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Current time:0:00Total duration:11:02

Electronegativity and chemical shift

Video transcript

if you look at methane we have four equivalent protons so we would expect one signal on an NMR spectrum and here's the signal for the protons on methane so this signal occurs at approximately one part per million and remember from the first few videos on proton NMR what that signal is talking about it's talking about the energy difference between the Alpha between the Alpha and the beta spin state so this is the alpha spin state and this is the beta spin state there's an energy difference between those two spin states right and this energy difference corresponds to a frequency because E is equal to H nu and the energy difference also corresponds to the effective magnetic field felt by a proton so if I draw in a magnetic field here and so the effective magnetic field right controls the energy difference so let's think about this if I have a certain effective magnetic field I get a certain difference in energy between the Alpha and the beta spin States the energy corresponds to a frequency that's absorbed and so this this signal is the is a certain frequency right we said before this was a lower frequency this is a lower frequency signal right here so this is a lower frequency signal and in earlier video we talked about how to compare frequency to chemical shift so a low frequency gives a low chemical shift so one is a low chemical shift here and so the protons in methane are shielded compared to the protons in chloromethane so let's look at chloromethane next down here so for chloromethane we have three equivalent protons so one signal on our NMR spectrum and this signal occurs just past three so approximately approximately 3.1 parts per million and let's see if we can understand why this occurs so we now have an electronegative atom right chlorine is much more electronegative than carbon so chlorine is going to withdraw some electron density and so the chlorine gets partially negative we give the carbon partial positive here so the chlorines withdrawing electron density from these protons so these protons are d shielded from the applied filled alright so here we have D shielded protons right so D shielding protons right the protons are D shielded from the from the applied magnetic field that means those protons experience a greater effective magnetic fields let me go ahead and draw this and I'm just going to exaggerate to get the point across so a greater effective magnetic field for a proton means a greater difference in energy between your alpha and your beta spin States so the alpha and beta spin States here now since we have a de shielded proton right we have a greater difference in energy between our spin States and energy corresponds to frequency so a greater effective magnetic field means a greater energy difference which means a larger frequency right so a higher frequency absorbed and so this right this would be a higher frequency compared compared to the previous example alright so everything is relative here so a higher frequency signal compared to the protons in methane and therefore we get a higher value for the chemical shift so let's say let's just sum this up really quickly so a shielded shielded protons right are going to give you a lower frequency signal and therefore a lower value for the chemical shift right ad shielded proton is going to give you a higher frequency signal and a higher chemical shift alright so once again just comparing these two things that's what electronegativity does right so the more here we have an electronegative atom that's D shielding the protons giving a higher chemical shift so that's the idea and let's apply this to a chart that has it has a bunch of different functional groups here let's think about the different chemical shifts for protons in different environments alright so we just said that if you if you de shield a proton right you're going to get a higher frequency signal and therefore a higher chemical shift and this is called downfield so the left side of this NMR spectrum right these are more d shielded protons to the right side of the NMR spectrum we're talking about more shielded protons therefore a lower frequency signal therefore a lower chemical shift and you could use the the older term upfield if you wanted to as well so we just talked about methane all right so we're talking about an alkane type type environment here so the proton the proton on on a carbon in an alkane type environment the chemical shift this is this is a this is a shielded proton alright so we would expect a low frequency signal or a low chemical shift so somewhere in the range of 0.5 to 2 is where we'd expect the signal for a proton in an alkane type environment so somewhere in that range alright so those are those are more shielded next we talked about chloromethane right in chloromethane we had an electronegative atom on a carbon that was bonded to our proton so that's that's this situation let me use these yellow for this so here we have why is an electronegative atom so you could think about something like chlorine or fluorine so a halogen or you could think about oxygen also also electronegative right so if Y is an electronegative atom Y withdraws electron density from this carbon and that DeShields this proton that's directly on that carbon so D shielding the proton gives you a higher chemical shift and so you'd expect this shift to be approximately 2.5 to 4.5 so if you see a signal and in 2.5 to 4.5 range right it could be a proton that's on a carbon that's directly bonded to an electronegative atom like a halogen or like oxygen alright so in between those two examples right so the signal for this proton right here alright this proton would show up approximately 2 to 2.5 and so this proton is directly bonded to a carbon but this carbon is not directly bonded to an electronegative atom but it is bonded to this carbon which is which is a carbonyl here so this oxygen right is more electronegative this oxygen withdraws some electron density but not quite as much not quite as much as in this example with with this electronegative atom directly on this carbon and so the signal the chemical shift is in between here so a proton that's on a carbon that's next to a carbonyl look for that approximately 2 to 2.5 again all these are just a submit ranges here so I try to give nice easy numbers to remember next let's look at the proton on alcohol so right here well alcohols have hydrogen bonding and hydrogen bonding has a d shielding effect so increased hydrogen bonding increased d shielding the problem is the amount of hydrogen bonding depends on things like concentration and temperature and since those things can vary or you get different amounts of hydrogen bonding you get different amounts of D shielding you get a different range you know a pretty broad range here for your possible signal so approximately approximately two to five for the signal on an alcohol but it might not even be in that range so just just think about two to five for the proton on an alcohol as an approximate region next let's look at the proton on a double bond here so proton all right bonded to a carbon a proton on a double bond the the shift is approximately four point five to six point five so let's see if we can understand why one way to think about it is using electronegativity and so if we think about this carbon here this carbon is sp2 hybridized if we compare that carbon to this carbon right this carbon is sp3 hybridized remember from hybridization videos that an sp2 hybrid orbital has more s character than an sp3 hybridized orbital therefore the electrons are held closer to the nucleus so you can say that an sp2 hybridized carbon is more electronegative than an sp3 hybridized carbon so if you want to think about it that way that's one way to think about it and so this sp2 hybridized carbon is withdrawing more electron density from this proton that means a different color here so this sp2 hybridized carbon is withdrawing more electron density D shielding this proton and giving you a higher chemical shift than for a proton bonded to an sp3 hybridized carbon so that's one way to explain this but it doesn't that line of reasoning isn't exactly that doesn't hold up completely because if we next look at a proton on a triple bond here so if I draw a triple bond and this proton right here so you might think okay well this carbon is SP hybridized and I know that SP hybridized carbon and SP hybridized orbital has even more s character than an sp2 hybridized orbital so therefore you can think about an SP hybridized carbon being more electronegative and these electrons are closer to this carbon and so you might think oh that's going to be sealed right that's going to that's going to de shield this proton and we would expect a signal that's an even higher chemical shift than for this proton and that's not what we observe so the proton on a triple bond actually this shows up somewhere in this range so somewhere around 2 to 2.5 approximately and so it's not just electronegativity that you have to think about so there's another effect that's causing that's causing the chemical shift for this proton that we'll talk about in next video and it's the same thing it's actually the same thing for the proton on a benzene ring so well we will save will save we'll save that discussion for the next video so we'll talk about this and we'll talk about this in the next video if we move on to an aldehyde right so for an aldehyde we have this carbonyl here the oxygen is withdrawing electron density right away from the the proton on the aldehyde and so it's d shielding that proton right therefore would expect the signal for that proton to occur to higher chemical shift so somewhere around 9 to 10 is where we'd expect the shift for this proton finally let's look at a carboxylic acid so the signal for this proton approximately 10 to 12 so once again we have we have this carbonyl here withdrawing electron density we have another oxygen here withdrawing some electron density so you could think about electronegativity effects you could also think about resonance effects and you could also think about there's some hydrogen bonding effect so there's there's all kinds of things going on here with the carboxylic acid and and and pretty much if you're just looking in the 10 to 12 region and you see a signal think think the proton on a carboxylic acids