Okay, so if anyone's seen the the boron
video out there, um you guys know that I
love boron. Um I also put uh boron
trihalide on the Christmas tree at
Christmas. Um and this is because boron
is is still an element that's very close
to my heart uh because that's what I did
u my PhD on and I also did a postto. So
I was researching boron for a long long
time and I'm still very interested in
the chemistry of boron.
I've never seen boron tri bromide though
I lecture about it and about molecules
like it of b bro bro bro bro bro bro bro
bro bro bro bro bro bro bro bro bro bro
bro bro bro b b b b b b b b b b b b b b
b b b b b b b b b b b b b b b b b b b b
b b b b b bon with three chlorines or
florines or
broines. So I thought I'd show you the
way I teach about it. I have this model.
So it's boron with three broines on it.
Allegedly boron tri bromide is a
colorless liquid. It's supposed to be
when it's pure. So this is our boron tri
bromite.
Tada. As you can see, it's orange. You
can see it's a triangle of metal. And
the idea is that this is a model of the
molecule with boron in the middle. And
then I can take a pen and draw on the
broines
here, here, and
here. You put the boron in the middle
and you can draw the bonds.
So, it's it's actually light sensitive.
So, we're storing it under um under foil
usually if it's if it's out in the
light. Um it's a little bit light
sensitive. So, what we're getting is a
little bit of bor bromine formed in our
boron triromides. Um this could also be
left over. You buy it and it is orange
as well actually. So, it's probably a
little bit of bromine left over from
when it was synthesized in the first
place by the chemical company. What I'm
hopefully going to show you today, it
usually does it anyway. It usually
reacts very uh quite quite quickly with
air and the moisture in the air. Well,
mostly the moisture um is that boron
will boron triromide will fume. Okay. Um
it usually blocks up our syringes and
stuff. So I'm hoping it's going to fume
today. So the reason I have this is to
show um my students that when I rotate
the molecule round, it looks the same.
So the molecule is just like a flat
triangle with three bonds and the boron
in the middle. And if you look at its
edge on, it's completely flat.
Well, there's a little knob here. You'll
notice that we've got this fancy fancy
getup going again. So, we have what is
called a shake line setup. This is
because we can't have boron in in in
oxygen, boron in in especially in wet
oxygen because it'll react with it. And
we want to keep our our compound as nice
and pure as possible so my students can
use it for their
reactions. So, if I put a bit of argon
on
hopefully and find out which line this
actually comes from. But the knob here
is quite useful because it also
indicates the students that the boron
doesn't have the maximum amount of
number of electrons around it. It could
do. So there's a possibility of some
molecule that has extra electrons like
ammonia coming in and attacking and
giving some of its electrons to there.
And when it does
happen, you get a molecule like this one
where here you have the boron which has
had two electrons given to it by the
ammonia. This is a syringe. Obviously,
boron tri is a liquid. It actually melts
at I think it's about - 46°. Um, so at
room temperature, it's a liquid. So, we
can actually syringe it around. I'm
going to put this super seal in the top.
Um, just so we don't let any air go in.
And then I'm going to syringe out a
little bit and just put it in this watch
glass. and hopefully it will do some
fuming. We're now opening up our
tap. Okay, so actually if you can see
carefully, you can see the fumes coming
out of the top
already. So this donates electrons and
the boron accepts electrons. It's a
so-called Lis acid. That's a molecule
that accepts a pair of electrons.
And you can see when it pairs accepts
these electrons, the bromines are forced
downwards. So instead of being flat now,
it's shaped like a
tetrahedrin. That's a ball with four um
equally spaced groups around it. Hope
that top is going to stay on. So what
we'll do is we'll flush this out with
argon a few times. So you've got argon
gas going through this. Okay. So argon
is an inert gas. It doesn't react with
anything. It's not not a room
temperature
anyway. So, if we flush it out with a
bit of argon, what I'm going to do is
just take a little bit cuz we don't need
much of this. Don't want to waste too
much of it cuz once it's in the air,
it's useless. So, I'm hoping that some
of it will come out. There's a little
bit of it. Lewis, after whom the Lewis
acids were named, was a very interesting
chemist. He died in the late 1940s,
about the year I was born. He was found
dead in his
lab, possibly with his mouth smelling of
cyanide. He may have been poisoned.
Nobody's ever quite
sure. But he was a very influential
chemist. He worked in Berkeley in
California. And he was the person that
defined what a chemical bond was in the
modern sense. That is two electrons
being shared by two atoms. And so every
time I think of bor boron tri bromide
and similar molecules lis acids I think
of Lewis and what he gave
chemistry. Pull this out of here. It's a
big thick needle. So hopefully that
won't
get bunged up. That went everywhere.
Then let's pull that fume hood down. So
you can see I splashed a bit there and
it's all there and you can actually see
it reacting with the supercal. Okay,
this is a really really reactive
compound. So it's it's everywhere. I'll
put that in there. Okay, so what's
actually happening here is that the
boron triromite is reacting with the
water in the air. Okay, it's mostly the
the moisture in the air that it's
reacting with. Actually, it's pretty
reactive. What we're getting off is HBR
gas. So it's hydrogen broomemide gas.
Okay. And um what you'll probably see is
a white residue of some sort of boric
oxide left behind. Um but HBR gas, this
is why um uh boron triromides is is is a
toxic chemical. Um is because HBR gas
what it gives off as well is also toxic.
Um so it's not this is why we're using a
fume cupboard because it's not not a
very good thing to breathe in at all. I
lecture about boron tri bromide and
similar ones because of their shape
because of the way they behave and their
simple
reactions. Chemists like Debbie who make
boron compounds use this as a useful
starting material. They can react the
comp the broomemide. One of the bromines
comes off and they put on some other
group and make more and more complicated
molecules. So it's like a starting
building block which they can make more
complicated. But I don't study boron
chemistry. So to me it's just a triangle
of aluminium.