hey guys so for this practice problem we have a few molecules and each has a pair
of protons circled and what we need to do is determine the relationship of
these protons we want to say whether they are homotopic enantiotopic
diastereotopic or hetero topic and so if you didn't see my tutorial on these
terms and their definitions go ahead and check that out now otherwise see what you can do.
so let's take a look at this example first now we remember when we're
trying to assess the relationship between two protons what we do is we
redraw the molecule in two ways and in each one of those two we swap one of
those hydrogens with some other element and so we can take this one and if we
imagine drawing this molecule again but with one of these hydrogens as let's
say chlorine now what we've done is we've definitely generated a new chiral
center right and if we draw the other version we can see that we would be
generating the opposite stereochemistry at that chiral center however there is
another chiral center over here which maybe wasn't a chiral center before but
because of the chlorine here this is now also a chiral center and the chirality
does not change in between you know between these two molecules so these
clearly have a diastereo... these are definitely two diastereomers
so these have a diastereotopic relationship. so we are gonna say that
these protons are diastereotopic. okay so because these two are diastereomers
they're diastereotopic now we're going to go over here and we
are going to say that for the same situation let's draw we can even
just draw one of these and we'll see what's going on let's say we replace the
one on the wedge with let's say chlorine now what will immediately notice is that
we actually have not generated a chiral center right because of the symmetry of
the molecule right there these are not two different directions so all we have
to do is flip this on the other side and we would get the other situation right
these are actually the same molecule we can just take this one flip it on the
other side and we would get that and so because these are the same molecule
these protons are homotopic right same molecule those protons are homotopic now
let's look at this one we have this situation group where we have a benzene
ring we have a bromine we have bromobenzene and we're looking at these
two hydrogens so let's draw what we would get so we have the bromine here
and that's very important because let's say we change this to fluorine right or
we can change the other one to fluorine or something else doesn't matter what it
would be now these are clearly different structural isomers because these two
have a meta relationship whereas over here they have a pair of relationship so
those are those are two they are structurally different so these clearly
have a heterotopic relationship and then lastly over here we've got this
situation now notice that there is a pi bond here that is very structurally
significant because without this pi bond we would just have this situation again
and certainly they'd be homotopic but because of that pi bond we are actually
generating a chiral center right let's say we put a fluorine there
versus a fluorine here these are actually not the same molecule in the
same way that those were because what we could do is we could take this molecule
and we could flip it over and the fluorines would line up but the pi bond
would be on the other side of the molecule so we are generating a chiral
center so we have one chiral center and it's the opposite stereochemistry at
that one chiral center so these are certainly enantiomers of
one another and for that reason these protons have an enantiotopic
relationship so diastereotopic homotopic hetero topic and enantiotopic.