When I first started teaching, I always found cracking to be one of the more
stressful practicals to manage. The students don't always necessarily see
what you want them to see I found and then, of course, there was always the
spectre of suck-back and the potential risky consequences thereof to
contend with. So when I moved schools back in 2014 and therefore, of course,
science technicians – hi Gwen, by the way if you're watching – I ordered a cracking
practical and was kind of surprised when Gwen showed up with some ... essentially
some Pasteur pipettes and bits of tubing. I didn't really know what I was
supposed to be doing with them.
It turned out that she'd been on a course with Bob Worley, of course,
and had seen how to do the reaction in the microscale version.
We never quite got the reaction to go quite as well as Gwen had seen it on
Bob's course. So I sort of battled with this reaction over the last couple of
years, learned some lessons and in today's video what we're going to do is...
It's gonna be the first of a two-parter. We're gonna really sort of take a deep dive
into getting this reaction to work really well for you,
whether or not you want to run it as a practical or generate large quantities
of alkenes for further demonstrations, which we'll be using next time.
So let's get straight into it.
So CLEAPSS members should be consulting PPO61 for this practical.
Essentially what it describes is turning one of these Pasteur pipettes into a little
test tube by sealing off the end and then you can load this up with your
substrate, bit of mineral wool, aluminium oxides and then the gases that you
produce can be bubbled through a delivery tube directly into some bromine
water or some potassium permanganate for example. And you can watch it decolorise.
What I'm gonna be showing you today is actually a slightly modified version for
collecting larger quantities of gases for demonstration purposes and we're
going to be using one of those gases and something really cool that's coming up in the next video.
So here's the first tip. If you try to seal the pipette as is,
you'll end up with a very narrow, weak seal. On top of this the paraffin won't
be able to get into the end of the pipette so before you seal the pipette,
simply snap a little piece off the end and you'll get a really nice finish.
Next, load in about half a centimetre cubed of paraffin. Tip number two here. This caused
me no end of misery over the last few years. In different places, paraffin might
be called different things. I've wasted hours and hours trying and
failing to get good results out of lighter paraffin, sometimes sold as
kerosene. You want to be using heavy paraffin sometimes called medicinal
paraffin. It should be reasonably viscous.
Now a little bit of mineral wool can be
inserted to absorb the paraffin. If you have it, a microspatula might be handy
for this, but I always just snip a wooden splint down to size. If you don't happen
to have mineral wool, glass wool really doesn't do the trick.
It's worth getting in some proper mineral wool – you won't regret it.
Onto the wool you can load some aluminum oxide, 1 to 2 microspatulas is plenty.
To set up the heating simply take your pipette, clamp it horizontally, having
connected it through a piece of silicon tubing to your delivery tube.
Students can run the product gasses straight through bromine water to watch it
decolourise, but if you want to collect larger quantities for demonstration
purposes, a set-up more like this one is handy. I've added a bit of food colouring
to the water to help it show up on camera.
I'm using three-way Luer lock stopcock valves,
but in principle you can accomplish the same thing with a Hoffman
tubing clamp. Likewise you can get syringe caps or you can just use bits of
sticky tack. The valves do make the process a little bit easier though.
An ethanol spirit burner gives a flame hot enough to crack the paraffin, but not hot
enough to melt the pipette. Here's another tip from Bob: for cracking
paraffin, position your ethanol spirit burner right at the intersection between
the catalyst and the mineral wool. An almost identical set-up can be used to
dehydrate propan-2-ol to give propene which we'll need in the next
video, but for the propan-2-ol the point of heating should be a little
bit further from the substrate.
You will see the catalyst darken as carbon begins to deposit on the surface at the start of the reaction.
Of course, you will want to discard the first bits of expanded air coming off before filling your
syringes with the product gas as you see fit.
If for some reason students didn't get to see what you wanted them to see,
this is the nice thing, you can always just squirt some of your collected gas into
some sample vials containing potassium permanganate or bromine water and shake to decolourise.
The product gas will burn, of course, with a smoky flame.
Now the great thing about this version of the practical is, of course, that there's real limitations for
how bad things can get with your suck-back. First of all is the fact that
you've got much smaller volumes of gas to deal with and therefore you would get
very little suck-back actually coming back into the tube. And even if you do,
there aren't any nasty consequences, because the tube cools down so quickly
that even if water does get back in there, you tend not to actually see
anything too exciting happening. The other cool thing about the fact that you
are using a smaller scale apparatus is that it cools down really quickly,
allowing for tidy-up to be much more efficient, leaving you more meaningful
time for discussion at the end of the practical.
So that's it for cracking.
Next time we're going to use basically the same set-up to create some propene gas
and hydrogenate it in a demonstration so simple and effective you will not believe it.
In between now and then make sure you get some practice in using the
microscale set-up. Good luck and I'll see you in a couple of months for part 2.