manganese dioxide. This has an extremely high surface area. To begin with weigh out
50 grams of potassium permanganate into a large 600 ml beaker. Then add a magnetic
stir bar and place this on a hot plate. Now add 150 ml of warm water and
commence stirring vigorously whilst heating the mixture gently. It is very
easy to dissolve. Otherwise we'll have permanganate crystals in our product
and these will mess up subsequent reactions. At around 60 degrees and
adding a little more water if you need to you should get all the crystals to
dissolve. At this point the crunching sounds from the stir bar will disappear.
Now weigh out 70 grams of manganese sulfate monohydrate crystals into
the water. If you have anhydrous salt or a different hydrate then adjust accordingly.
Dissolve these in as small an amount of water at room temperature as you can.
This will take quite a bit of stirring so be patient. In our case it took around 150 ml of
water to dissolve and form a nice pink solution. Now slowly add the manganese sulfate solution to
solution the reaction will produce a bit of heat so go slowly and don't allow
this to boil you'll see a dark colored precipitate forming in the beaker this
will gradually get thicker and thicker as you add the manganese sulfate
solution and you will see the purple color of the permanganate slowly
disappear allow to stir for a good hour and you'll end up with a dark brown mud
a bit like this ours was quite warm and bubbled when agitated we think this may
be oxygen gas being produced by slight decomposition now filter the mud we use
the strengthened filter paper for this but be aware that the mixture is quite
acid and may dissolve paper filtration is slow
and takes a lot of time.
It takes a few hours to get a semi-dry product don't worry if the filtrate is
slightly pink with unreacted permanganate now watch the semi-dry
product with 400 mils of cold water and green filter your filtrate at the end of
the wash should be colorless the brown mud will stain all your glassware and
equipment here's a tip on how to clean it off
prepare a dilute solution of sodium bisulfide in water use this to wash your
glassware or stained equipment magic after a few more hours of filtration you
will have a semi-solid attractive-looking brown paste place this
into an oven at 150 degrees C every hour check on the solid and use a spatuline.
to break it up eventually you'll get a fine powder and some small solid lumps
pulverize these into a very fine dry powder and here's our chemical manganese
dioxide around 65 grams are produced from the preparation which is pretty
much quantitative considering that it's probably not 100% dry and maybe contain
some salt impurities as well it's good enough for the
the next step though
in part two we're now going to use our active manganese dioxide to prepare
benzaldehyde we decided to use all the manganese dioxide we prepared in the
next step in order to do a test of its oxidizing power in theory two moles are
needed to oxidize one mole of benzaldehyde
alcohol but the efficiency may depend on the available surface area. We measured
out 30 mils of benzyl alcohol. This was added into a three-neck flask fitted
with a stir bar and on a water bath. We then measured out 80 mils of
dichloromethane to act as the solvent. Our theory was that dichloromethane
would dilute the alcohol and thus prevent overoxidation to benzoic acid. In addition,
we figured that its low boiling point would stop the reaction mixture getting
too warm and again protecting against overoxidation. Once this was added we set
up the flask with a reflux condenser. Using ice and the cooling water and with
vigorous stirring we bought the mixture up to reflux. We then oven dried a small
glass of water for 20 minutes, taking it out of the oven, and then placed it in the
funnel which would fit in one of the spare necks of the flask. Removing the
stopper and working quickly we then added a small amount of the manganese
dioxide into the flask and then restoppered the neck. Either due to a
reaction taking place or a seeding effect we noticed an immediate increase
and reflux. We continued with the process of slowly adding the manganese
dioxide powder over the following hour. Once it was all added the magnetic
stirring continued to work well and we left the flask refluxing for a further
hour on the water bath then allowed it to cool slowly down to room temperature.
Cooling took place.
It took a while as if the flask was still producing some heat due to the
reaction taking place. Here's the final reaction mixture in the flask.
By the way the manganese dioxide gets everywhere you least expect it to so
have some sodium bisulfite solution handy for cleanup.
We then filtered the reaction mixture using a filter pump compared to the
filtration in the flask.
In part one this was pretty simple with the dichloromethane solvent and took a
few minutes. The reaction flask was a mess so we used a further 50mL of DCM to
wash this out thoroughly.
As you can see the filtrate had a yellow-orange color.
We then switched off the pump and thoroughly mixed the black solid with a
further 40ml of DCM.
in order to get all our product out then filtered again we then repeated the DCM
wash and then got the solid completely dry on the pump the black solid is now a
mixture of manganese oxides and we saved it because we're sure we can recycle
this back into manganese sulfate and then more manganese dioxide here's our
filtrate we placed this on a hot water bath and evaporated off the DCM using a
vacuum pump you could also distill the solvent off if you wanted to recycle it
we ended up with an amber colored oil which we measured and was 27 mils and
volume this had a very strong sweet almond aroma
you
to purify this you could vacuum distill it but we decided to do a bisulfite
purification
prepare 150 mils of a saturated sodium bisulfite solution in water
this takes around 80 grams and lots of stirring
add the crude benzaldehyde to this solution and then stir well for a period
of 15 minutes
be patient as it can take a while for the adduct to form and it requires a lot
of stirring and breaking up any lumps to get it to completely react the adduct
isn't completely crystalline so we can see that there's probably some residual
benzal alcohol in here as well as benzaldehyde we gave this a really
thorough stirring and then filtered the solid adduct to form we have this very
through a glass sinter.
We then washed this twice using around 20 ml of dichloromethane.
Thoroughly stirring together, the idea was that this would remove any benzyl alcohol
and other organics trapped in the adduct.
The adduct as you can see was quite a nice, pure white color and a crystalline solid.
Now we prepared a saturated solution of sodium carbonate and 150 ml of water as before, make
sure the solution is saturated.
Add the crystalline bisulphide adduct from the filter into this solution and stir well,
again being patient.
As it can take a while to completely react, the adduct first floats and then over 10 minutes
with stirring dissolves and separates into a layer of our benzaldehyde product.
We placed the mixture into a separating funnel and drained off the bottom aqueous layer.
We allowed the mixture to stand to let as much water separate as possible, then drained
the product into a container.
We then poured the mixture into a small flask.
We used some anhydrous magnesium sulfate to remove all water, present, and then filtered
into a storage container.
And here's our product, 12.6 grams of benzaldehyde.
This is a 43% yield on our starting benzyl alcohol, which isn't too bad and is actually
comparable to the real-life yields produced by some other oxidizing agents.
The benzaldehyde is a very good solution for the adduct.
It can be used in a variety of ways, such as per sulfate and copper or iron nitrate.
We'll keep playing around and seeing if we can get the stoichiometry just right on this
one because it seems to a very convenient method and doesn't seem to over oxidize.