Say for the sake of argument you find yourself stuck at home for an extended
period of time and you still wanted to be able to demonstrate some cool chemical
phenomena. Well, as it turns out of course the kitchen is absolutely jam-packed
full of all kinds of things that you can be showing off and so I decided this
month to narrow things down just to a very simple thing, associated with some
fruit. I've got a good stash of these around, I'm taking care of myself
and we're going to be looking at one of the chemicals present in the skin of
some citrus fruits. This is limonene.
So let's get into it.
So I've got the closest thing I could come up with for some molecular models
at home. I hope you'll excuse the liberties that I've taken with my bond
lengths, I don't think there's going to be a future for me as a balloon artist,
but we can but try. I've made some models here of a molecule called
isoprene and essentially what we have here that I'm trying to model out is two
double bonds joined together by a carbon-carbon single bond and coming out
at the end we have a CH3 group here, you'll excuse my little sort of hydrogen,
closest thing that I could come up with associated for this.
This is the building block for many molecules in natural products called the terpenes, so
it turns out that limonene, that smell that we associate with that
fresh orange peel, lemon peel etc, added to many cleaning agents, is an example of
a terpene, as also are many of the carotenoids that give oranges etc
their orange color. Also of course famously are carrots.
So there are many, many examples of terpenes that we can look out for and they all
come from these sort of isoprene building blocks. What do we have here?
Carbon-carbon single bond, double bond double bond, and here's most of the CH3.
Now, if I get two of these together we can kind of see how, potentially, we can
begin to form like a six membered ring here and coming off the top we would
therefore have a carbon-carbon double bond and a CH3
and then a CH3 coming off of the other end, and essentially that is the structure of
limonene. We get two of these isoprene molecules and we stick them together.
Now, when we look at that we see that we've got like a hydrocarbon there
essentially with a six membered ring in the middle and that's actually pretty
similar in terms of its structure to cyclohexane, so we might of course expect
similar kinds of properties from this molecule that we can find in the peel of
our orange. Now essentially what you want to be able to do is cut off a section of
peel, a little bit like this, and you want to get as much of the flavedo, fancy
name, there for the outer layer of the orange peel that's the sort of colored
section, and as little as possible of the actual flesh of the orange which of
course is going to be full of moisture. We want to maximize how much
actual limonene that we've got present in there. So here you can see quite a
good sort of section of peel that I've generated here. Now the only other thing
essentially you're going to need is a candle and what you want to do is heat
up a section of your orange peel over the candle flame to warm up those oil
sacs that are present in there and give it a squeeze and you'll immediately be
able to see that really cool effect that you're going to get from the
surprisingly flammable content of your orange peel.
Again, big smoke coming off the top, showing those
unsaturated molecules in the flame.
Now of course, any molecule that is similar in terms of structure to cyclohexane is
going to have the same kind of effect on our balloons as cyclohexane.
And in an earlier Exhibition Chemistry we saw how devastating cyclohexane can be
for the structure of balloons, because it turns out that balloons are
made of latex, which is itself made from isoprene building blocks. So any
combination of limonene with our balloons we might find that we're going
to get some rather surprising results. It's not as good in terms of its
reliability as cyclohexane, but if you play that up in the right direction the
tension that you can build from that as a demonstration opportunity can open up
some excitement for your class.
It tends not to be as dramatic as the cyclohexane,
that gives you this terrifying thing of 'Is it going to pop?
and if so, when will it pop?'
There we go.
So there you have it, simple demonstration
that you can show off with the surprising result of a compound that you can find
in the outer layer of an orange and its similarities to a balloon from which
we've modelled our isoprene. Give it a try, hopefully the next time I see you
we'll maybe be back in a lab, who knows. Take it easy.