HNMR splitting question

[diene]

---

Members don't see this ad.

I get confused when noncarbon atoms are involved.

#243 from destroyer confused me

CH3 - O - CH2 - CH2 - O - CH3

The answer is a pair of singlets...can someone explain that?
Do we only consider carbon neighbors of a carbon and see how many hydrogens they have? Do we also look at O and see how many hydrogen its carbon neighbors have?


Also, I made up a random question but it will help me understand if someone can answer it

CH3 - CH2 - NH - CH3

 

[jhc83]

you only look at carbon neighbors...therefore CH3 on the left and right will have singlets...whereas CH2 in the middle will each have triplets...

 

[diene]

you only look at carbon neighbors...therefore CH3 on the left and right will have singlets...whereas CH2 in the middle will each have triplets...

But the answer is 2 singlets..no triplets? Also, CH3 on the left and right don't have a carbon neighbor? so how does that work?

 

[kpreet]

equivalent protons dont split each other. the compound is symmetric. so it will only have two different hydrogen signals and I believe that the structure your made up is not symmetic so it will have four different signals in the NMR.

 

[diene]

equivalent protons dont split each other. the compound is symmetric. so it will only have two different hydrogen signals and I believe that the structure your made up is not symmetic so it will have four different signals in the NMR.

Okay I get the symmetry, but what is really confusing me is the fact that there is an oxygen. For CH3, on both sides, there are no adjacent carbons, so then how do we get a singlet for them? Aren't we only looking at hydrogens on adjacent carbons?

 

[R1Guy]

Okay I get the symmetry, but what is really confusing me is the fact that there is an oxygen. For CH3, on both sides, there are no adjacent carbons, so then how do we get a singlet for them? Aren't we only looking at hydrogens on adjacent carbons?

This is a symmetrical molecule, there is a line of symmetry splitting the two CH2's. Therefore, we would expect two total signals, one signal for the CH3, and another signal for the CH2.

As far as splitting, the CH3 would be a singlet that would show up in the 4ppm range because it is being pulled downfield by the electronegative oxygen. Splitting patterns are based on the n+1 rule (where n is the number of hydrogens on the adjacent atom). Now for the CH3, the adjacent atom is an oxygen, which has no hydrogens on it. So n in this case is zero, and 0+1=1 (singlet!)

Now for the CH2. Remember, splitting patterns are based on the n+1 rule, where n is the number of hydrogens on the adjacent ATOM. So you should be thinking, well the adjacent atoms in this case are an oxygen with zero hydrogens, and on the other side a carbon, that has two hydrogens attached to it. So n in this case is 2 and n+1=3 (so you would initially think the CH2 would show up as a triplet). BUT... because of symmetry these two CH2's are chemically equivalent, and chemicaly equivalent protons DO NOT SPLIT EACHOTHER. So n is essentially zero in this case and n+1=1 (singlet!)

So, long story short hydrogen signals are split based on how many hydrogens are attached to the atom immediately beside them. However, if those protons are equivalent because of symmetry, they will not split eachother.

Hope that helps.

 

[diene]

This is a symmetrical molecule, there is a line of symmetry splitting the two CH2's. Therefore, we would expect two total signals, one signal for the CH3, and another signal for the CH2.

As far as splitting, the CH3 would be a singlet that would show up in the 4ppm range because it is being pulled downfield by the electronegative oxygen. Splitting patterns are based on the n+1 rule (where n is the number of hydrogens on the adjacent atom). Now for the CH3, the adjacent atom is an oxygen, which has no hydrogens on it. So n in this case is zero, and 0+1=1 (singlet!)

Now for the CH2. Remember, splitting patterns are based on the n+1 rule, where n is the number of hydrogens on the adjacent ATOM. So you should be thinking, well the adjacent atoms in this case are an oxygen with zero hydrogens, and on the other side a carbon, that has two hydrogens attached to it. So n in this case is 2 and n+1=3 (so you would initially think the CH2 would show up as a triplet). BUT... because of symmetry these two CH2's are chemically equivalent, and chemicaly equivalent protons DO NOT SPLIT EACHOTHER. So n is essentially zero in this case and n+1=1 (singlet!)

So, long story short hydrogen signals are split based on how many hydrogens are attached to the atom immediately beside them. However, if those protons are equivalent because of symmetry, they will not split eachother.

Hope that helps.

Thank youuu!!! That makes sense! Just a quick question though, I just rewatched chad's video and he states that "nitrogen and oxygen are meanies- nobody is their neighbor and they are nobody's neighbor" What do you think he means by that? because clearly you have to consider oxygen/nitrogen neighbors The example he used was CH3OHCHCH3 ...he didn't consider the oxygen neighbor

 

[n27s]

Also, I made up a random question but it will help me understand if someone can answer it

CH3 - CH2 - NH - CH3

I'm a little confused as to what the answer to the one you made up would be.. anyone?

 

[diene]

I'm a little confused as to what the answer to the one you made up would be.. anyone?

I am thinking a triplet and two quartets. The reason I am saying that is because the hydrogen on the nitrogen can be easily replaced by another hydrogen. Based on the example chad showed, it would make sense to ignore the nitrogen neighbor is this case.

Anyone wanna confirm that?

 

[R1Guy]

Thank youuu!!! That makes sense! Just a quick question though, I just rewatched chad's video and he states that "nitrogen and oxygen are meanies- nobody is their neighbor and they are nobody's neighbor" What do you think he means by that? because clearly you have to consider oxygen/nitrogen neighbors The example he used was CH3OHCHCH3 ...he didn't consider the oxygen neighbor

meanies lol.... I never watched chad's videos but that's a good way to memorize it I guess.

Let's talk about the oxygen first. He just means when you have an alcohol, the hydrogen that is attached to the oxygen (the OH) will always show up as a singlet (it does not get split by neighbors, and it does not split its neighbors). Now, that sounds confusing so let's walk through the molecule you listed, which is 2-propanol. If you draw a condensed structure of 2-propanol, you will see that it is a symmetrical molecule (the two CH3's on the end are chemically equivalent). Because of symmetry, we would expect three total signals in the HNMR, one for the CH3's, one for the CH, and one for the OH.

The CH signal would show up in the 4ppm range because it is being pulled downfield by the electronegative oxygen. Now, let's talk about the splitting pattern for this signal. When we look at the adjacent atoms, we have a CH3 on the left, a CH3 on the right, and an OH up top. So n in this case is 7....BUT!!!! the hydrogen attached to that oxygen does not cause splitting, that is what he means by they are meanies (he just means they are the exception to the rule). Since the hydrogen attached to the oxygen is an exception, n in this case is only 6, and n+1=7 (septet).

Next, let's talk about the chemically equivalent CH3's on the ends of the molecule. Since they are chemically equivalent, we will get one signal for the two CH3's, and since it is further away from the oxygen it will not get pulled downfield as much; it would show up around 1.7ppm ball park. Now for the splitting, when we look at the adjacent atom, there is only one carbon with ONE hydrogen attached, so n is one and n+1=2 (so this signal will be a doublet)

Next, let's talk about the OH. Let's talk first about the splitting pattern for this signal. Remember, splitting is based on how many hydrogens are attached to the adjacent ATOM. The adjacent atom in this case is a carbon with ONE hydrogen attached to it. So you would initially think that n is 1 and n+1=2 (doublet). BUT....oxygens are "meanies", meaning they are the exception to the rule. The hydrogen on the adjacent carbon does not split in this case, so n is essentially zero, and n+1=1 (singlet). Now, as far as what ppm this signal would show up in the HNMR, alcohols have a pretty broad range. They typically show up anywhere in the 2-5ppm range, and they look very different than normal signals...they are broad singlets.

Now, we walked through an examble for an alcohol, but the amines (NH) behave the exact same way... they are exceptions to the normal rule, they do not split or get split.

Hope that helps.

 

[diene]

meanies lol.... I never watched chad's videos but that's a good way to memorize it I guess.

Let's talk about the oxygen first. He just means when you have an alcohol, the hydrogen that is attached to the oxygen (the OH) will always show up as a singlet (it does not get split by neighbors, and it does not split its neighbors). Now, that sounds confusing so let's walk through the molecule you listed, which is 2-propanol. If you draw a condensed structure of 2-propanol, you will see that it is a symmetrical molecule (the two CH3's on the end are chemically equivalent). Because of symmetry, we would expect three total signals in the HNMR, one for the CH3's, one for the CH, and one for the OH.

The CH signal would show up in the 4ppm range because it is being pulled downfield by the electronegative oxygen. Now, let's talk about the splitting pattern for this signal. When we look at the adjacent atoms, we have a CH3 on the left, a CH3 on the right, and an OH up top. So n in this case is 7....BUT!!!! the hydrogen attached to that oxygen does not cause splitting, that is what he means by they are meanies (he just means they are the exception to the rule). Since the hydrogen attached to the oxygen is an exception, n in this case is only 6, and n+1=7 (septet).

Next, let's talk about the chemically equivalent CH3's on the ends of the molecule. Since they are chemically equivalent, we will get one signal for the two CH3's, and since it is further away from the oxygen it will not get pulled downfield as much; it would show up around 1.7ppm ball park. Now for the splitting, when we look at the adjacent atom, there is only one carbon with ONE hydrogen attached, so n is one and n+1=2 (so this signal will be a doublet)

Next, let's talk about the OH. Let's talk first about the splitting pattern for this signal. Remember, splitting is based on how many hydrogens are attached to the adjacent ATOM. The adjacent atom in this case is a carbon with ONE hydrogen attached to it. So you would initially think that n is 1 and n+1=2 (doublet). BUT....oxygens are "meanies", meaning they are the exception to the rule. The hydrogen on the adjacent carbon does not split in this case, so n is essentially zero, and n+1=1 (singlet). Now, as far as what ppm this signal would show up in the HNMR, alcohols have a pretty broad range. They typically show up anywhere in the 2-5ppm range, and they look very different than normal signals...they are broad singlets.

Now, we walked through an examble for an alcohol, but the amines (NH) behave the exact same way... they are exceptions to the normal rule, they do not split or get split.

Hope that helps.

Wow, great explanation!!! 👍 Thank you soo much

So now going back to CH3 - CH2 - NH - CH3

triplet, quartet, singlet, and a singlet...did I get it right?

 

[Kingsta2426]

no not quite

CH3 - CH2 -NH-CH3

triplet, quartet, singlet, singlet in that order with respect to the molecule

 

[R1Guy]

Wow, great explanation!!! 👍 Thank you soo much

So now going back to CH3 - CH2 - NH - CH3

triplet, quartet, singlet, and a singlet...did I get it right?

Correct, you got it now!

 

[R1Guy]

And that's because of the N/O exception. It's crazy that I've never heard or seen that before!

If it was CH3-CH2-CH=CH-CH3

Then it would be triplet, multiplet (5 peaks), multiplet (4 peaks), multiplet (5 peaks), doublet (from left to right in the order I wrote above) right? Will the double bond cause things to be downfield? What are the chemical shifts?

I put the (# peaks) because I feel you guys might say that instead of multiplet. I was taught (and on the NMRs I have seen) that if there are adjacent H's that are asymmetrical the splitting of the signal will be crazy (doesn't have uniform peaks, fitting Pascal's) and won't yield any real information, so you just call it multiplet. Sometimes I've seen that's true, other times clearly unequivalent H's split like they were regular. What am I missing?

Are there any other exceptions to keep in mind for NMRs? All I really look for are:
1. Adjacent H's
2. How many are equivalent
3. How many there are, and what the splitting will be

Yup, the splitting patterns you stated for 2-pentene are correct. For clarity purposes, let's number the carbons from right to left (just like we would for nomenclature purposes to put the double bond at the lowest number). The two hydrogens attached to the sp2 carbons (carbons number 2 and 3) will show up in the 5-6ppm range. The hydrognens attached to carbon 1 and 4 will show up at approximately 1.7ppm because they are being pulled downfield by the double bond. Lastly, the hydrogns on carbon 5 will show up at about 1ppm becuase they are far enough away from the double bond to not really be effected.

Now as far as splitting, yes hydrogens attached to sp2 carbons that appear in the 5-6ppm range often show complex splitting patterns. DO NOT WORRY about this for the purposes of the DAT. The NMR questions on the DAT are very, very basic questions; they are not going to ask you about complex splitting. In reality, if you ran an NMR of 2-pentene, the two hydrogens showing up in the 5-6ppm range would most likeley be overlapping, so you can imagine a quartet and a pentet overlapping with eachother would look pretty crazy, and no, that splitting would not help you piece the molecule together very well. What is CRUCIAL is the integration of that signal in the 5-6ppm range. The integration will tell you that it is two hydrogens, so you would know that it is a DISUBSTITUTED alkene! This is conjunction with the CH3 doublet (due to hydrogens on carbon 1) only leaves one way this molecule could be pieced together.

For the purposes of the DAT, you should be able to look at a molecule and know how many signals you will get(think about symmetry), what approxiamte ppm range they will show up, and the splitting based on the n+1 rule. Again, don't worry about complex splitting for the DAT.

Hope that helps.