Leah here from leah4sci.com and in this video we're going to look at the acidity and basicity
of alcohols for the entire alcohol video series, visit my website leah4sci.com/alcohol
Alcohols are amphiprotic where amphi tells us it can go two A's and protic is referring
to it's proton ability, if a molecule is amphiprotic, it means that it can act as both an acid and
a base, in other words it can be a proton donor or a proton acceptor.
For a quick review of arrhenius, bronsted lowry and Lewis acids and bases, study the
tutorial link below.
Let's take a look at a simple alcohol, ethanol.
If the alcohol takes up a proton, we get a protonated oxygen with a positive charge where
the alcohol that picks up the proton is a base and the protonated alcohol or the oxonium
is the conjugate acid, if the acid reacts to give up the proton, the resulting molecule
is an oxygen with three lone pairs and a negative charge and the proton in solution are attached
to another base.
The resulting molecule is an alcohol with three lone pairs as a negative charge, the
alcohol that gives up the proton is our acid and the resulting negative alkoxide is our
conjugate base.
Which reaction is preferred?
Remember that molecules just like anything else, if they're happy, they're stable, unreactive
and therefore more likely to form.
If we look at the oxonium, it's relatively stable but the alkoxide you should remember
from your substitutional elimination reactions is very strong and very reactive, it is not
very stable.
This tells us that equilibrium lies to the right for the acidic reaction and lies to
the left for the basic reaction.
Why is that?
The PKA of an alcohol like ethanol is 15.9, remember from general chemistry that when
you have a high KA or a low PKA, that gives you a very strong acid, in the case of 15.9,
this is a relatively high PKA and therefore a relatively weak acid.
Compare that to something to ethanoic acid, ethanoic acid has a PKA of 4.8 where every
PKA unit is a tenfold increase in acidity.
Let's look at acidic alcohols in more detail, when alcohol picks up a proton, the resulting
positive oxygen is called an oxonium.
This is something that you'll see a lot in reactions or rather this is something that
will be implied in a lot of organic chemistry reactions.
For example, think of the SN1 reaction in which you're given an alkyl halide reacting
with sulfuric acid in a methanol solution.
Anytime you see sulfuric acid, understand that it's an acid catalyst that donates an
H+ and solution where we don't care about the resulting conjugate base but what happens
next, what does methanol actually do to this reaction?
Not only does it act as the solvent, but this is a solvolysis reaction.
When sulfuric acid dissolves in solution, the protons don't just randomly float around
that solution, they get picked up by the solvent molecule, in this case the methanol where
the reaction looks something like this, The H2SO4 is going to react with a methanol solvent
molecule to give us HSO4- as a conjugate base that does nothing plus the protonated methanol
which is holding on to the proton that came off in the solution, looking at this reaction
carefully, you'll recognize that this is an acid base reaction, sulfuric acid forms HSO4-
which is the conjugate base, and methanol because it reacts with the acid to pick up
a proton is the base which forms the protonated conjugated acid.
Alcohols will easily be deprotonated to form the conjugate acid but they're less easily
deprotonated to form a conjugate base.
This is because of weak acid is going to form a stronger conjugate base and if given the
opportunity, the strong conjugate base will seek out a proton in an attempt to reform
the weaker and more stable acid.
There are two common reactions to form the alkoxide anion.
The first is to react the alcohol with a very strong base in a way that does not allow equilibrium
to shift back to the left, for example using NAH or sodium hydride, recognize that sodium
is a spectator and hydride is a very strong negative base, the hydride grabs the hydrogen
off of the oxygen, giving oxygen back it's electrons forming the alkoxide where oxygen
now has three lone pairs and a negative and the hydride attacker is now bound to the alcohols
hydrogen to form H2 gas, this is what makes the reaction irreversible because H2 as a
gas bubbles out of solution and if you try to reverse this reaction, they're nowhere
to be found.
Another reaction you'll see is reacting alcohol with a neutral metal like sodium, even though
the two reactions look very similar, they're actually very very different, in a case of
sodium hydride, sodium is simply a positive spectator, it's there to balance charge and
nothing else, it's not reactive.
In the case of sodium metal, we're dealing with a neutral sodium atom, which you look
on the periodic table, you'll see that it's in group one, which means it has one valence
electron, this gives us a very reactive sodium for a radical reaction, most of you are not
required to know this mechanism just understand what is happening.
Sodium donates it's one electron to the hydrogen atom so that the two electrons binding oxygen
to hydrogen remain with the hydrogen.
This once again gives us an oxygen atom with three lone pairs and a negative charge.
This reaction will also form H2 gas to bubble out of solution, how?
We have to use everything twice.
If we start with two alcohols, two sodium atoms then we have two hydrogen atoms each
getting one radical electron which gives us the two electrons to create that H2 gas bond
for a very different reaction with a very similar outcome.
A deprotonated alcohol is called an alkoxide where the out portion refers to the alkyl
chain and oxide is the negative oxygen, some common alkoxides to recognize are methoxide
which is just a methyl group, ethoxide has an ethyl group, and tertbutoxide or tertbutyloxide
which you'll often hear me referred to as the chicken foot base.
You should recognize these from your substitutional elimination reactions where methoxide and
ethoxide are both a very strong nucleophiles and very strong bases and the tertbutoxide
as a big bulky base, we'll go for the E2 reaction or the non-zaitsev or Hofmann product.
For detailed reviews, see the substitutional elimination series link below.
When it comes to ranking, the strength of acids and bases, some professors will provide
a PKA table and most will not, does this mean you have to memorize PKA values?
No, instead this means you have to understand the basic CARIO principles that I teach in
the acid base series link below to help you rank the molecules based on what you see,
rather than on based on a number that you memorized.
As a reminder, CARIO stands for Charge of the atom, the Atom that we're comparing, Resonance
stabilization of the atom, Induction or the Inductive effect, and Orbital Hybridization
specifically of the acidic or basic atom.
Let's do a quick review as it relates to alcohol starting with charge.
Charge is pretty straightforward because you should remember that something more positive
tends to be more acidic and more negative tends to be more basic, if everything about
the two molecules in question looks the same, where the only difference is the charge, we
look for the more positive to be the more acidic one and the more negative to be the
more basic.
For atom, if we're comparing two alcohols, the atom is going to be the same, an oxygen.
But what if we're comparing an alcohol to something else?
I asked you to memorize the following ten atoms of the periodic table, hydrogen, carbon,
nitrogen, oxygen, fluorine, phosphorus, sulfur, chlorine, bromine, and iodine.
This helps you remember that the electronegativity increases up to the right and size down to
the left.
What if I take these three atoms and put them on a carbon chain, for example, if I have
CH3 bound to a carbon, nitrogen or oxygen, oxygen should have four bonds, so we'll give
it three hydrogen atoms, nitrogen should have three for two hydrogen atoms, oxygen two bonds
for one hydrogen atom.
Let's rank them in terms of most to least acidic, what we're looking at here is an atom
holding the acidic hydrogen, where carbon is the least electronegative and oxygen is
the most electronegative.
Electronegativity tells us how strongly the atom wants to be negative or rather how stable
the atom will be with that negative charge.
If you're looking at these three neutral atoms and thinking I can't compare them, sometimes
it's easier to look at the charged molecule, so ask yourself what happens if I remove an
H+ from each molecule and each case losing an hydrogen gave the atom an extra lone pair
of electrons with a negative charge.
Looking at the electronegativity, carbon being least electronegative is most unhappy and
therefore most unstable with that negative charge and if it's very unstable it's gonna
be highly reactive, making it less likely to form and most likely to return to its neutral
state.
If the molecule does not want to form a base, that makes it a very weak acid.
Nitrogen of intermediate electronegativity is in the middle so its not happy with that
negative but not as unhappy as the carbon that puts it in the middle and oxygen as the
most electronegative atom is most comfortable holding on to that negative charge, so even
though it's still very strong and very reactive, it's a lot more stable than negative carbon
or nitrogen, making it the least reactive of all three and the most likely to remain
with that negative charge.
If it's most likely to keep the negative charge, it's most likely to react in a way that makes
it give up the proton to form that negative charge making it the strongest acid in the
series.
If we look at the PKA values and alkane has a PKA of 45 to 50, the amine has a PKA of
35 to 40 and alcohols have a PKA between 15 and 19, but we didn't solve it using PKA,
we solved it by understanding and this is where it gets interesting.
For resonance, we're not just looking at the atom and its charge, we're looking at what’s
going on nearby.
Let's take a look at ethanol once more and compare it to a molecule that is very similar
except for the fact that there is a pi-bond sitting on the carbon that is holding the
oxygen atom, you should recognize this molecule as an enol because it has an alkene and an
alcohol on the same carbon atom, recognize that the oxygen can resonate it's lone pair
of electron onto the carbon atom, kicking the pi-bond over to the left we’ll show
the resonance over here with a lone pair and a negative charge on the left carbon and a
pi-bond between the oxygen and right carbon.
Oxygen now has three bonds, one lone pair, and a positive charge.
Based on this resonance, which molecule is more acidic?
It's hard to tell, they're both neutral, so let’s look at the conjugate base.
Removing a proton from ethanol gives us ethoxide, removing a proton from the enol gives us an
enolate.
This is where we can see the difference, the ethoxide oxygen is stuck holding that burden
of negativity by itself, but the enolate which is a deprotonated enol can share that negative
charge between oxygen and the nearby carbon atom, if oxygen is not left carrying that
negative charge by itself, it's more stable than the ethoxide and if it's more stable,
it's more likely to form making the enol the stronger acid.
Anytime we say that something is a stronger acid, the conjugate base of the weaker acid
is going to be the stronger base.
If the enol is the stronger acid, ethoxide becomes the stronger base.
The inductive effect takes a look at what else is going on on the molecule.
Once again, we're comparing two molecules of ethanol but the second ethanol has a chlorine
atom on the carbon that is holding the oxygen, how does that chlorine atom impact the acidity?
We know that chlorine is a halogen and halogens are highly electronegative but they're not
just electronegative, they're greedy.
Chlorine starts to pull on the electrons between itself and carbon, making the chlorine partially
negative and the carbon partially positive.
If the oxygen is sitting next to a partially positive carbon, it's electron density starts
to be pulled towards that carbon atom which doesn't look like much in the neutral molecule
but look what happens when we deprotonate, the products look exactly the same except
for chlorine sitting on the second molecule.
But if the electron density is being pulled from the oxygen towards the carbon, that means
that even though oxygen isn't getting any real help meaning there is no resonance to
help it carry that negative charge, having some of its negativity, some of it's burden
pulled in the direction of carbon makes the oxygen less burdened or less overwhelmed with
that negative charge and therefore makes it more stable.
If this conjugate base is more stable, that means it's more likely to form, making chloro
ethanol more acidic whereas the ethanol that just forms ethoxide is less likely to form,
making the ethoxide more basic.
If you look up the PKA values, you'll see that ethanol has a PKA of about 16 whereas
chloroethanol has a PKA of about 14 meaning the acid is almost a hundred times stronger
than ethanol.
Last but not least is the Hybridization or Orbital Hybridization of the atom.
Orbital Hybridization is less likely to come up during the topic of alcohols, one specific
alcohol that we do want to look at in more detail is the phenol and specifically how
resonance impacts the acidity which is exactly what we're going to cover on the next video.
First, make sure you give this video a youtube thumbs up and then visit leah4sci.com/alcohol
for the complete alcohol video series along with the practice quiz and cheat sheet.