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MCAT
Course: MCAT > Unit 9
Lesson 14: Alcohols and phenols- Alcohols and phenols questions
- Alcohol nomenclature
- Properties of alcohols
- Biological oxidation of alcohols
- Oxidation of alcohols
- Oxidation of alcohols (examples)
- Protection of alcohols
- Preparation of mesylates and tosylates
- SN1 and SN2 reactions of alcohols
- Biological redox reactions of alcohols and phenols
- Aromatic stability of benzene
- Aromatic heterocycles
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Properties of alcohols
Created by Jay.
Want to join the conversation?
- How does it become H2 if there is only one H on the phenol ? Where is the other H coming from?(6 votes)
- What is alkoxide?(3 votes)
- Alkoxide is the conjugate base of an alcohol. It consists of an organic group that is bonded to a negatively-charged oxygen atom, and is written as R0-, with R being the organic component. Further properties of alkoxide include the fact that it is a strong base, and is not stable in protic solvents (such as water).(6 votes)
- Jay, you're awesome! love your ochem videos, I am studying for the MCAT right now and I cannot thank you enough.(2 votes)
- can you please draw an example of a non covalent bond interaction?(1 vote)
- NaCl is an ionic compound.....ionic compounds differ form covalent molecules because the atoms in ionic compounds are held together through electrostatic forces......see Coulomb's law.....(2 votes)
- What is the chemistry behind slow/stuck fermentation?(1 vote)
- Wouldn't the OH be in equilibrium with the alkoxide form of it if you have the alcohol by itself or in water or some other protic solvent?
Also since H is very reactive(because it is a free radical) wouldn't it form these products:
Original alcohol
Open chain alcohol if the original was cyclic
Metal hydride
H2(1 vote) - In the reaction of an alcohol with a metal(sodium,for example) why does sodium bond better with the alkoxide than the hydrogen bonds with the alkoxide?Why does sodium replace the hydrogen?(1 vote)
- What is the difference between alcohols and phenols? Thank you in advance!!(1 vote)
- In terms of classes of compounds, phenols refer specifically to alcohols that have a hydroxyl bound to an aromatic. Most of the time you hear the term "phenol," however, will be in reference to the simplest molecule in this class of compounds (C6H5OH). You can learn more about it here: https://www.khanacademy.org/test-prep/mcat/chemical-processes/alcohols-and-phenols/v/alcohol-nomenclature-rules(1 vote)
- if hydrogen bonds with another atom , doesnt it form helium?(1 vote)
- Why does the last reaction shown not create NaH as a product?(0 votes)
- because H here is H+ & NOT H-(ANION) . So, Na+ reacts with O-(0 votes)
Video transcript
Let's start with physical
properties of alcohols. And so we're going to
compare, in this case, alcohols to alkanes And this
alkane on the left here, two carbons, so this
is of course ethane. On the right, if we take
off one of 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
is somewhere around 20 to 25 degrees Celsius,
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 large differences
in 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 force that's 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
has such a higher boiling point. So if I show two ethanol
molecules interacting, so here is 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. And hydrogen is going to lose a
little bit of electron density, leaving it relatively positive. So we give it a partial
positive charge. It's the same thing for the
other ethanol molecule, right? 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 force that holds those two
molecules together. And that, of course,
is hydrogen bonding. So there's hydrogen bonding
between alcohol molecules. And since hydrogen bonding's 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 it 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 and in between the
alcohol molecules. 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 the
molecule is 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 of environment, a non-polar type of environment. So this part of the
molecule is 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 molecule. Now, we know that
like dissolves like, so non-polar will not
dissolve in polar. But as long as we have a
relatively small number of carbon atoms in
our alkyl group, the OH group is polar
enough for the alcohol to be soluble in water. Now, if you have a large
number of carbon atoms, your molecule is more
non-polar 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 a 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 alkoxide. So this is an alkoxide
ion right here. So a chemical
property of alcohols, they are acidic if you
use a strong enough base. And the conjugate base to an
alcohol is called an alkoxide. 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 plus, 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 in this reaction. And this is how you
form an alkoxide. This molecule is
called sodium ethoxide. So we have sodium
ethoxide over here on the right, sodium
ethoxide, which is a relatively
strong base that is used in 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 1 alkali metals. So here we have our
alcohol, like that. And if we react our alcohol
with a group 1 metal, so an alkali metal. Those all have one valence
electron, being in group 1 on the periodic table. So something like lithium
or sodium or potassium. We're going to form an alkoxide. So we're going to
form, let's see, three lone pairs of electrons,
a negative 1 formal charge. In 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 react cyclohexanol. So we're going to react
cyclohexanol with sodium. So actually, let's
go ahead and redraw that cyclohexanol
molecule here, because I want to show a little
bit of the mechanism. 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 will donate its one
valence electron very easily, because it will then
have the stable electron configuration of a noble gas. So the first step
in 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 bottom 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 one from one hydrogen,
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.