explains hey guys I wanted to work through some more stuff on ChemDoodle
today specifically I wanted to go through some problems involving proton
NMR spectra and in doing so I'll show you some cool features on here most
notably we can generate NMR spectra for any molecules so this will be very
useful for let's say assigning peaks to a spectrum from a molecule so we can
take some example like this any molecule you want and will go right here to
spectrum and we will notice we can do either proton or C-13 right now let's do
proton NMR and so this is the spectrum this is the proton NMR spectrum that
corresponds to this molecule that we've drawn right here and so this is very
useful because now we can practice this with any molecule we want you don't have
to rely on my examples you can put in any of the molecules from your problem
sets from your quizzes or exams or whatever and and you can practice here
you can generate the spectrum and then you can assign the peaks so just as a
warm-up right here before we do some trickier stuff let's just go ahead and
assign these protons to their to the peaks on the spectrum so let's take a
look over here we've got this we've got this guy right there we would expect
that to be integrated to 3 and we would expect that to be a singlet because
there is no neighboring hydrogen's so that is uh that is that is one peak we
would expect over here we've got our carboxyl proton and that's certainly
going to be integrated to one and it's certainly going to be a singlet so just
looking at some of the integration and splitting behavior first then over here
we've got these two now these two are on the same carbon and usually when we have
protons on the same carbon they're chemically equivalent here it's a little
bit different so let's go ahead and assign these peaks first then we'll talk
about these alkane protons so we have got this methyl group right here that is
going to be integrated to 3 it's going to be a singlet and we would expect it
to be relatively up yield because alkane old protons usually
are a bit upfield so if we're gonna call that a let's understand that this right
here that's gotta be a right there right this is definitely the most upfield
it is a singlet it is a very tall peak because it's integrated to three so that
makes perfect sense we're gonna assign that one right there
likewise the carboxyl proton is very easy to assign because we know that it
is always going to be very very downfield it's gonna be passed to the
aromatic region and there it is right here at past 11 almost 12 so if we call
that one B then that has got to be B right here so we have got this methyl assigned
and we've got this carboxyl proton assigned so this is a little bit new in
my previous NMR tutorial we didn't really fully talk about this but this is
this is an important thing to understand because once again normally the protons
all that are on the same carbon are chemically equivalent but it's going to
be a little bit different for alkene protons and the reason is this when you
have Sigma bonds Sigma bonds result from the direct overlap of orbitals and so
there is free rotation on that inter nuclear axis and so the groups are
spinning and if the protons are spinning around then if they're not seeing if
they're seeing part of the molecule at one moment then they're seeing another
one at another moment and so all three or two or having many protons they're
gonna as they as they flip around see the same environment but when we have a
pi bond like this recall that the pi bond we've got a
sigma bond but then we have a pot we have a pi bond which is the result of
the lateral overlap of those unhybridized p orbitals so those are
locked into place there is no rotation about this pi bond so what that means is
that these protons are locked into a specific chemical environment so this
proton is necessarily closer to that methyl group and this proton is
necessarily closer to that carboxyl group so they are
to experience a different chemical shift because they are experiencing a
different chemical environment or they're gonna exhibit a different
chemical shift rather and so what we want to understand is that one of these
peaks here corresponds to one of these protons than the other corresponds to
the other so which is which well this one is closer to the methyl group which
is less deshielding this one is closer to the carboxyl group which is more
deshielding because it's got those oxygens in there so if we call this one
C we would expect C to be more downfield so that's C right there and then let's
go ahead and call this D and then D would be the one that is a little bit
more upfield and we can see there's a little bit of splitting going on they
are likely splitting each other but in terms of chemical shift that's where
they're at and this does make sense in terms of a range for alkyne protons and
we're just understanding that the discrepancy in their chemical shift is
due to the difference in the chemical environment what they are seeing on the
other side of that double bond so that was a quick warm-up we just saw how easy
it is to take any molecule and on chem doodle generate the NMR spectrum and
then we can do any kind of assignment or analysis from there so let's go and do
some other examples now okay let's try one where we're given a structure and we
have to predict the spectrum we have to figure out what the spectrum is going to
look like so let's go ahead here and take a look at how many resonance we
might expect let's find all the chemically equivalent protons so over
here we've got a CH3 so we're gonna expect that to integrate to 3 and what
is the splitting going to be we know that we've got two hydrogens next door
right here so these two hydrogen's are going to split this signal into a
triplet alright because of the n plus 1 rule so to next-door this should be a
triplet right here we've got something that should be integrated to 2 and what
is what is the splitting pattern gonna be so we've got 3 right here we've got 2
right here that's a total of 5 so this peak should be split into
smaller peaks which is starting to get pretty messy so we can even just call
that a multiplet you could call it a sextuplet but it might we sometimes over
five we just sort of call it a multiplet then once again over here we've got
these are these are two protons so we are gonna expect that to be integrated
to two and we've got two protons next door right there two protons next door
right here that's a total of four so we should expect this resonance to be split
into five peaks again that's we maybe we can just call it a multiplet it'll look
you know they're little be split quite a bit and then right here we've got
another peak we've got two H's we've got two chemically equivalent protons right
here and we've got only two next door there are no protons next door on this
carbon this carbon has its other bonds to oxygen not to hydrogen so there's two
next door that will split it into three we're gonna expect this to be a triplet
and then lastly we've got our carboxyl proton that has got to show up as a
singlet and it'll be integrated to one of course there's only one of them so
we've got some information here about integration we've got some information
some predictions we've made about splitting patterns in terms of chemical
shift again we should we're talking about the proximity of these protons to
this oxygen containing functional group so the further away the protons are the
more upfield they're going to be and so as we go down this molecule closer to
this functional group the the resonances should get further and further downfield
so as we go this way we should be going down field we're gonna start over here
this is pretty far away from anything that would be deshielding so this should
be close to around 1 ppm and then as we get a little closer we're gonna get a
little up to 2 maybe a little past 2 ppm so you know maybe around 1
to 2, 2.5 maybe some somewhere in that region right because we're not very
deshields it maybe this one a little bit more but this this is all pretty upfield
then over here this guy the carboxyl proton we know that that's gonna be very
far downfield usually around 11 something like that so this is this is
giving us a very clear picture of what this structure what this or what this
spectrum should look like so let's take a look at our let's generate our NMR
spectrum let's go to generate and then generate our proton NMR spectrum and so
that's what this thing is gonna look like ok so I went ahead and put that
down here so we can see all of our data and we can see that we were really spot
on here we do have the most upfield the most
upfield peak is this triplet we've got a triplet and that does look to be
integrated a bit higher than the other Peaks because it does correspond to
three protons so there's our triplet we can see those three smaller peaks and
next we do have this wee a slightly more downfield downfield we have this
multiplet right there and we can even count the six peaks one two three four
five six there are all six peaks that correspond to the splitting pattern for
this resonance then a little more downfield we have this other multiplet
or we could call it a quintuplet so there's that we've got this 2H right it
seems to be integrated to about the same height as this other one one two three
four five smaller peaks and then we do have this triplet we have this last
resonance for the aliphatic protons and so that is up to two and it is a triplet
and we do see that everything falls pretty neatly into that region that we
expected a little under one two a little over two that makes a lot of sense what
in terms of corresponding with these types of protons and then over here we
do have our carboxylate on that is way up there past 11 as we expected
and so that is clearly distinct from the rest of this data here but so that's
what we've done we took this structure and we predicted the NMR spectrum we
predicted the integration we predicted the splitting patterns and we predicted
chemical shift and we saw that it matched the generated proton spectrum
very nicely okay so let's try one here where we are going from the spectrum to
the structure so this is a different type of question that we may have to
answer and so for something like this let's say we are given this spectrum and
we have this data and we're also given a molecular formula so let's say that we
know we the molecule is C402H8 so there's four carbons this data is going
to tell us about these eight protons but then we have to figure out what the
molecule looks like now with these oxygens it can be kind of tricky because
we have to figure out are those two separate hydroxyl groups are those two
separate carbonyls are they part of the same functional group are they part of
an ester or carboxylic acid and if we had some IR data that might tell us more
about what kinds of functional groups those oxygens can be found in so without
that it's a little bit trickier but this is a pretty simple molecule so we should
still be able to do it we'll just sort of go fragment by fragment we'll look at
each piece and and figure out what that is telling us about what must be present
on the molecule so let's start over here we have a triplet C this is a triplet
and let's say it's integrated to three so the thing is that almost always when
we have a signal that is integrated to three that's got to be a methyl group so
it's it's very difficult to find three chemically equivalent protons that are
not part of a methyl group so usually we can just go ahead and bet that this
means there's a methyl group somewhere and it is a triplet so it must be
adjacent to a carbon with two protons on it so that's something that we want to
be aware of over here we have we have another we have another
residents that disintegrated to 3 so this has got to be another methyl group so
we've got two methyls the only difference here is that this one is a
singlet so it is next door to a carbon with no hydrogens hydrogen's on it so
let's start - let's start to fill this in so we've got a methyl over there
we've got a methyl over here and we've got some stuff in between we don't
really know what's in between yet but we've got a methyl and we've got a
methyl over here this is this is integrated - - and it's a quartet so
that means we'd probably have a CH2 right usually when something is
integrated - - it means that those are two protons on the same carbon so we
have a CH2 somewhere and now the thing is this is a quartet so you can clearly
see the four peaks right here so if it's a quartet it means that it is probably
going to be next to a CH3 or it could it could be a slightly different scenario
but here we have two methyl so that makes a lot of sense for this CH2 to be
adjacent to a methyl so let's just arbitrarily pick one of these let's say
we put a carbon here and we do that so this is already starting to make some
sense now we're missing one carbon right we have three we have three carbons
placed and we have all of the hydrogen's placed but so we're missing one carbon
and two oxygens so given that this methyl remember this methyl represents
this singlet right here and the singlet means that it must be next to a carbon
with no hydrogens on it right so there's because there's no splitting at
all there are no neighboring hydrogens and that actually makes a lot of sense
here because if we have two oxygens then we can go ahead and place that final
carbon right here now how do we get away with having no hydrogen's on this carbon
well we've got two oxygen atoms so let's put the
best of the bonds to this carbon going from this carbon going to oxygen so what
if we make a carbonyl and then an oxygen there now we've got an ester right so
this works perfectly we've got this guy is corresponding to this one all right
that's gonna be this methyl over here and then we've got these two
corresponding to this one and then we've got this methyl corresponding to this as
well and all of the all of the chemical shifts also makes sense because we've
got this this methyl is clearly going to be the the most upfield because it's the
furthest away from any oxygen containing functional groups and and so there there
we see that it is almost up to one there and it is a triplet because it is next
to a carbon with two hydrogen's on it and then over here we've got a little
bit more upfield this methyl is next to this ester group and it is a singlet and
then this one is going to be the most downfield because it is right next to
this oxygen and and it's a quartet because it is next to this methyl group
so we've got three hydrogens splitting these two protons into a quartet so that
is our that is our guess here and that ends up working out pretty well so
that's that example so as you can see this is a tremendously useful utility
you can take any molecule can be very complicated whatever it is and you can
go ahead and get that spectrum you can generate that spectrum and you can you
can you can just make up any molecule if you're thinking what what would a
spectrum look like if it had this functional group next to this functional
group you can think of whatever you want and and you'll see what that spectrum
would look like so we just use this utility to assign peaks we did one to
predict a spectrum and we did one where we use the spectrum to predict the
molecular structure so there's a lot of ways to use this once again if you want
to check out chem doodle and use this feature or any of the features that I've
shown you so far check out the link in the description
it's very affordable perfect for students and so I hope you'll check it
out, until next time.