it's professor Dave let's talk about Fischer projections.
so we've talked about stereochemistry chirality we've talked about how to assign R/S by the
cahn-ingold-prelog convention we've covered that in previous tutorials
however I like to do that using regular line notation and while it's not my
favorite it's not the way that I like to do it I know that other introductory
organic chemistry courses often do this using Fischer projections and so I
wanted to talk a bit about that just for students who are learning
stereochemistry that way in their courses I have talked about Fischer
projections in my biochemistry tutorials because this notation is typically used
with linear sugar so linear carbohydrates or a simple sugars like
monosaccharides and so that is typically the application of this notation but for
whatever reason some courses like to use this to introduce stereochemistry so own
it to talk about this a little bit to make sure everybody understands what
this notation means so when we see Fischer projections where we're not
going to any longer see those wedges and dash bonds we're gonna see things flat
like this and so if we're still understanding that every vertex and end
point is a carbon atom so for example this is implied to be a carbon atom but
there's something about the stereochemistry of these bonds that is
also implied and what is implied by this notation is that any horizontal bonds
are implied to be wedge bonds and any vertical bonds are implied to be dash
bonds so the geometry of this right this implies this okay and so you can if you
forget you can remember that by you can see a little bowtie there's a little
bowtie on every carbon essentially is one way to remember that and so this
means this and this also looks a little strange we're used to including at least
one of the bonds in the plane of the board so having all four of these bonds
either projecting outwards or inwards is strange we can envision it this way
though imagine this is the molecule we're talking about an imagined urine
plane right here looking at it this way you would
see D and B coming towards you like this that's those and you would see A and C
going away like this so this is a this is another way of drawing this molecule
but written that way okay so that's just the necessary information we do need to
know horizontal implies wedge vertical implies - we're typically going to see
this with longer molecules like this so imagine something like this we've got
methyls here and then we've got a hydroxyl on one side or the other on
each of these three carbons so we've got carbon carbon carbon and again we have
to remember that this is not implying that this is a flat molecule these are
sp3 hybridized carbons those are tetrahedral centers and so
another way of thinking of this is when we have a longer one like this what can
get confusing is because these are all implied to be dash bonds it might seem
strange how can we have all these dash bonds in a row that might seem illogical
so one way you can think of it is sort of a horseshoe shaped thing where you're
almost going through a Rolodex you can imagine yourself riding this like a
rollercoaster and pulling each carbon up to you but notice that each successive
bond is going further and further away from you so this is this right if we're
looking at this we know that we've got if we're looking down upon this molecule
we are seeing this we've got OH to the right which would be the wedge we've got
OH to the left which would be the dash and OH to the right which would be
the wedge so we want to be able to visualize the the actual molecule we
want to be able to understand that this is not showing the accurate geometry of
the molecule and see it this way but sometimes we will be using this notation
for various purposes whether we're looking at sugars or non sugars just
regular molecules and so now that we understand what Fischer projection
notation implies now we can start to assign R/S on molecules in that
notation
okay so we're gonna want to be able to assign R and S on molecules written in
Fischer notation and so to do this we need the cahn-ingold-prelog convention
if you haven't seen my tutorial on that convention yet watch that one first
that explains the convention but if we're up to speed with that then we can
look at a molecule here and what we're gonna do it's not that different from
what we already know and the tricks that we talked about there so first we just
have to remember assigned priority based on atomic number right so we know that
the highest atomic number that's bromine so that'll be one two is chlorine so
that'll be two, F is next that'll be third and then hydrogen is
fourth so the key thing here we remember is that we need to have the lowest
priority group facing away from us right that's what we have to do so hydrogen
has to be away from us and the thing with Fischer projections is if the
lowest priority substituent is on a vertical bond that is implied to
be a dash bond so it is away from you so in this case right this hydrogen is on that
vertical bond so it is away from us in such a case we can just trace from one
to two to three exactly as is shown and we can get R and S from there so here if
we go from one to two to three we're gonna go like this
that's clockwise so this is R okay because the lowest priority group was
already away from us now that will be the case once again when the lowest
priority substituent is on a vertical bond because the vertical bond is
implied to be the dash bond now here's the case where the lowest priority group is
on a horizontal bond if the if the lowest priority group is on a horizontal
bond that is implied to be a wedge bond and that is not facing away from us so
in such a case we just do our regular trick that we talked about in the other
tutorial we can pretend as though two groups have been swapped although we
don't even have to swap them once again we're just going to assign this is
priority one this is priority two this is priority three now let's just trace
from one to two to three exactly as shown we're gonna go like
that so it looks like s however this was not the appropriate way to do it because
the H was facing towards us recall that anytime you swap two groups on a
stereocenter you're inverting the stereochemistry so
if we had swapped say hydrogen and fluorine then we would have had H on go
on the on the dash bond however in doing so we would invert the stereochemistry so
the way that we can account for that is we can simply pretend that hydrogen is
away from us assess it as is and then invert our answer so looks like s
therefore is R so this one is also R so once again when the lowest priority
group is on a vertical bond on a Fischer projection you can assess R and s as is
just trace from 1 2 to 3 and clockwise this our counterclockwise is s however
when the lowest prior priority substituent is on a horizontal bond that
is the case where we trace from 1 to 2 to 3 and then the easiest way to do it
is just do that and then invert your answer to the other option that will
account for the fact that the lowest priority group was actually towards us
rather than away from us so that's the easiest way to assign R and s to
stereocenters when molecules are drawn in Fischer projection form