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Video transcript

for this NMR the molecular formula is c9 H 10 Oh let's go ahead and calculate the hydrogen deficiency index so if we have nine carbons the maximum number of hydrogen's we can have is two times nine plus two and two times nine plus two is equal to twenty so for nine carbons 20 hydrogen's is the maximum number here we have only ten hydrogen's so we are missing 10 hydrogen's or we're missing five pairs of hydrogen's therefore the hydrogen deficiency index is equal to 5 with an HDI of 4 or higher you think benzene rings so I'm going to go ahead and draw a benzene ring in here because I'm pretty sure there's one in our molecule and if we look down here for this very complicated looking signal right it's really a bunch of overlapping signals from protons on the benzene ring I know that because we're in the aromatic proton region for our NMR all right so right in here so I have five aromatic protons let me go ahead and draw them in so I put my five aromatic protons in slightly different environments giving us overlapping signals which give us this complicated looking one down here alright that takes care of an HDI of four but we would H to have five so we have one more thing and it's of course going to be a double bond and I know that because of this signal down here between nine and ten remember a signal between nine and ten that's the region for an aldehyde a proton so we have we have an aldehyde proton over here so I draw in my carbonyl I draw in my hydrogen and then this is a this is another piece of the puzzle here so this aldehyde is connected to something alright let's go to these other two signals down here so this this signal represents two protons so I'm going to write a ch2 here how many neighboring protons do we have for those two ch2 protons well there's one two three Peaks so if there's three peaks just subtract one to figure out how many neighboring protons you have so three minus one is equal to two so this is a ch2 with two neighbors it's the exact same thing for this signal right so this represents two protons this be a ch2 and once again we have three peaks 1 2 3 so 3 minus 1 is 2 so this is a ch2 with 2 neighbors and so these 2 CH 2's must be right next to each other right so let me draw that out here so if we have if we have 1 ch2 next to another ch2 each of those ch2 protons have 2 neighbors for example if I think about these right here right how many neighboring protons well this is the carbon next door and I have two neighboring protons so that makes sense when we look at the signal on the NMR there's only one way to put together these different pieces of the puzzle all right so we would have to put we'd have to put a ch2 coming off this coming off this place on our benzene ring and then another ch2 and then finally our aldehyde so go ahead and draw in our aldehyde like that and so that must be the structure of our molecule so a little bit about this aldehyde proton let me go ahead and highlight it over here so this aldehyde proton right here are right here we only see a singlet on the NMR spectrum but it does have two neighbors right so let me go ahead and draw in the neighboring proton so there on this carbon right here so if we're thinking about the aldehyde proton right this is the this is the carbon next door and so we have two neighboring protons and with two neighbors you might think we would get some splitting for the signal for this aldehyde proton here but we don't notice any on the NMR and normally don't see any splitting because the coupling constant is usually very small and so therefore the signal looks like often it looks like it's a singlet but sometimes if you zoom in you can observe some splitting for the aldehyde proton for this in tomorrow we have a signal for two protons so that must be a ch2 and how many neighboring protons well for this signal we have four peaks 1 2 3 4 so 4 minus 1 is 3 so 3 neighboring protons for these 2 ch2 protons what about the chemical shift for this signal right so the chemical shift is getting close to 4 parts per million and in that range that makes us think about those protons being bonded to a carbon that's bonded to an electronegative atom and if we look at our molecular formula the only electronegative atom we see on here is oxygen so that oxygen must be bonded to this carbon so let's go ahead and draw that so that oxygen is bonded to that carbon and that carbon is bonded to two hydrogen's so that's what we have so far so the oxygen right the oxygen is more electronegative than carbon the oxygen is withdrawing some electron density from these two protons and give us giving us a higher value for the chemical shift all right so the signal for these two protons in magenta is right here next let's look at this signal so we have three protons so a ch3 how many neighboring protons well for our signal we see one two three Peaks so three minus one is two so two neighboring protons and so two neighboring protons must be the ones in magenta right so we can go ahead and put our methyl group on here and let's use red for the methyl protons so these three methyl protons are giving us this signal all right we predicted two neighbors and those are those neighboring protons are the ones in magenta right here so those are the two neighbors for the ones in magenta right we predicted three neighbors and so that's one two and three finally let's look at the last signal right so we only have one more proton to think about there's only one place to put it right must go on the oxygen so this represents this is the NMR spectrum for an alcohol for ethanol all right so this this proton in blue is this signal on the NMR spectrum and the chemical shift is hard to predict for an alchoholic proton right usually you see two to five parts per million but it's really hard to predict exactly where this signal is going to appear and also let's uh let's think about how many neighboring protons this proton in blue has right so the carbon next door has two neighbors so you would think you'd expect two plus one Peaks right if n is equal to two two plus one gives us three so we'd expect a triplet for this signal but we only see a singlet and that's because this proton this alcoholic proton rapidly passes from one molecule to another and this proton transfer is so fast that the proton never stays in place long enough to interact with these neighboring protons and so the NMR machine usually doesn't show any splitting under the right conditions it is possible for for splitting of the alcoholic proton to occur and you might see a triplet here but on most NMR's you're only going to see a singlet which is which is another clue on your NMR spectrum