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Current time:0:00Total duration:10:44

Video transcript

let's see how to determine the number of expected signals in an NMR spectrum and the best way to do this is just to do a lot of practice problems and we'll start with methane and methane has four protons in the same environment therefore we say those four protons are chemically equivalent and we would expect to see only one signal on our NMR spectrum so for methane we would expect to see one signal because all four protons are chemically equivalent if we move on to propane alright so this carbon right here has two protons on it and these two protons are the same environment therefore they are chemically equivalent and we would expect to see only one signal for these two protons if we look at these protons over here so let me go ahead and draw them in so these methyl protons these methyl protons are in their own environment here so we'd expect one signal for these methyl protons but for a molecule like this you need to think about symmetry so if I draw a line right here it's easy to see that these three protons are in the same environment as these alright so really really we have six protons in the same environment so these six protons are chemically equivalent and we would expect to see one signal for all six so for propane we would expect to see a total of two signals alright let's move on to this one all right so let's let's draw on our protons here so let's so let's just go ahead and draw on all of them first so we're drawing in all of the all of the protons alright so we have these methyl protons here and then and then right here we have a proton on this carbon and then we have all of these protons so a lot of them let's focus in on the methyl protons first right so these methyl protons here are in their own environment and that environment is the same as all of these methyl protons they're all in the exact same environment they're all right next to a carbon with a hydrogen on it and so we would expect to see only one signal for all these protons so one signal for the magenta protons alright so again think about symmetry think about the symmetry of this and then if I look at this proton right here right this is in a different environment from the magenta protons but it's in the same environment as this proton so we'd expect one signal for these red protons here so one signal for the red protons so a total of a total of two signals for the molecule alright let's do let's do some more examples so let's look at let's look at this one next alright so we have we have a carbon right here with two protons on it and these two protons are in there are the same environment they're chemically equivalent these two protons are next to are next to this oxygen here we know oxygen is more electronegative than carbon so oxygen is going to withdraw some electron density and so these two protons are in a different environment from these two protons these two protons right here let me go ahead and change colors these protons in red are further away from the oxygen so they're in a different environments would expect one signal from the protons in red and we would expect one signal from the protons in yellow all right we still have our methyl protons so let me go ahead and draw those in so we have three protons over here on this carbon they are further away from the oxygen they're in their own environment right they're chemically equivalent so one signal expected for these protons finally don't forget about the proton right here the proton on the oxygen so that's in its own environment so we would expect one signal for that proton so really a total alright if you count them all up a total of four signals right so we would expect four signals not sure why I put an S on here only one signal so total of four for this alcohol let's look at this alcohol down here so we have these two protons right and these protons over here and these protons over here so let's think about that so symmetry helps for an example like this these two protons right are on the same environment they're between two CH 2s and so we'd expect one signal null for these protons so they're chemically equivalent let's look at let's look at these protons next so these right here right are next to a ch2 they're also next to an H that's the exact same environment as these protons right there next to a ch2 and next to an O H so if you think about symmetry right here we'd expect only one signal only one signal for these four protons and then finally we still have the we had the proton on the oxygen and once again symmetry helps us think about the fact that this proton is in the same environment as this proton so we would expect would expect one signal there so we would a total of three expected signals for for this molecule all right let's move on to this compound over here on the right and we can see that there's a chiral Center right here on this carbon and they're therefore must be a hydrogen a proton on that carbon so let's go ahead and draw in all of our protons and then let's analyze them here so we have three protons on this carbon on this carbon right here we have two protons and I'm drawing them in with a wedge and a dash because we're going to need to think about those two protons and then over here right we have we have three like that alright so let's think about how many different signals we're going to get alright so in the past let me go back and use a different color here we talked about a ch2 group right and we didn't have any chiral centers like these two protons right here we didn't have any chiral centers in this molecule these two protons are generally equivalent alright but if I if I think right over here I have a chiral Center at this carbon and that's going to affect these two protons these two protons are no longer chemically equivalent right because a chiral Center is present so in general if a chiral Center is president is present the two protons on a methylene group on a ch2 group are generally not equivalent so we would expect two different signals so this one might give us one signal I'll put that in magenta so one signal here and this one would expect a different signal there in slightly different environments because of this chiral Center that is present all right so let's keep on going and let's figure out how many more signals we would expect so over here right we have these three protons are chemically equivalent and they're in their own unique environment so we would expect one signal for those three protons all right and those are in a different environment from from these over here so we would expect one signal for these so for a methyl group so methyl protons are always in the same environment so they're chemically equivalent so you don't have to worry about them all right and then let's see what else do we have here we have we have the the proton on the oxygen so we would expect one signal for that and then finally finally we're going to have let's see what color should I choose here let's go with orange we still have a proton right here in its own unique environment so you would expect one signal for that proton so how many total signals do we expect so let's count them up so one two three four five and six so a total of six expected signals for this alcohol all right let's move on to let's move on to a benzene ring here so let's look at benzene and we know that benzene has six protons right so I could draw in I could draw in the six protons here and those six protons are all in the exact same environment they're all chemically equivalent and sometimes it's easier to to just go ahead and draw your circle in here all right and that just helps you to see that these protons are equivalent allows you to see the symmetry a little bit better all right so again thinking about symmetry all six protons are the same environment therefore we would expect only one signal for benzene on an NMR spectrum so let me go ahead and write that here so one only one signal for benzene because all six protons are chemically equivalent move over here to to this molecule down here all right so let's think about what we have so we have a methyl group here a methyl group here so that's three protons right still three protons for each methyl group and those protons are same environment alright so these three protons in this methyl group on the same environment as these three protons right there right next to an oxygen which is next to this benzene ring so next to an oxygen next to a benzene ring and so we'd expect only one signal right only one signal for these six protons right here if you look at our ring if we look at our ring we know that there's a proton here we know there's a proton here a proton here and a proton here and if we think about those protons that are all in the exact same environment right so if I think about let's say this proton this proton is next to a CH right with a double bond it's next to it's next to a carbon right here with an oxygen this is in the same environment as this one right this one's in the exact same environment this one is this one is so again symmetry helps you realize that all four protons are in the same environment they're chemically equivalent therefore one signal so we would expect to see two signals we expect to see two signals for this molecule on an NMR spectrum finally let's just for fun let's just look at cubane here right so one of the one of the most interesting molecules in my opinion there are eight protons on cubane cubane is just this this cube here and those eight protons are all equivalent alright so if you think about if you just rotated this cube right you wouldn't be at all a difference here so we have eight protons all equivalent therefore only one signal so Q Bains NMR should have only one signal on it all right so that's that's hopefully some insight into into how to look at a dot structure and how to figure out how many signals you would expect to see on your NMR spectrum