hey guys I wanted to talk to you about chirality today so let's use the Nanome
building tool and we're gonna start with methane and we're gonna build a couple
molecules so let's take we're gonna take fluorine we're gonna replace two of
these hydrogens with fluorine right there want a piece and now chlorine
let's put a chlorine there and a chlorine there and bromine so a bromine
there and a bromine there and we'll leave the hydrogen as it is so let's
recall when we're talking about chirality we molecule is chiral when in
it you typically were talking about a chiral center so we're looking at these
here these are clearly mirror images as we'll set up in a moment here and and
this usually occurs when we have a central atom bonded to four different
groups so each of these carbon atoms has a hydrogen a fluorine
a chlorine and a bromine and now we're positioning these in such a way that we
can very clearly see that they are mirror images of one another okay so we
have we can mirror each atom over to the other molecule and we can see that they
are mirror images of each other and then a molecule is chiral when its mirror
image is not the same so let's let's rotate this other one here so that we
can line up the hydrogen the carbon and the chlorine right that's the white the
gray and the green we can see that those are gonna be able to line up perfectly
if we try to place this one over the other so let's put that there but what
we're finding out is that the fluorine and the bromine do not line up right so
we turned one around and by trying to overlap three atoms the other two just
can't do it so these are not the same molecule they are enantiomers of each
other but sometimes we can actually have a chiral molecule that has no chiral
centers so take a look at this molecule there are no chiral centers those
carbons that that are connected to the chlorines they don't have four different
groups right they have double bonds to another carbon so those are not really
considered chiral in the way that we typically talk about
chiral centers and yet this molecule is chiral and this has a lot to do with the
geometry of the p orbitals that are that are allowing for the pi bonds to occur
see how they are on they are on they're on perpendicular planes and so this is
this is called an allene it's a dichloroallene and this is an
example of a molecule that is chiral despite having no chiral centers so just
a few more looks at this here keep turning it around you could see if that
back carbon had the chlorine in the other direction we would have the other
isomer the other stereoisomer of this molecule so that is one example let's
put that away and now let's take a look at another one this is an interesting
feature of nanome here you can you can there are a number of databases put in
any pubchem ID or something like that or another database and we can get pretty
much any molecule we want here let's go and let's take that over to
ball-and-stick view I prefer the ball-and-stick view so this is called a
biphenyl and let's just modify this little here we're gonna change this
around let's make some hydroxyls and now let's make a couple of methyl
groups so let's take those hydrogen's down there let's swap them out let's we
have to go to change atoms so let's make a carbon and a carbon and now let's put
the hydrogens
so we're drawing this kind of however we can just place the items wherever we
like and then what we're gonna do is we're going to go over here and we can
minimize
and that will make the molecule adopt the the energetically favorable
conformation it will just become how it's supposed to be so there we go we
have minimized it and now that looks about right so this is a biphenyl right
we've got two phenyl rings here connected by a sigma bond as we can see
and we know that sigma bonds are typically going to be able to rotate and
so we wouldn't think that this would be a chiral molecule but the problem is
that we have this hydroxyl here in this hydroxyl here right and we also have
this methyl this methyl down here in this methyl down there and those are
very sterically hindered groups and that is going to prevent rotation along this
sigma bond so it's true that sigma bonds can rotate but it is the presence of
these substituents on the molecule that makes rotation impossible and it is kind
of locked into this conformation sort of like with the allene with the pi bonds
but here it's not it's not the pi bonds that are preventing rotation it's the
steric hindrance of these functional groups and so this is locked in to this
orientation and therefore this is a chiral molecule despite having no chiral
centers so let's that's that one let's put this away okay let's do this other
one so this is an example of a this is a spiroheptane this is a dichloro spiro
heptane so these are two four membered rings and they're connected they share a
carbene right so we talked about nomenclature of these spiral compounds
it's a heptane because there's seven carbons but then what ends up happening
again so we're seeing that it's it's about the rigidity of a molecule the
restriction of rotation because these share that carbon right there there is
no rotation there's no way for that chlorine atom and this other chlorine
atom to have any spatial orientation other than the one that is exhibited
here so for that reason this is a chiral molecule and we would have to move one
of those chlorines in order for that to be the other stereo isomer
okay let's do one more once again let's let's input this pubchem ID this is a
fascinating molecule called a hexahelicene so let's take this we can see that
it's a bunch of benzene rings in a particular orientation let's go to
ball-and-stick once again and so we can see that we might expect this to be
planar and if you're just looking at the line notation of the molecule you could
expect it to be planar but once again sterics are going to
dictate that it adopts this sort of corkscrew looking geometry because those
that write those the the edge most benzene rings would be right on top of
each other if this was a planar molecule and that's just not possible
energetically speaking and so we can look at this in a few other ways wire
view we can look at this in vander waals view which is the space-filling view so
we're looking at all of the electron clouds really taking up all the space
that they can and so this looks pretty interesting now let's go back to let's
go back to ball-and-stick okay and so this is a chiral molecule once again
despite having no chiral centers if we keep flipping this around and around we
we keep seeing the same it the corkscrew is winding the same way you see how the
the right side is always on the top like that and so if it was winding the other
way we would have the enantiomer but so that is an example of another chiral
molecule with no chiral centers and let's send this away and I hope you
enjoyed that little lesson about chirality.