Main content
Organic chemistry
Course: Organic chemistry > Unit 7
Lesson 1: Alcohol nomenclature and propertiesPhysical properties of alcohols and preparation of alkoxides
The physical properties of alcohols. How an alcohol can be deprotonated to form an alkoxide. Created by Jay.
Want to join the conversation?
- Why does the Hydrogen choose to bond with another hydrogen in the final step instead of to sodium, when sodium is more electronegative.
Thanks(12 votes)- Because once the sodium gave up its electron to hydrogen, it has a full shell like a noble gas, meaning it doesn't want to react. The only thing left for that H to react with is another H, forming H2(32 votes)
- at 6.14...H is told to be proton.why?(4 votes)
- I'm not sure exactly what part you're referring to, but a hydrogen atom is comprised of a proton and an electron. If this electron is donated, then all that remains is the single proton.(6 votes)
- Atwe react the alcohol with a strong base - why are we not reacting with a weak base? 5:44(5 votes)
- Both comments are correct, I would also like to remind you that alcohols are typically around 16 pKa (weak acids). A strong base is used because a weak base will not react with a weak acid.(4 votes)
- Hello, can anybody please help clear my huge doubt? I was learning about acidity of alcohols and my textbook gave the reaction between alcohol and a metal to form a metal alkoxide and hydrogen gas.
My Q is: How can we say alcohol is acidic by this reaction because it is not giving a proton, rather hydrogen gas is being formed.. I know Bronsted acids are proton donors, but it's hydrogen liberated here, not h+ (proton). Please help..thanks(3 votes)- You can consider it as giving up a proton, but the proton then reacts with Na:
RO-H → RO⁻ + H⁺. Then
2Na + 2H⁺ → 2Na⁺ + H₂(5 votes)
- With 3 R groups we have a tertiary alcohol..
If we replace an R group with a hydrogen, we get a secondary alcohol.
If we replace an R group from a secondary alcohol with a hydrogen, we get a primary alcohol.
If the R group of a primary alcohol is just a single carbon, we get methanol.
Replace the carbon in methanol with a hydrogen, and we get water.
So...
Is water considered a special class of alcohol?(3 votes)- No. An alcohol is defined as an OH group attached to an alkyl carbon. Since water contains no carbon, it is not a special case of an alcohol.
That said, we can still consider how the replacement of an H atom in water with an alkyl group will affect the properties of the remaining OH group. That is, we can compare the properties of an OH group in an alcohol with the properties of an OH group in water.(3 votes)
- Does he have the boiling points mixed around or something for ethanol and ethane? It's really confusing(3 votes)
- No, they are correct. Think about it, one is a gas at room temperature, one is a liquid. Which would you expect to have lower boiling point considering this observation?(4 votes)
- Hi!
What if I react an alcohol with NaOH or KOH...would I get an alcoxyde and water? Thanks!(3 votes)- You would get an equilibrium reaction. There will be no significant amount of alkoxide formed.(3 votes)
- atJay Jay calls the H in the NaH a hydride ion. I dont understand this usage of ion. Isnt it just a polar molecule until it comes into contact with the alcohol? I wouldnt have though the electrnegativity (1.3) would be enough for H to totally take the Na electron for that bond to be ionic. Or can we call any atom in a molecule with a polar charge an ion? 7:00(2 votes)
- It's not so much the electronegativity difference that makes NaH act like a salt (an ionic compound). The difference is only (0.7). H- (hydride = hydrogen with 2 electrons) is extremely basic. It forms extremely stable H2 gas. (not stable if you light a match).
In fact, the pKa of H2 as an acid can never be measured as it so high and complicated.(3 votes)
- why is the hydrogen attached to oxygen the most acidic? why not the hydrogen on the carbon atoms acidic....!(1 vote)
- Use the element rule. The more electronegative the atom hydrogen is attached to is, the more acidic the hydrogen.(4 votes)
- atwon't that molecule have a double bond with the rest of the carbon chain? 5:54
-C=O(2 votes)- No because the C there already has 4 bonds, one to the O one to the next C and 2 H. It is in a stable configuration and wont take O's extra electron to form a 5th bond.(2 votes)
Video transcript
Let's start with physical
properties of alcohols. And so we're going to
compare, in this case, alcohols to alkanes. In this alkane on the left
here, two carbons, so this is, of course, ethane. On the right, if we take
off on those hydrogens and replace it with an OH, we of
course have ethanol right here. So let's start with
the boiling point. So the boiling point of ethane
is approximately negative 89 degrees Celsius. And since room temperature's
somewhere around 20 to 25 degrees Celsius, we're--
At room temperature, we are much higher than the
boiling point of ethane which means it's already boiled. It's already turned into a gas. So at room temperature and
room pressure, ethane is a gas. Ethanol however, has a
much higher boiling point somewhere around
78 degrees Celsius. And once again, since
room temperature is somewhere around 20 to 25,
the boiling point of ethanol is much higher than
room temperature. So at room temperature and
pressure, ethanol is a liquid. It hasn't boiled, yet. And these a large differences
and boiling points between these two
molecules can be attributed to the intermolecular
forces that are present. So if two molecules of
ethane are interacting, really the only
intermolecular forces holding those molecules together
would be London dispersion forces which are the weakest
of the intermolecular forces. So it's relatively easy to
pull part two ethane molecules. And that accounts for the
very low boiling point, doesn't take a lot of
energy to pull them apart. So it's easy for it
to turn into a gas. Ethanol, however, has a
much higher boiling point which means it's much harder
to pull those molecules apart. It takes more energy. So let's look at why ethanol
such a higher boiling point. So if I show two ethanol
molecules interacting-- So here's one ethanol molecule. And let's go ahead and draw
another ethanol molecule right here. And if I think about the
oxygen-hydrogen bond, I know that's a
polarized covalent bond. I know that there's a large
difference in electronegativity between the oxygen
and the hydrogen. Oxygen's much more
electronegative which means the electrons
in the bond between oxygen and hydrogen are going to be
much closer to the oxygen atom, giving the oxygen atom a
partial negative charge. So these electrons in this bond
between oxygen and hydrogen are going to move away
from the hydrogens. So hydrogen is going to lose a
little bit of electron density, leaving it relatively positive. So we give it partial
positive charge. It's the same thing
for the other ethanol molecule, partially
negative oxygen, partially positive hydrogen. And we know that
opposite charges attract. So the partially
positive hydrogen is attracted to the
partially negative oxygen. And so there's a strong
intermolecular forces that holds those two molecules
together and then, of course, is hydrogen bonding. So there's some
hydrogen bonding. So there's hydrogen bonding
between alcohol molecules. So hydrogen bonding. And since hydrogen bonding is
the strongest intermolecular force, it's relatively difficult
to pull those molecules apart. It takes a lot of energy,
takes a lot of heat. And that's why the
boiling point of ethanol is so much higher than the
boiling point of ethane and also accounts for
the state of matter. What about solubility? So is ethanol soluble in water? And of course, it is. And the reason why is
hydrogen bonding, once again. So if we draw a water
molecule in here, I know that the water molecule
is polarized in the same way that the alcohol molecule is. So the hydrogen is
partially positive, and the oxygen, right over
here, is partially negative. And so once again,
opposite charges attract. The hydrogen is attracted
to this oxygen here. And so because of
hydrogen bonding, there's interaction
between the water molecule in between the alcohol molecule. So the water molecule is polar. So if you want to think about
it in terms of polarity, because of the difference
in electronegativity, water is a polar molecule. Ethanol is a polar molecule. And like dissolves like,
so these two molecules will be soluble in each other. So if I look at the
structure of ethanol, the reason why it
is soluble in water is because of this
portion of the molecule, this hydroxyl group, this OH. It's the differences
in electronegativity that allow the hydrogen bonding. So this portion of the
molecule is the polar portion of the molecule. And this portion of
molecules the part that loves water which is
why it is soluble. So if it loves water, we say
it's hydrophilic, hydro meaning water, phil meaning
love, so hydrophilic. Whereas this portion
over here on the left, this is more of an alkane-type
environment, a nonpolar type of environment. So this part of the
molecule scared of water. So it's hydrophobic. So we have the
hydrophobic portion of our alcohol molecule. And we have the
hydrophilic portion of the alcohol molecules. Now, we know that
like dissolves like, so nonpolar will not
dissolve in polar. But as long as we have, in
a relatively small number of carbon atoms in
our alcule group, the OH group is enough-- is
polar enough for the alcohol to be soluble in water. Now if you have a large
number of carbon atoms, your molecule's more
nonpolar than polar. And so alcohols will stop
being soluble in water if they have a lot of
carbon atoms on them. So let's look at, now, the
preparation of alkoxides. So let's look at an alcohol. So here we have our alcohol. And if we react our alcohol
with a strong base-- So we'll give it a lone pair
of electrons, a negative 1 formal charge. So we have a strong base here. And our strong base is
going to take this proton and leave these electrons
behind on this oxygen. So now we have an
oxygen that used to have two lone
pairs of electrons and now has three lone pairs. That gives it a negative
1 formal charge. And the base is
going to form a bond with that proton like that. So this is an acid
base reaction. So if we react an alcohol
with a strong base-- So this is a strong
base here-- we're going to form the conjugate
base to an alcohol which is called an alkoxides. So this is an alkoxides
ion right here. So it's a chemical
property of alcohols. They are acidic if you
use a strong enough base. And the conjugate base to an
alcohol is called alkoxides. Let's look at an example. So let's take ethanol. So here I have my
ethanol molecule. And we'll react that with
a strong base something like sodium hydride, so NaH. So Na+ and H with 2. Hydrogen with two
electrons around it which makes it a
negatively charged ion, so that's called
the hydride anion. So we have the basic portion,
the negatively charged hydrogen. It's going to
function as a base. It's going to take
these two electrons. It's going to take that
proton right there. So the acidic proton on alcohols
is the one on the oxygen. And the electrons
in here are going to kick off onto our
oxygen like that. So we're going to
get for our product an alkoxide with three lone
pairs of electrons around it, giving it a negative
1 formal charge. The sodium is floating
around positively charged. So it's going to
electrostatically, ionically, interact with our
alkoxide anion. And the hydride anion
picked up a proton. So those two hydrogens combine
to form hydrogen gas which will, of course, bubble
out of your solution. So the formation of hydrogen gas
will be observed this reaction. And this is how you
form an alkoxide. This molecule is
called sodium ethoxide. And so we have sodium
ethoxide over here on the right which is a
relatively strong base that is used a lot of organic
chemistry reactions. And let's see we used a
strong base to form it. We used sodium hydride over
here to form that molecule from ethanol. So there's another
way to form alkoxides. So let's take a look
at a generic way to form alkoxides from
Group One, Alkali Metals. So here we have our
alcohol like that. And if we react our alcohol
with a Group One metal-- So an alkali metal, those
all have one valence electron being in Group One on
the Periodic Table, so something like lithium
or sodium or potassium. We are going to
form an alkoxide. So we're going to
form-- Let's see, three lone pairs of electrons,
a negative 1 formal charge. And the mechanism, the metal is
going to donate its one valence electron, leaving it
with a plus-1 charge. So it's going to interact
with your alkoxide like that. And this also releases
hydrogen gas like that. So that's your general reaction. Let's look at an
example where we'll react-- Let's use cyclohexanol. So we're going to react
cyclohexanol with sodium. So let's actually--
Let's go ahead and redraw that
cyclohexanol molecule here because I want to show a
little bit of the mechanisms. So let's go ahead and
draw it like that. And put our lone pairs of
electrons on the oxygen. So sodium metal has one
valence electron like that. So if we think
about what happens, sodium is-- Sodium will
donate it's one valence electron very easily
because it will then have the stable electron
configuration of a noble gas. So the first step
of the mechanism is donation of this
one valence electron. So we're going to show the
movement of one electron. So we're going to use
a half-headed arrow. And then the two electrons
in the bond between oxygen and hydrogen, we're going
to use a two-headed arrow to show the movement of
those two electrons off onto that oxygen. So let's go ahead and
draw what we have now. We have our cyclohexane ring. And we now have three lone pairs
of electrons around my oxygen which makes it
negatively charged. And the sodium donated
its one valence electron. So now it has a plus-1 charge. So it's going to interact with
that negatively charged oxygen. And what happened
to the hydrogen? That hydrogen there is going
to pick up one electron. So now we have hydrogen
with one electron around it. That is extremely reactive. Hydrogen prefers to have
two electrons around it. So when two of those hydrogen
atoms get close to each other, they're going to, of course,
react and share their electrons to form hydrogen gas. So I could draw it like that. So that's where those
two electrons are. The two electrons are here. So each-- One from
one hydrogen and one from the other hydrogen. So hydrogen gas is going to
be produced in this reaction as well. So that's an overview
of physical properties and also the preparation
of alkoxide anions.