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Current time:0:00Total duration:9:45

Video transcript

the n plus one rule allows us to predict how many Peaks we would expect to see for a signal in an NMR spectrum so if we think about the signal for one proton if that proton has n neighboring protons we would expect to see n plus one Peaks on the NMR spectrum the n plus one rule only applies when the neighboring protons are chemically equivalent to each other so in the last video we looked at this molecule and we focused in on this proton right here in red and we said that this signal right here was due to the proton in red and we also talked about this proton right here in blue and this signals due to the proton in blue let's look at the proton in red all right so let's see how many neighboring protons we have so if you think about this carbon right and you think about the carbon next door so this carbon right here so this proton is a neighbor right so we have one neighbor so far we go over here to this carbon this next door carbon here there are no protons on that carbon so we have a total of only one neighboring proton for the red proton so one neighboring proton so n is equal to one so we're going to see one plus one Peaks so one plus one is equal to two we expect to see two peaks for the signal for the red proton alright so here's the signal and we see our two peaks so this is called a doublet so the signal for the red proton is split into two peaks because of the presence of the blue the neighboring blue proton alright let's do the same thing for the blue proton alright so we think about the signal for the blue proton how many neighboring protons do we have well if we go to the carbon next door so this carbon there are no protons on this carbon and on this carbon that we of course have one proton so only one neighbor so n is equal to one we're going to see n plus one Peaks so one plus one is equal to two we expect to see two peaks so we go over here to the signal for the blue proton and we see our two peaks so we get a doublet one neighbor gives us a doublet here what about the protons over here these protons in genta right so how many neighbors do we have for those protons go over here to the oxygen right no protons on that no neighbors so n is equal to zero n plus one Peaks so zero plus one is equal to one would expect only one peak for the signal for these three equivalent protons and of course this is the this is the signal right only one peak and we call this a singlet alright let's do let's do another one so we also saw this molecule in the last video alright so over here and let's look at our protons we expect one signal for the blue proton and we would expect one signal for these red protons here let's think about the red protons first so how many neighboring protons do we have alright so we go to the carbon next door and then we have one proton so one neighbor so n is equal to one so we expect one plus one Peaks so we expect two peaks so the signal for the red protons needs to have two peaks so this one right here so there's one peak and there's the second peak so we get a doublet alright what about the signal for the blue proton right so we get a signal for the blue proton how many neighboring protons do we have so we go to the carbon next door and we have one two neighbors so for the blue proton we have two neighbors and is equal to two expect n plus one Peaks so two plus one is equal to three so three peaks for this signal so this signal is right here and we get one two and three three peaks which is called a triplet alright let's go to the next one so let's look at this one right here okay so we have bromo ethane so let's first draw in the protons so on this carbon we have two protons and on this carbon we have three protons alright so let's start let's start with the signal let's start with the signal for these protons right here so these are equivalent would expect one signal for those protons how many neighbors do those protons have right we go to the carbon next door and we have one two we have two neighbors so n is equal to two would expect n plus one Peaks so two plus one is equal to three would expect a triplet for this signal and here is our triplet right one two and three peaks for that one next let's think about the signal for these two protons right so how many neighbors do those two protons have so we go to the carbon next door one two three neighbors so n is equal to three so three plus one is equal to four we would expect a signal with four peaks for the protons in blue and here's our signal with four peaks one two three and four so we call this a quartet alright so with four peak four Peaks we call it a quartet let's do another one let's look at this molecule right here alright and let's first draw in the protons alright so on this carbon there are three protons so here are the three protons on this carbon there are three protons and on this carbon there would only be one a proton so here's this one let's think about how many signals we would expect to see so not thinking about the n plus one rule or spin-spin splitting here so a signal for these protons these protons are in the same environment as these protons that's one signal for those and then we would expect a signal for this proton right here in a different environment alright let's think about let's think about the signal for the red protons first how many neighboring protons do the red protons have we go to the carbon next door right and this is the carbon next door for both of those red protons and there's one neighbor alright so one neighbor n is equal to 1 so 1 plus 1 is equal to 2 would expect a doublet for this signal and so that must be this alright so this is kind of like a zoom in and it's not the exact same drawing I hand drew all this stuff so it's not exactly perfect but you can see there are there are two peaks here two peaks so this is a zoom in this is supposed to represent a zoom in of those two peaks there so that's that signal what about the blue proton right so how many neighbors does the blue proton have all right so we look at this carbon and we look at the next door carbon so this is the carbon next door one two three this carbon is also next door to this carbon so one two three so a total of six so n is equal to six so expect n plus one Peaks so six plus one is equal to seven right so we would expect seven peaks call they say septet let me go ahead and rewrite that here so let me go ahead and write septet here and let's look at this signal it must be this right here alright and this is this is pretty hard to draw and once again this is a blow-up of what you're looking at so we'd expect seven peaks so one two three four five six and seven so that's that's the idea of the n plus one rule and let's talk a little bit more about spin-spin splitting and when to expect a signal so if you have if you have chemically equivalent protons they don't show spin-spin splitting alright so if I look at these two protons right here these two protons are in the same environment as these two protons right think about symmetry so we would expect only one signal we expect only one signal on the NMR spectrum and and these protons are not going to split these protons even though they're next door because they're chemically equivalent to each other so chemically equivalent protons do not show spin-spin splitting alright let's look at let's look at these examples right here so what we've been talking about is nextdoor protons right so this this proton and this proton right will split each other signal if they're in different environments so splitting is observed right we're talking about these next-door these next-door protons here but if you have if you have this situation so let's just once again make this the red proton and make this the blue proton right there's an extra carbon in between so splitting is generally not observed for this situation so the red and blue protons won't spoil each other generally again there are exceptions but for simple NMR spectra you really really have to think about you really think about the situation on the left here so these are just these protons are too far apart for them to feel any effect and then finally it is possible to have splitting from protons on the same carbon so if these are in different environments right so if this proton is in a different environment from this proton it's possible for splitting to be observed and we'll talk more about a possible talk more about that in the next video