phosphorus together can reduce
secondary alcohols to alkanes we've done
using a slight variation on this using
hypo phosphorus acid and it worked well
we got a viewer question which was
whether or not aluminium would
substitute in place of phosphorus after
all it does seem to work to some extent
for making alkyl iodides
initially we thought no way but then we
researched some more about the iodine
and red phosphorus reduction mechanism
and we figured that we'd give it a go
we'll explain a bit more about the
reaction here so let's go through the
reagents first we've got aluminium foil
and we've weighed out 1.4 grams of it
this is going to be a fairly large
excess to what we need in theory to
react with all the iodine and next we've
got our iodine we're using 4.9 grams of
iodine powder here note that this is
recovered from another reaction so it's
pure but might be very slightly damp
and here's our reduction substrate as
in the previous reaction we're going to
use mandelic acid firstly because we've
proved it works in a previous reaction
secondly because the product of
reduction is phenyl acetic acid and this
should be very apparent by its sickly
honey-like aroma and here's 2 grams of
mandelic acid we prepared this in a
previous video so check that out for
details if you want to know how to make
this
ok here's the lineup before we did the
reaction we just wanted to test and see
how aluminium and iodine reacted
together in elemental form just to check
it wasn't going to be dangerous so
here's some iodine on some aluminium
foil and to spice it up here's some
aluminium metal powder as well
let's wait and see if anything happens
and the answer was very little we could
feel the bottom of the sheet warming up
so there's an exothermic reaction but
it's not violent at least at low
temperatures so we figured that with a
solvent we should be okay so let's get
set up we're going to use a 250 mil
flask on the heating mantle and equipped
with magnetic stirring for the reaction
that gives us lots of
space in case anything exciting does
happen we've got a little reflux
condenser attached to the top let's get
this started
ok we did some research on solvents and
we found that the iodine and red
phosphorus reaction has been done very
effectively using acetic or propionic
acid aluminium isn't particularly
reactive towards these acids so we'll
try this out
let's just dismantle the condenser
for a moment and here's 25 mils of pure
anhydrous propionic acid that should
dissolve our iodine and substrate
effectively as well
ok now we're rocking let's start off
with the iodine and get this into the
reaction flask
it dissolves in the propionic acid
rapidly forming a brown colored
solution
ok all in now so next for our substrate
the mandelic acid
ok
ok
ok
we'll start by adding about a third of
the aluminium foil and now we're going
to leave this stirring at room
temperature just to see what happens and
if any spontaneous reactions starts this
is going to be boring to start with so
in the meantime we'll talk a little bit
about the theory here and why we figured
that actually this isn't totally
a crazy idea
here's the iodine and red phosphorus
reaction very simply the phosphorus in
iodine react forming pi3 this first of
all reacts with the alcohol to iodinate
forming an intermediate compound the
hydroxyl from the alcohol ends up
irreversibly tied to the phosphorus and
eventually forms phosphorus acid
but in a second step a reduction can
occur
presence of hydrogen iodide the intermediate can be reduced forming an
alkane, in our case, phenylacetic acid.
The result of this reaction is iodine, which then reacts with more red
phosphorus to form PI3 again.
So here's a simplified overall schema.
This works because the phosphorus can react with iodine, the PI3 can react with
the alcohol, and with a tiny bit of water present high will be generated which
acts as the reducing agent, and keeps the cycle going.
What we figured was that we now know that aluminium does work together with iodine
and can react with simple alcohols such as ethanol to form ethyl iodide.
We've tested this in a previous video.
We also know that aluminium iodide is highly reactive with water and will
generate hydrogen iodide.
And the aluminium hydroxide formed is highly stable.
Like the phosphorus acid, it's not going to participate
any further.
So we've in theory potentially got everything we need in one place to perform the same reduction,
only using aluminium in place of red phosphorus.
Of course, theory is one thing.
Well, at room temperature after 15 minutes there's no visible reaction so we're going
to start to gently warm the mixture now.
And
some smoke-like fumes are starting to appear in
the flask.
A few more minutes of heating and the colour
seems to be changing as well .
And soon we notice white
needle-like crystals forming on the inside of the flask definitely a
reaction occurring at this point we add the rest of the aluminium foil and we
slowly raise the temperature until the mixture starts to reflux and pretty soon
the color of the iodine starts to fade to a paler yellow
the mixture is becoming slightly frothy
after a while the frothiness died down we're probably about 50 minutes into the
reaction now to give this the best chance of reacting we then allowed the
mixture to stir and reflux for another two hours here we go
we then allowed to cool back down to room temperature again slowly we
transferred into a beaker and in order to quench any remaining aluminium iodide
we added a couple of ice cubes
no real obvious reaction at this point
well pretty much everything apart from fenyl acetic acid should be water soluble
so we're going to start by adding 80 mls of cold water to the reaction mixture
yum-yum, looks like coffee with luxury aluminium flakes
most of the iodine color is gone which is encouraging if the aluminium had simply reacted to that mixture it would have been better
with the propionic acid then we would guess that there would still be a huge
amount of iodine. There's also an insoluble scum of something floating on
the surface. Let's start by adding a little bit of sodium hydrogen sulfite
which should reduce any residual iodine and make this a bit easier to work with.
We then added a small amount of 50% sulfuric acid to see if this would
dissolve the gray cloudiness in the mixture. It did, but not much, so we
covered the beaker and left it in the fridge for a few hours to chill down and
for the precipitate to settle.
Here's the result a few hours later. There's a gray solid on the bottom of
the beaker which we are assuming is unreacted aluminium. We're going to
decant as much of the clear liquid we can into a separating funnel now. The
solids might contain some organic products, but if there's any phenylacetic
acid in here we should also be able to get some out of the liquid as well.
We were fairly cautious.
not to get any solids into the funnel as you can see. So let's see how this goes.
We'll first try to extract the aqueous mixture using dichloromethane. So here's
25 mils. Let's give this a really good shake for a few minutes. And now we'll
allow to separate. One issue we're going to have is that the DCM will also
dissolve out some of the chloromethane. We're going to have to separate the
propionic acid as well, but let's see what we get. The aroma of phenylacetic
acid is very strong and potent and should be easily recognizable above even
the sickly propionic aroma. Looking good. So let's now drain off our bottom DCM
layer. And now we'll place this in a petri dish and evaporate the DCM to the
hot water. And let's see what we've got. And here we are. A fair bit of liquid remaining,
and we can tell immediately from the aroma that unfortunately it's pretty much propionic
acid. This reaction hasn't generated any phenylacetic acid in a significant amount. This isn't
too surprising, and is basically what we were expecting. But there is a surprise on going
on. We've got a very aqueous layer now that a chunk of the propionic acid has been extracted
out. It's not strong, and definitely faint, but it's unmistakable. There's a rubbery
petrochemically sickly honey acacia aroma in there in the background. Call us crazy
if you will, but it's there. We did another extraction of the mixture in the beaker using
diethyl ether. Again, this seemed to only give us a small amount of propionic acid.
So it's clear, this reaction as it stands doesn't reduce mandalic acid. But we can't
help but feel a sense of unfinished business here. Perhaps our choice of solvent was totally
wrong and the acid just dissolved the metal. Perhaps you need a little bit more water,
or perhaps anhydrous conditions. Maybe you need a catalytic amount of hydroiodic acid
in there as well to start with to get things moving? This might all be bullshit, and there's
really no chance.
but the background aroma is there and it tells us that maybe just maybe with the
right conditions this can yield something
it's a cliffhanger and unfortunately we don't have time to play with it any
further perhaps someone else out there would like to take on the challenge
this is our last reaction video but we've got a few more interesting things
to show you so there will be a few more videos coming up thanks for watching and
stay tuned