it's professor Dave let's talk about different types of protons.
so in chemistry and in particular in the context of NMR spectroscopy we will
often hear these terms homotopic enantiotopic diastereotopic and hetero
topic and we want to understand what they mean. so first let's just go over
the definitions of these terms so let's start first with homotopic what we're
gonna do is let's say we have this molecule of ethane and if we take two
protons let's say we're examining these two protons here and we want to
understand the relationship between those two protons what we can do is we
can draw two new molecules and in each one we'll replace one of those two
protons with some other element let's say chlorine arbitrarily and so for one
of them we'll take the hydrogen on the wedge and we'll turn it into a chlorine
and for the other molecule we'll take the hydrogen on the dash and turn that into
a chlorine and what we're gonna do now is we're gonna examine the relationship
between these two molecules and what we can see is that this this carbon here
even with the chlorines there is not a chiral center because there are two
identical hydrogen's here right so these actually are the same molecule there are
actually just different confirmations of the same molecule we could take this
sigma bond here and we could rotate it some number of degrees to have it end up
looking like that other conformation so these are the same molecule and if we've
performed this and we end up with the same molecule then the relationship
between these two protons is homotopic there they have a homotopic relationship
and so that's the definition of homotopic. now let's go over here and
let's look at enantiotopic so what does enantiotopic mean and so let's perform
the exact same thing let's notice here that instead of ethane we're actually
looking at butane so we have these two methyl groups there instead of hydrogen
and we'll do the exact same thing we will compare these two hydrogens and on
the top one we'll turn the one on the wedge into chlorine on the
we'll turn the one on the dash into chlorine now let's compare the
relationship between these two molecules and actually in this case we are
generating a chiral center because there are four different groups we've got
methyl hydrogen chlorine and the rest of the molecule and in fact we are
generating the opposite stereochemistry at this center depending on which
hydrogen we turn into chlorine and so because these two molecules are
enantiomers of one another the relationship between these two protons
is an enantiotopic relationship so these are enantiomers
therefore these protons are enantiotopic. so as you can guess from the pattern
here probably we're getting to diastereotopic and so now with
diastereotopic let's notice the key difference here we've actually added a
hydroxyl group right there on the wedge of this carbon right here
and we're gonna do yet again the exact same thing we're gonna take these two
hydrogens and for one molecule we'll turn the one on the wedge into chlorine
and for the other one we'll take the one on the dash and turn that into chlorine
now this is a different situation than for enantiotopic because it is true that
we are generating a new chiral center and it is it is true that we are
generating the opposite stereochemistry at that new stereocenter depending on
which one we turn into chlorine but there is a pre-existing stereocenter
this carbon right here is also a stereocenter and that stereochemistry is not
affected it is not altered by this activity right here and so because this
stereocenter stays the same but we have a new stereocenter
that is opposite depending on which one we're looking at we are actually
generating two different diastereomers right these are not enantiomers of one
another because recall that diastereomers are molecules where one at
least one but not all of the stereocenters are inverted so this stereo
center is inverted but not this one so these are diastereomers of one another
and therefore these hydrogens have a diastereotopic relationship
and so that's these three. let's round it off here and the last one is
heterotopic and so heterotopic is a little bit different and so let's say
we've got this nitrobenzene and we'll arbitrarily look at these hydrogens
where we could be looking at any of them but we'll take a look at these two and
on this one let's take the bottom hydrogen right here and turn that into
fluorine again arbitrarily could be any other elements but we're just saying
we're gonna change this one into fluorine over here and then we're gonna
change this one into fluorine over here and now we're going to compare these two
molecules and what what we see is that these are no longer mere stereoisomers
of each other these are actually structural isomers of each other these
are completely different molecules because the the position of the fluorine
is in a different location and so this is actually gonna make these protons
that means these protons have a heterotopic relationship and so we've
got these molecules are the same homotopic these molecules are
enantiomers so these were enantiotopic these molecules are diastereomers so
these were diastereotopic and these are structural isomers of one another
so these hydrogens are heterotopic so hopefully that makes the definitions of
these words pretty clear before we move on let's just point out one other thing
because this is used often in the context of NMR spectroscopy one thing
that we want to understand about the chemical shifts of these kinds of
protons is that anytime you have homotopic
or enantiotopic protons these will all have they will all show the same
chemical shift okay so these protons and these protons will all show up on an NMR
spectrum with exactly the same chemical shift because of the the way that
they're experiencing exactly the same chemical environment on the other hand
diastereotopic and hetero topic protons will show up with a different chemical
shift so this is a very important under to understand because it's going to help
us interpret NMR spectra so this is not too difficult to see
because these hydrogen's are actually occupying different positions on the
benzene ring so that seems relatively intuitive how these could show up with a
slightly different chemical shift for diastereotopic it's a little bit harder
to see but just in terms of the spatial orientation of the groups they are
experiencing a slightly different chemical environment at any given time
so same chemical shift with when protons have this relationship and different
chemical shift when they have one of these two relationships so to put this
into context let's look at one example molecule and see what we can assess here
so let's say we have this interesting compound with a six membered ring and
then another three membered ring right there and so let's count up the
hydrogens we have two four six eight and nine we have nine protons here what
could we say about the relationships between any pair of these protons and so
obviously if we are examining protons a pair of protons on different carbons
those would all be heterotopic because in replacing those hydrogens by some
other element we would definitely be creating different structural isomers
but for any of these carbons for any of these pairs of protons let's say we took
these two or these two or these two or these two if we were to examine the
relationship between those pairs we would find that all of those pairs of
protons are diastereotopic so the reason is because of this existing chiral
center right there because we have a bromine right there anytime we try to
take one of these pairs of protons and replace one of them with some other
element so let's say we put a fluorine here versus a fluorine there we would be
generating a chiral center because of the other structure on this molecule
there are other groups there other functional groups and so if we took a
put a floor in there or a fluorine there we would be generating two different
diastereomers and we could say the same thing for these two or these two or
these two so in this case we can see that we have a lot of
diastereotopic relationships on this molecule and so from this example
hopefully now we have a little more confidence in being able to assign
one of these relationships to any pair of protons.