today we're in a lab because I want to
tell you about a really smelly class of
compounds
you'll hear they're not the smelliest in
the world but they're pretty close
they've been in the news in the last few
days because these compounds which are
called macaque times or files were
subject to release from a factory in
France about a hundred kilometers also
from the French coast and the smell was
so strong that it's blown right across
southern England and everybody thought
they were gas leaks and there were
terrible smells these compounds have two
names for the same compound sometimes
people call them mcapples and I'll
explain why in a minute and chemists
usually call them thiols they all
suggests sulfur and these all these
compounds contain sulfur I've got a
model here the yellow is a sulfur and
this is a hydrogen and here's a carbon
atom and then you have other groups
attached onto here the characteristic of
these compounds is that nearly all files
smell dreadful even in very very low
concentrations over here I've actually
got a bottle with a little bit of a tile
in it this one is called 2 methyl
propane tile it is the compound that is
added to natural gas
so that people can smell it and smell a
gas leak because gas itself natural gas
doesn't smell so if you had the leak you
wouldn't know until you lit a match and
blew yourself up and then it'd be a bit
late I'm not going to open this bottle
because if I do the smell is so strong
that the air going up into the fume
cupboard will start people thinking that
there's a gas leak somewhere so you can
understand that you don't need to
release very much of this compound as
was done in France to start widespread
concern that there's gas leak in the
newspapers they said the number of
people telephoning the gas company to
say there was a leak went up more than
10 times compared to an ordinary day
instead of 8,000 calls they had a
hundred thousand calls in southern
England that there was a gas leak and
this was from a factory that had
released the file in France when anybody
in this building is using a file or
mercaptan an email goes around to
everybody warning them that there is a
stench reaction going on so that they
don't fall phone the gas board if they
smell something but they check up that
it isn't this particular experiment but
I must say that most of my colleagues
are very good and the smell very rarely
gets out I never knew why these
compounds were called macaque tans but
just as we were preparing to film this
sample professor moody whose lab this is
came up and explained to me that the
reason it's called maccaption comes from
the words mercury and capturing because
the captain's react quite strongly with
mercury and you could use it for
trapping mercury these compounds all
have rays from smells
and you find them in nature in very
small quantities but where they have
quite distinctive effect for example you
find a particular mercaptan which is
called grapefruit mercaptan in
grapefruits and this is why grapefruits
have their characteristic flavor or
taste ever since I read about this gas
leak I have been trying to contact my
friends and people who know a lot about
science and ask them why derma captains
and I all smell so dreadful and the
answer is that none of them know because
there are all sorts of different
theories about how the human nose detect
smell one theory is that it depends on
the shape of the molecule which fits
into a particular receptor in the your
nose and this is clearly true in some
cases because you can have left-handed
molecules that smell different from
right-handed molecules but dials have
lots of different shapes the only thing
they have in common is this sulfur
hydrogen bond so it's difficult to
imagine that they're all going to fit
into the same receptor in your nose
another theory is that it is due to the
frequency with which the atoms vibrate
in the molecule in every molecule the
atoms are not stationary but they
vibrate and each group sulfur hydrogen
or oxygen hydrogen has characteristic
region of the infrared spectrum where
they vibrate there is some evidence that
this can affect smell and in fact two
days ago a new paper was published in
which the author's showed that if you
took the same molecule with hydrogen
atoms or deuterium atoms that's heavy
hydrogen so they heavy hydrogen vibrates
at a different frequency from ordinary
hydrogen some people can tell the
difference in the smell
and the deuterium compounds smell more
had a bird smell we always have
difficulty in describing smells very
well but they said a bird smell I think
it's really quite unlikely that our
noses are so sensitive to tiles and
mcoppins
just because of the vibrational
frequency I think it's probably because
a lot of foodstuffs eggs cabbages and
things like that when they go rotten
give off hydrogen sulfide or sometimes
mercaptans
and so over the long period of thousands
hundreds of thousands years of evolution
it has been an advantage to be able to
detect these gases so that you don't eat
rotten food
but whatever the reason these compounds
are very very smelly
they're not the smelliest compounds
because the selenium and the logs that
Sulli with selenium hydrogen bombs and
tellurium hydrogen bombs are even worse
in this lab before it was refurbished
and new benches were put in there was a
small spillage of a selenium tile at a
weekend and the fire brigade had to come
in and the firemen were dressed looking
like spacemen just to remove this tiny
quantity of chemical and the whole
building had to be evacuated and there's
a story that in 1909 and the centenary
of Charles Darwin there was some chemist
working with selenium compounds in the
city and they released some of these
compounds and the smell was so bad that
garden parties all over the city of
Cambridge were disrupted and so Darwin's
centenary was not celebrated in quite
the way that people had wanted tiles are
very useful for making sulfur containing
organic
the sulfur hydrogen bond is not very
strong so it can break and you can make
carbon sulfur bonds very easily so files
are used quite a lot in the lab that's
why there is one here but if you're
using it properly in a lab it's quite
easy to destroy the smell because tiles
react with bleaches the sort of bleach
you use for cleaning bathrooms and so on
and the bleach oxidizes the sulfur
hydrogen to sulfur oxygen bonds and you
get a so-called sulphonic acid which
doesn't smell and is not volatile so it
just stays in your flask without
smelling
one tell you that Brady's starting a new
channel about psychology and how your
mind works
and mine but the exciting thing is from
my point of view that one of the people
appearing on this new channel is my
daughter Ellen so it's a real
opportunity for you to go to this
channel to see how her hair compares
with mine if you want to go and watch
these videos either click on the link on
the screen or if you're watching on your
mobile the link will be down below are
you looking forward to them I am looking
forward to it and as ever I have no idea
what's going to be in them so be quite
exciting to see Ellen speaking on screen