sodium salt of hypophosphorus acid which can apparently be used together with iodine instead
of red phosphorus to reduce alcohols to the corresponding alkane first we wanted to see
if we could prepare the acid from the salt so we started by measuring out 100 grams of
sodium hypophosphate crystals these are the dihydrate salt the theory was to see if we
could perform the displacement reaction using a strong mineral acid
so we measured out 30 mils of concentrated hydrochloric acid initially this is half the
molar equivalent we placed the salt on a hot plate and then added the acid to the crystal
these seem to dissolve and a much lighter colored solid appeared in the suspension this is a good
sign that sodium chloride is being created and precipitated
due to the concentration we gave the mixture the good stir and heated it on a hot plate we
then prepared a further 30 mils of concentrated hydrochloric acid which should make up to one
mole of HCl per mole of sodium hypophosphate we added this to the hot mixture and stirred again
you can see that the white crystalline deposit is quite thick
and different to the original salt we stirred the mixture for a few minutes
and heated up to around 80 degrees C then allowed it to cool right down to
nearly zero degrees in order to precipitate as much of the sodium
chloride salt as possible on chilling for a few hours the salt fell to the
bottom of the beaker leaving the clear solution so we set up for vacuum
filtration and removed the white salt from the liquid
we allowed all the liquid to filter through and for the salt to become reasonably dry
we ended up with covering 40 grams of sodium chloride as you can see here it's slightly damp
but even so according to the stoichiometry of the reaction we would expect 47 grams to be produced
in total so this is not bad at all
so this is not bad at all
so this is not bad at all
so this is not bad at all
so this is not bad at all
our filtrate is slightly yellow due to our filter not being totally clean
however it tells us that we've got a lot of hypophosphorus acid as
hypophosphite and hydrogen ions in this solution so we transferred this into a
250 ml flask to see if we could distill of some of the water we added a small
magnetic stirred bar so that we could stir to prevent the mixture from bumping
and then used a hot plate to heat the mixture with a
simple distillation app with a setup
and then used a hot plate to heat the mixture with a simple distillation app with a setup
we also used a vacuum pump to help drive off the water at a lower temperature
we know that hypophosphorus acid decomposes temperatures higher than about 100 degrees
c to form explosive and toxic phosphine gas so we wanted to be very careful about this
soon we saw some condensation appearing
and we insulated the flask using some aluminium foil to make the distillation more efficient
and we insulated the flask using some aluminium foil to make the distillation more efficient
Despite all these measures the distillation of water was incredibly
slow and after an hour we had only managed to obtain around 10 mils of
water distilled.
At this point we noticed a white precipitate, presumably more sodium
chloride forming in the boiling flask, so we allowed the mixture to cool at this
point and chilled it down again to zero degrees.
Once again we filtered this on a vacuum and we noticed that the liquid was now actually
quite viscous and syrupy.
We got a further four grams of sodium chloride from this filtration, so we
figured that we must have a relatively strong hypophosphorus acid solution by
now. And here's the filtrate. Exactly 100 grams
in weight.
And the filtrate is now 100 grams in weight.
We started measuring 74 mils giving a density of 1.35. The pure acid has a
literature density of 1.49, so this is looking good.
So let's first put it to the test by reducing some iodine. We placed a small
amount in a beaker and added a spatula of solid iodine at room temperature.
I'd say that worked pretty well. Maybe a bit too well.
Let's now see if the acid can be decomposed by heat. This will also tell
us how safe it is to boil a reflux. We heated a small amount in a beaker on a
hot plate. First we got some condensation.
Then very slight boiling. Then tiny explosion.
So we've definitely got some good stuff here, and we'll have a go in a future
video at using it to reduce an alcohol.
In the meantime we'll leave you with our final experiment, evaporating the liquid
to dryness on a glass petri dish. Enjoy.
The hot plate is only at about 150 degrees C, so this is a good warning about
the reactivity of this compound. Huge clouds of white phosphorus pentoxide are
generated as the phosphine gas produced spontaneously combusts in air.
Stay tuned.
Closed Captioning by Kris Brandhagen. brandhagen.com