Oxidation of alcohols (examples) - Chemistry Video Lecture - Chemical Engineering

In the last video, we took
a look at the mechanism
for the oxidation of alcohols.
In this video, we'll
do specific examples
for different types of alcohol.
So we'll start with
a primary alcohol.
And we identified the
carbon attached to the OH
as my alpha carbon.
And in order for this
mechanism to work,
we need at least one hydrogen
attached to our alpha carbon.
So if we react to
our primary alcohol
with sodium dichromate,
sulfuric acid, and water,
which we call the Jones
reagent, in that mechanism
we're going to oxidize
our alpha carbon
and lose one of those
protons attached
to the alpha carbon,
which would give us
an aldehyde functional group.
So we increased the number
of bonds of carbon to oxygen,
we lost a bond of
carbon to hydrogen.
Now, the difficulty is trying
to isolate this aldehyde.
Usually, it's very
difficult to isolate,
and oxidation will continue.
And you get a second oxidation
to produce a carboxylic acid
as your final product.
So if you react a primary
alcohol with the Jones reagent,
you're going to end up
with a carboxylic acid.
Let's look at an
example, and we'll
use ethanol as our
primary alcohol here.
So if we react ethanol
with the Jones reagent,
the chromium in the
sodium dichromate
is chromium-6 plus,
which has kind
of an orangish color to it.
So when you're starting
off with your reaction,
it's going to look a little bit
orangish due to that chromium
presence.
And when we oxidize our
primary alcohol, when
we oxidize our
ethanol, we're going
to turn it into a
carboxylic acid.
We're not changing
the number of carbons,
so there's still going to
be two carbons like this,
but we're now going to change
it into a carboxylic acid.
So acetic acid will
be the product.
So we went from this carbon
having one bond to oxygen,
and we oxidized it
so this carbon now
has three bonds to oxygen atoms.
And in that process, if we
oxidized that alpha carbon,
we're going to
reduce the chromium.
So the chromium is going to go
from an oxidation state of 6
plus and eventually it's
going to reach an oxidation
state of 3 plus like we talked
about in the last video, which
has kind of a greenish color.
So it's very easy to
monitor this reaction
by just looking for
the color change.
And this is a very,
very fast reaction,
so this was originally used
for the breathalyzer test
to determine if
ethanol is present.
So let's see what would happen
if you wanted to actually stop
it at the aldehyde?
You don't want the
oxidation to continue
to the carboxylic acid.
Let's say you wanted to actually
stop it at the aldehyde.
Well, to do that, you would
have to use a different reagent.
So let's go ahead
and look and see
how we could stop the reaction
after the first oxidation.
So if we started with
a primary alcohol--
so I'll just re-draw a primary
alcohol really fast here
like this.
And if we wanted to
oxidize it only once,
so that we end up
with an aldehyde,
the best reagent
to use for this is
something called pyridinium
chlorochromate, or PCC.
So let's take a look at the
structure of the PCC reagent
really fast.
So pyridinium, let's go ahead
and show what that looks like.
So it's derived from
pyridine, so let's go ahead
and sketch that in like that.
So pyridine as a base is
going to pick up a proton
to form a positive charge here.
And then we have a CrO3
and then Cl and then
with a negative charge.
So this would be the pyridinium
part, so let's go ahead
and write it.
Pyridinium.
And then we have chlorochromate
over here on the right,
so I'll go ahead and
write chlorochromate.
And then that makes it easier to
see where the P-C-C comes from.
So this is the
PCC reagent, which
is a much more mild agent
than the Jones reagent.
It'll oxidize your
primary alcohol
and stop at your aldehyde.
So let's go ahead and
react to ethanol again.
This time, we'll use
PCC instead of Jones.
So if we started with ethanol
and we added PCC-- so here
we go-- we're going to
end up with an aldehyde.
And it's a two-carbon
aldehyde, so we
can say those two
carbons are still there.
And we are going to
form a double bond,
and this time it's
going to be an aldehyde.
So this is ethanal,
or acetaldehyde,
which will be the result
of this oxidation reaction.
So that takes care
of primary alcohols.
Let's look at the oxidation
of secondary alcohols now.
So we'll start with a
general reaction over here.
So we'll have a
secondary alcohol, so
two different alkele
groups, or they
could be the same, attached
to our alpha carbon.
Our alpha carbon is
attached to an OH,
and remember for the
mechanism to work,
you must have a hydrogen
attached to that alpha carbon.
So this is my secondary
alcohol like that.
Now, for secondary alcohols,
we can only get one product.
We saw in the last
video that when
you oxidize a
secondary alcohol, you
are going to end
up with a ketone.
So for a secondary alcohol,
you could use either Jones
or you could use PCC.
So either one of
those two reagents
will oxidize a secondary
alcohol to a ketone.
So let's take a
look at an example.
So let's start with
a secondary alcohol.
So I'm just going to draw
a benzene ring on here
and then attach
that benzene ring.
There will be a secondary
alcohol presence.
So there's my secondary alcohol.
And if I were to add
either Jones or PCC,
I look at my secondary alcohol.
I identify my alpha carbon.
It's the one attached to the OH.
And I can see there is
one hydrogen attached
to that alpha carbon.
This is a secondary alcohol.
So when I draw the
product, I'm going
to convert that secondary
alcohol into a ketone.
So if I were to do that,
I would just real quickly
redraw my benzene
ring here, and I
would convert that alpha
carbon into a ketone,
so that would be my product.
All right.
So let's look at a
tertiary alcohol.
So if I had a tertiary alcohol,
something like tert-butanol
here like that,
and if I attempted
to oxidize that tertiary alcohol
with either Jones or PCC,
we saw in the last
video no reaction.
Because if I find the alpha
carbon, this carbon right here,
there are no hydrogens
attached to that alpha carbon.
And again, we saw
in the mechanism
that that was necessary.
So something like
tert-butanol would not
be able to be oxidized
in this fashion.
So that sums up the
oxidation of alcohols.