Solubility of organic compounds

- [Voiceover] You often hear the phrase

like dissolves like when you're talking

about solubility and even though

this idea isn't perfect, it does allow

you to predict the
solubility of compounds.

For example, a polar solvent will dissolve

a polar compound in general,
so like dissolves like.

I also have here a polar
solvent will dissolve

in ionic solute because you don't usually

describe ionic compounds as being polar.

Next, a nonpolar solvent will dissolve

a nonpolar compound,
so like dissolves like,

but a polar solvent will not dissolve

a nonpolar compound, so this would

be like and unlike here.

An example of a polar solvent is water.

An example of a nonpolar compound

could be something like oil.

We know that water will not dissolve oil.

Let's go back to this first idea

of a polar solvent being able to

dissolve a polar compound or a

polar solvent dissolving an ionic compound

like sodium chloride.

We know from experience
that sodium chloride,

or salt, is soluble in water.

Over here on the left we
have part of a salt crystal.

We know that crystals are held together

by attractive forces,
the positively charged

sodium cation is attracted to the

negatively charged chloride anion.

Opposite charges attract and our

crystal is held together
by these attracted forces.

If we get some water
molecules to come along,

we know that water is a polar solvent,

water is a polar molecule.

The oxygen is more electronegative

than this hydrogen, so the oxygen

pulls some of the electron density

in this bond closer to it giving it

a partial negative charge.

If we are withdrawing electron density

from this hydrogen, this hydrogen

gets a partial positive charge.

Since opposite charges
attract, the partially positive

hydrogen in water is attracted to the

negatively charged chloride anion,

so there's an interaction here.

If we get a bunch of water molecules,

here's another one right here,

so partially negative oxygen, partially

positive hydrogen, so there's
another attractive force.

We can pull off these chloride anions

from the solid and bring
the anion into solution.

On the right here we
have our chloride anion

in solution surrounded by
a bunch of water molecules

and we have all these partially positive

hydrogens interacting with our

negatively charged chloride anion.

For the sodium cations let's go back

to our solid on the left.

Since the sodium cation
is positively charged,

that's going to interact with the

partially negatively charged oxygen

in the water molecule, so opposite charges

attract and if you get
enough water molecules

you can pull off these sodium cations

and bring the sodium
cations into a solution.

We have the partially negative oxygens

on water interacting with our

positively charged sodium
cations in our solution.

Our polar solvent, water,
needs to be able to

interact with our solutes and in this case

the polar solvent attacks the solid

over here on the left and it replaces

these ion interactions of our crystal

with ion-dipole interactions
in our solution.

By ion-dipole, I mean we
have a cation right here,

so that's our ion and then our di-pole

would be water, water's a polar molecule,

it has di-pole moment, so we have all

of these ion di-pole interactions.

Ionic solutes that are able to participate

in these interactions
will dissolve in water.

If you have a polar compound,
right, a similar idea,

you have attractive forces that allow

the polar compounds to be dissolved

in a polar solvent like water.

Let's move on to a nonpolar compound,

so a nonpolar compound, something

like this molecule on the left here

and this molecule's called naphthalene.

Naphthalene is a solid with a

very distinctive smell to it.

The first time I smelled
naphthalene in the lab

it reminded me of my grandparents' house

because my grandparents, when I was a kid,

had mothballs that were
made of naphthalene,

so it's a very distinctive smell.

Now naphthalene is nonpolar because

it's composed of only
carbons and hydrogens,

it's a hydrocarbon, so
naphthalene is nonpolar

and you would need a nonpolar solvent

to get it to dissolve.

Toluene is a nonpolar solvent, again,

this is a hydrocarbon, so if you

take solid naphthalene and liquid toluene,

naphthalene will dissolve in toluene,

so like dissolves like, our nonpolar

solvent will dissolve
our nonpolar compound.

Finally, let's look at
this last idea here,

so a polar solvent, something like water,

should not dissolve a nonpolar compound,

something like naphthalene,
and that's true,

naphthalene will not dissolve in water,

so water doesn't interact well enough

with the naphthalene molecules to

get them to dissolve and form a solution.

This concept of like
dissolves like is important

because it allows you to predict whether

or not a compound will
be soluble in water.

Let's look at several organic compounds

and determine whether or not those

compounds are soluble in water.

We'll start with ethanol.

Ethanol has a polar oxygen-hydrogen bond,

the oxygen is more
electronegative than hydrogen,

so the oxygen withdraws
some electron density

making the oxygen partially negative

and leaving the hydrogen
partially positive.

If water comes along, I'll draw in

a water molecule here, and we know that

water is a polar solvent,
water is a polar molecule,

the oxygen has a partial negative and the

hydrogens have partial positive charges.

We can see that there's an opportunity

for an attractive force, opposite charges

attract, so the partially positive

hydrogen on ethanol is attracted

to the partially negatively
charged oxygen on water.

This is an example of
hydrogen bond density,

remember hydrogen bonding
from earlier videos.

Here is a good example of that.

We can even have some
more hydrogen bonding,

I could draw in another
water molecule down here,

so let me go ahead and do that,

we know that the oxygen
is partially negative,

hydrogens are partially positive,

so here's another opportunity
for hydrogen bonding

between partially
negative oxygen on ethanol

and the partially positive
hydrogen on water.

This portion of the
ethanol molecule is polar

and loves water, so
this is the polar region

and this portion loves water, we call this

hydrophilic, so let me
write that down here

so this portion of the
molecule is hydrophilic,

or water loving.

Let's look at the other portion
of the ethanol molecule,

so this portion on the left.

We have a CH2 here and a CH3 here,

so carbons and hydrogens which

we know are nonpolar, so this region

is nonpolar, this region
doesn't like water,

it's scared of water, we
call this hydrophobic,

or water fearing.

We know that ethanol is soluble in water

just by experience, so that must mean

this hydrophobic region doesn't

overcome the hydrophilic region,

so the hydrophilic region is polar region

of the ethanol molecule, it's enough

to make ethanol soluble in water.

If you think about that same concept

and look at a different molecule,

so on the right here's 1-octanol.

1-octanol has an opportunity
for hydrogen bonding

we have this OH here, so it's the

same situation as the ethanol on the left,

so we have a polar or hydrophilic
region of the molecule.

However, the difference is this time

we have extremely large nonpolar

hydrophobic portion of the molecule.

This nonpolar region overcomes

the slightly polar region making the

1-octanol molecule nonpolar overall,

so 1-octanol will not dissolve in water.

This one is a no and this one over here

was a yes, ethanol is a yes.

Next, let's look at cinnamaldehyde,

so down here on the
left is cinnamaldehyde,

let's focus in on, let's focus in on

this carbon oxygen double bond first.

We know that oxygen is more

electronegative than this carbon here,

so the oxygen withdraws
from the electron density

making it partially negative and this

carbon would be there
for partially positive.

This very small portion of the molecule

is polar, this small portion
could interact with water.

However, we have an extremely large,

nonpolar region of the
molecule, all of these

carbons and hydrogens
over here on the left.

This very hydrophobic
region, or nonpolar region,

overcomes the small polar region

making cinnamaldehyde overall nonpolar.

Since it's overall
nonpolar, cinnamaldehyde

will not dissolve in water.

If it's nonpolar, you would
need a more nonpolar solvent

to get cinnamaldehyde to dissolve

and there are several examples of

nonpolar organic solvents
that will do that.

Next let's look at sucrose, so over here

on the right is sucrose or one way

to draw or represent the sucrose compound.

Now we see lots of carbons and hydrogens,

so all of these right here, let me just

go ahead and highlight all
these carbons in this ring

and so all these carbons in these rings,

all these hydrogens, so
at first you might think

okay, there's lots of
carbons and hydrogens,

this might be nonpolar,
but of course we have

lots of these OH groups, so I'm gonna

go ahead and circle a few of them,

right, we have all of these OH groups

in the sucrose molecules, so lots of them.

That means opportunities
for hydrogen bonding.

Because of all these opportunities

for hydrogen bonding,
sucrose is soluble in water

which we know from experience.

Of course sucrose, or sugar, sugar will

dissolve in water, so the opportunity

for hydrogen bonding
is the reason for that.

Benzoic acid is a solid
at room temperature.

If you take some benzoic acid crystals

and you put them in some
room temperature water,

the crystals won't dissolve.

We can explain that by looking

at the structure for benzoic acid.

While we do have this
portion of the compound

which we know is polar and hydrophilic

due to the presence of the
electronegative oxygens,

we also have this portion of the compound

on the left which is
nonpolar and hydrophobic

due to the presence of all
the carbons and hydrogens.

Since the benzoic acid crystals don't

dissolve at room temperature water,

the hydrophobic portion of the compound

must overcome the hydrophilic
portion of the compound.

You actually can get benzoic acid

crystals to dissolve in water
if you heat up the water,

if you increase the
solubility of the compound

by increasing the
temperature of the solvent.

Let's think about benzoic acid crystals

in room temperature water
and let's add a base,

let's add sodium hydroxide.

The sodium hydroxide's going to react

with the most acidic
proton on benzoic acid,

so benzoic acid is acidic, it will

donate this proton right here.

That means the electrons
in red in this bond

are left behind on the oxygen,

so I'll show those
electrons in red over here.

That's gives this oxygen a negative charge

and we form sodium benzoate.

I won't get too much
into acid base chemistry,

but we took the most acidic
proton off of benzoic acid

to give us the conjugate
base sodium benzoate.

Sodium benzoate is highly soluble

in room temperature water.

That must mean we increase this

hydrophilic portion because now we have

a negative charge, so
the hydrophilic portion

now is able to overcome
the hydrophobic portion.

Sodium benzoate is soluble,

this negative charge is better able

to interact with our
solvent which is water.