron of this hydrogen.
- Or if you think about the Arrhenius definition of an
- acid, it is a proton donor, and this OH group
- can donate a proton.
- It takes away the electron of this hydrogen, gives the way
- the proton, either way.
- But the reason why this is more acidic than alcohol is
- once it gives away this proton, it is actually
- resonance stabilized.
- So let me show you what that means.
- To do that, let me actually show you the bond between this
- oxygen and this hydrogen.
- The oxygen has this pink electron, and the hydrogen has
- that magenta electron right there.
- If you put this in a solution of water, so you have some H2O
- over here, this oxygen right here really wants to take back
- this magenta electron.
- And as it takes back that magenta electron, it would
- essentially donate the hydrogen
- proton to a water molecule.
- So the water molecule would give one of its electrons to
- the hydrogen proton and then become positive.
- So once this happens, the next step would look like this.
- What was our carboxylic acid will now turn into the
- carboxylate ion, so it will now look like this, so it has
- our carbonyl group.
- Now, this oxygen just took an extra electron.
- So if I want to draw it, I have that one.
- Actually, let me draw.
- So this is the oxygen.
- To start off with, the oxygen had two lone pairs, so I want
- to draw those two lone pairs first. So to start off with,
- it had those two lone pairs, and now it had this pink
- electron from the get go, and now it took this magenta
- electron, so now it has one extra valence electron.
- We can even draw them here.
- We can count them: one, two, three, four, five, six.
- Six is just a neutral oxygen, but now it gained another one.
- It has seven.
- It now has a negative charge, and then the water has now
- become a hydronium ion.
- So you have the water here.
- We've increased the proton or the hydronium
- concentration in the water.
- This one water molecule is now a hydronium molecule, so this
- is now bonded with this hydrogen
- proton just like this.
- This oxygen gave away an electron to this proton, so
- now it has a positive charge.
- And this right here, this carboxylate ion right over
- here, the reason why this thing was a stronger acid than
- something that just had an OH group is because the conjugate
- base, the carboxylate ion, is actually resonance stabilized.
- It is more stable than the conjugate base of an alcohol,
- and let me show you that.
- This thing can share its negative charge.
- Let me draw it.
- It can take this magenta electron, give it to this
- carbon, then this carbon will have an extra electron, so
- then it can give back an electron to this top oxygen.
- So it is resonance stabilized with this
- structure right over here.
- Let me draw the same so it could look like this.
- That's too big.
- Let me scroll down a little bit.
- It could look like this.
- And now, this took back this blue electron, so now one of
- the bonds is gone, and it started off with two lone
- pairs, so I want to draw that there, and now it has
- another lone pair.
- It has this electron, this electron, and now it has that
- blue electron over there.
- And now this oxygen, this top oxygen, has a negative charge,
- and now the carbon has a double bond
- with this side oxygen.
- So now the carbon-- let me go back to the yellow-- that's
- the first bond with that oxygen.
- It had one, two lone pairs to begin with, and now it has
- this magenta bond.
- So this pink electron is at this end.
- And now this purple electron is at the other end, or this
- magenta electron, and now it has a double
- bond with this oxygen.
- And we know that when you have resonance stabilization, it's
- not like you're going back and forth.
- The reality is that you have a half double bond between both
- oxygens, that the electrons are just flowing across the
- whole place, and that stabilizes the molecule.
- And so to show that this is a resonance structure, let me
- put some brackets around it.
- And, in general, if this R group right here is actually
- even better at withdrawing electrons, so if you put
- something that was really electronegative here,
- something that likes to hog electrons, it would make the
- carboxylate ion even more stable.
- It would make the carboxylic acid even a better acid.
- So if you put something electronegative here, then you
- could imagine that some of this negative charge that we
- drew in these two resonance structures, can be sucked to
- that R group, and then that would make it even more
- stable, and would make the carboxylic
- acid even more acidic.
- Now, like in everything we've looked at, there are some
- common carboxylic acids that are not systematically named,
- that it's probably a good idea to know.
- And I'll start with one, just to see the pattern that we've
- seen in other things.
- We've seen that this thing over here, if we have this, we
- call this acetaldehyde.
- This was the very simple aldehyde we studied.
- we saw that if we have something like this, which is
- a ketone, we called this acetone.
- So you could imagine what we're probably going to name--
- let me do this in a different color-- this
- molecule right over here.
- This is a carboxylic acid, clearly, and it has just that
- one methyl group, just like the acetaldehyde, just like
- the acetone, and so this is acetic acid.
- So the acetaldehyde, acetone, and acetic acid for me are
- fairly easy to memorize just because they all have the
- acet- part.
- They all have that as their prefix, and this part of the
- molecule is identical in every case.
- And the difference is the hydrogen, the methyl group
- over here, or the OH group, making this one a carboxylic
- acid, an acetone, and all that.
- Now, a couple of other ones that it wouldn't hurt for you
- to know is this one right over here.
- This is, you could argue, even simpler than acetic acid, and
- this formic acid.
- And then another one, and actually, I recently did a
- general chemistry video with this, where we did a titration
- example, but this is essentially two carboxyl
- groups attached to each other.
- It looks like this, two carboxyl groups attached to
- each other and you can see them.
- This is one carboxyl group right over here, and then you
- have another carboxyl group right over here.
- And so this actually can be deprotonated twice.
- This hydrogen can be lost, and that hydrogen can be lost, and
- this is oxalic acid.
- I'll leave you there.
- And in the next video, we'll learn how to systematically
- name carboxylic acids.
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