NMR help

[tiny529]

Hi! I'm new here (will introduce myself shortly!) I'm in Organic Chemistry 2 and have a problem that no amount of pondering or doodling is helping me solve.

 

Here's the problem...

 

Below are the ¹H and ¹³C NMR spectra of a dichloroalkane. Analyze the spectra and propose a structure consistent with the data. Provide a COGENT explanation outlining how you arrived at the proposed structure.

 

The ¹H NMR isn't super clear, but it looks like there's a sextet between 4.5 and 4 ppm (small... maybe 1 H), a triplet between 4 and 3.5 ppm (maybe 3 H?), a quartet between about 2.3 and 2 ppm (maybe 2 H?), and a doublet around 1.5 ppm (tall... Maybe 6H).

 

The ¹³C NMR is also not very clear, but it looks like there's a singlet at 30 ppm, a quartet at about 47 ppm, and a doublet at 60 ppm. The 60 and 47 are about the same height and the 30 is about 3/4 the height of the other 2.

 

My problem is that I can easily make a structure that matches one or the other spectra, but I can't seem to make one that matches both. I'll get a great ¹H structure only to realize that it will give me 5 ¹³C signals instead of 3. Or my ¹³C will look good and I'll realize that I couldn't have a triplet in my ¹H NMR.

 

Does anyone have any insight to help me puzzle this out? It's part of a take home quiz that is due on Tuesday (it says that we are allowed to utilize any resources available, including the internet, and I've just about exhausted my resources!).

 

Thanks!

 

(edit) Adding the scan here. It's added at the bottom too since I didn't realize that I can't add a reply without it turning into the edit of my last reply. =)

 

Edited February 20, 2011 by tiny529

[hypervalent_iodine]

It might just be me, but I can't see any spectra? Until I see it, I won't be much help because I would need to see the integration and Jz coupling constants. Just looking at what you've said for the ¹H NMR, I'm not sure your integrations are right. Are you given an empirical formula or mass spec at all?

 

Is your ¹³C spectra proton coupled or what? You don't normally get a proton coupled spectra without a decoupled one as well. Also, peak heights in ¹³C don't mean much of anything. Certainly not in the same way as in ¹H NMR.

Edited February 20, 2011 by hypervalent_iodine

[tiny529]

It might just be me, but I can't see any spectra? Until I see it, I won't be much help because I would need to see the integration and Jz coupling constants.

 

 

 

Also, is your ¹³C spectra proton coupled or what? You don't normally get a proton coupled spectra without a decoupled one as well.

 

Also, peak heights in ¹³C don't mean much of anything. Certainly not in the same way as in ¹H NMR.

 

Sorry about that... Like I said, they are difficult to read. I could scan them in, but they would be even more difficult to read. There is no integration on it, unfortunately. That's why I gave general height. My prof is trying to make this as difficult as possible, I think. Pretty much what I said before is all of the information I have.

 

For the ¹³C, 1,3-dichloro-2,2-dimethylpropane seems reasonable... I'd get a singlet, a triplet (not a doublet, but the lines are so close together that it's hard to tell what exactly it is), and a quartet. The doublet/triplet (whatever the heck it is...) is the furthest downfield, which would make sense being so close to the chlorines... Though the singlet is further upfield than the quartet, so that's probably not right... Argh! This is driving me nuts. =b

 

(edit) I guess 1,1-dichloro-2,2-dimethylpropane works better... I get 1 doublet, a singlet, and a quartet in the proper positions, but it still doesn't even remotely match the ¹H NMR. =\

Edited February 20, 2011 by tiny529

[mississippichem]

You can approximate the integral heights of the peaks by treating them as simple triangles, if you're not the type that likes to integrate. This will be troublesome though if you don't have a decoupled spectrum. Could you give us a list of peaks with chemical shifts and multiplet designations?

 

For example: a triplet at 7.3 ppm. Or 7.3(t)

Edited February 20, 2011 by mississippichem

[tiny529]

You can approximate the integral heights of the peaks by treating them as simple triangles, if you're not the type that likes to integrate. This will be troublesome though if you don't have a decoupled spectrum. Could you give us a list of peaks with chemical shifts and multiplet designations?

 

For example: a triplet at 7.3 ppm. Or 7.3(t)

 

Unfortunately, I don't have that information. The best I can do it approximate. Like, the sextet at approximately 4.3 is about 1/3 of the height of the triplet at approximately 3.7. Maybe I'll try to scan in the page, even if it's impossible to read just so you can see how little information we are given.

 

Anyway, here are my best approximations...

 

1H sextet @ 4.3

3H triplet @ 3.7

2H quartet @ 2.3

6H doublet @ 1.5 (it actually looks more like 9H to me, but that would make it a tert-butyl singlet and that's impossible in light of the other signals...)

 

For the ¹³C...

 

doublet @ 60

quartet @ 43

singlet @ 30

 

I am, literally, given no more information... I can't help but wonder if my prof forgot something. He tell us, in this quiz, to look up carbon splitting in a textbook other than the one we are using!

 

(edit) If worse comes to worse, I'll pretend that I missed the "a dichloroalkane" and assign a separate structure to each spectra. 3,3-dichloro-2-methylpentane makes the most sense for the H NMR and 1,1-dichloro-2,2-dimethylpropane makes the most sense for the C NMR.

 

You can approximate the integral heights of the peaks by treating them as simple triangles, if you're not the type that likes to integrate. This will be troublesome though if you don't have a decoupled spectrum. Could you give us a list of peaks with chemical shifts and multiplet designations?

 

For example: a triplet at 7.3 ppm. Or 7.3(t)

 

Okay... I hope this works... Sorry I couldn't crop out the top half of the page. I'm on my Netbook and the touch pad is giving me grief. =) It's problem 5, at the bottom of the page.

 

Edited February 20, 2011 by tiny529

[Horza2002]

Is the carbon NMR proton decoupled? Because it looks to me that there are several peaks that are close together (although its very hard to tell) and not splitting patterns. Also, you have four different proton enviroments but are only quoting three carbons....so according to you at the moment, one carbon has two different enviroments on it; while this is possible (diastereotopic hydrogen's) it doesn't look like thats what you've got. This looks like a beginners type problem, and diastereotopic hydrogens is a little more advance. Additionally, there is no quaternary carbon in the NMR so I can't be 1,1-dichloro-2,2-dimethylpropane (the central carbon here would be quaternary).

 

The doublet which intergrates to 6 hydrogens implies that it is a 1,1-Dimethyl moiety. This would mean that what you think is a sextet is actually a higher order multiplet but you can't see the smaller peaks. If you could zoom in on those spectra and let us know if the carbon is decoupled or not, then we might be able to give you some better help

[hypervalent_iodine]

Horza2002 to the rescue!

[tiny529]

Yeah, this is just college Organic Chemistry 2, but that makes it no less difficult for a beginner like me! =)

 

Like I said before, my H NMR solution hasn't yet matched my C NMR solution. I hadn't even considered that there might not be just 3 carbons, but that what I interpreted as a doublet might actually be 2 separate signals. That's very helpful! I will revisit my H NMR solution of 3,3-dichloro-2-methylpentane and see if that makes sense with this new insight.

 

Here are my zooms...

 

 

[Horza2002]

Well, again, while the the 1H-NMR interpretation implies that it could be 3,3-dichloro-2-methylpentane, the carbon NMR doesnt imply that as there is no quaternary carbon (in this molecule, the 3-carbon would have no hydrogen bound to it so be quaternary). It is standard that practise while running carbon-NMR to decouple them from hydrogen so that they should all appear as singlets. So if you have a signal that looks like a doublet, I would almost certainly say that it is actually two signals very close to each other.

 

To be honest, if you were trying to do assign this structure in reality, you would use 2D experiments like COSY, HMQC and HMBC instead as well as a C-DEPT experiment. It would make the assignment soooo much easier!

[tiny529]

C₃ would be a quaternary, wouldn't it? There would be the 2 chlorines, an ethyl, and an isopropyl attached to it. Are you figuring that there should be more than one quaternary carbon to account for the peaks that I thought were a doublet?

 

Let's see (and please correct me if I'm wrong... I'm not very good reading this stuff yet)... 3,3-dichloro-2-methylpentane should show a quartet for C₁, a doublet for C₂, a quartet for C₃, a triplet for C₄, and a quartet for C₅.

 

....H₃C...Cl...H

........|....|....|

H₃C--C--C--C--CH₃

........|....|....|

........H...Cl...H

 

C_1..C_2..C_3..C_4..C₅

 

Since this isn't what is showing in the C NMR, I guess this couldn't be right, even though it matches the H NMR nicely (there is an isopropyl sextet/doublet and an ethyl quartet/triplet, plus the separation doesn't cause any messy multiplets).

 

What is this software of which you speak? Is it something that is readily available (and free, would be nice) or is it something that isn't going to be accessible to a lowly undergrad chemistry student such as myself?

Edited February 20, 2011 by tiny529

[Horza2002]

No I said that your C-NMR doesnt show a quaternary carbon in it. SO that seems to imply thats what it is. You are right that C3 is quaternary but I was saying that the NMR spectra implies that there no quaternaries.

 

In 3,3-dichloro-2-methylpentane, look at the file Ive attatched:

 

C1 - doublet

C2 - heptet

C3 - quaternary

C4 - quartet

C5 - triplet

 

Im starting to wonder if this is a predicted NMR and so the C-NMR is hiding the quaternary NMR. I didn't say anything about any software...what did u mean?

Doc16.pdf

[tiny529]

No I said that your C-NMR doesnt show a quaternary carbon in it. SO that seems to imply thats what it is. You are right that C3 is quaternary but I was saying that the NMR spectra implies that there no quaternaries.

 

In 3,3-dichloro-2-methylpentane, look at the file Ive attatched:

 

C1 - doublet

C2 - heptet

C3 - quaternary

C4 - quartet

C5 - triplet

 

Im starting to wonder if this is a predicted NMR and so the C-NMR is hiding the quaternary NMR. I didn't say anything about any software...what did u mean?

 

Brain fart on my part... Sorry about that. I was looking at C3 as a quaternary simply because it had no hydrogens attached. Wasn't thinking about the fact that it it didn't have carbons attached on all sides. =\

 

Also, there isn't a quaternary carbon showing at 30 on the C NMR? I'm confused... ='(

 

To be honest, if you were trying to do assign this structure in reality, you would use 2D experiments like COSY, HMQC and HMBC instead as well as a C-DEPT experiment. It would make the assignment soooo much easier!

 

Just assumed that these were software. What is COSY, HMQC, and HMBC?

 

Will try doodling a little more, but I'm starting to reach my boiling point with this one. It's worth 20% of the quiz grade though so I can't just let it go.

 

Does anyone have any ideas? Something that I am missing???

Edited February 20, 2011 by tiny529

[Horza2002]

Typically, quaternary carbons (those that have no hydrogens bound to it) have a very low intenisty peak in the carbon NMR. Typically, they would be about 1/5 the size of the peaks you currently have in the spectra. However, if these are computer generated spectra, it might not give a proper approximation. The H-NMR does seem to imply that it is indeed 3,3-dichloro-2-methlypentane so I'm thinking that is what it is. thb, I can't think of what else it could be because you only have four carbon enviroments and five hydrogen enviroments (thus it implies there are 5 carbons one of which doesnt have any hydrogens bound to it).

 

No COSY, HMBC, MHQC and PENDANT are different types of NMR experiments, that are 2D. For example, COSY tells you which protons are coupled to each other, HMBC tells you which protons are bound to which carbon atoms. PENDANT also tells you how many hydrogens are bound to each of the carbon atoms...so it makes it soooo much easier.

 

http://en.wikipedia.org/wiki/Correlation_spectroscopy

[tiny529]

Typically, quaternary carbons (those that have no hydrogens bound to it) have a very low intenisty peak in the carbon NMR. Typically, they would be about 1/5 the size of the peaks you currently have in the spectra. However, if these are computer generated spectra, it might not give a proper approximation. The H-NMR does seem to imply that it is indeed 3,3-dichloro-2-methlypentane so I'm thinking that is what it is. thb, I can't think of what else it could be because you only have four carbon enviroments and five hydrogen enviroments (thus it implies there are 5 carbons one of which doesnt have any hydrogens bound to it).

 

No COSY, HMBC, MHQC and PENDANT are different types of NMR experiments, that are 2D. For example, COSY tells you which protons are coupled to each other, HMBC tells you which protons are bound to which carbon atoms. PENDANT also tells you how many hydrogens are bound to each of the carbon atoms...so it makes it soooo much easier.

 

http://en.wikipedia....on_spectroscopy

 

There is one more thing I'm noticing about the H NMR... The triplet @ ~3.7 isn't in a 1:3:1 ratio. Just eyeballing it, it looks like 1:4:3, which doesn't make any sense... Could that possibly not be a real triplet?

 

I'll keep plugging away for now, but if I still haven't come up with anything by tomorrow night, I'll use 3,3-dichloro-2-methylpentane and hope that my prof gives partial credit if it's wrong.

[Horza2002]

The triplet, doublet and quartet are not in exact ratio for an effect known as roofing. It is possible that its two signals overlapping, but if your a new learning this area, then it seems a little mean. What Uni are you at what undergrad year?

 

Once the anwser revleaed to you, it will probably make perfect sence.

[tiny529]

The triplet, doublet and quartet are not in exact ratio for an effect known as roofing. It is possible that its two signals overlapping, but if your a new learning this area, then it seems a little mean. What Uni are you at what undergrad year?

 

Once the anwser revleaed to you, it will probably make perfect sence.

 

This problems seems a little mean anyway. =) If it were just the H NMR, I'd be fine. It's the C NMR that's throwing me off so badly. =\

 

I'm a senior at Temple. The kicker is that I haven't had Organic Chemistry 1 in about 10 years. This is the only class standing between me and my BS (which is partly why it's taken me 10 years to get back to it...) I've been studying on my own since about August, so I feel as comfortable as I can with the subject. I got a 100% on my last quiz, I am grasping most of it. Amazingly enough, I'm actually starting to enjoy it (something I never thought I'd say!), though this particular problem is making me want to pull my hair out.

[hypervalent_iodine]

This problems seems a little mean anyway. =) If it were just the H NMR, I'd be fine. It's the C NMR that's throwing me off so badly. =\

 

I'm a senior at Temple. The kicker is that I haven't had Organic Chemistry 1 in about 10 years. This is the only class standing between me and my BS (which is partly why it's taken me 10 years to get back to it...) I've been studying on my own since about August, so I feel as comfortable as I can with the subject. I got a 100% on my last quiz, I am grasping most of it. Amazingly enough, I'm actually starting to enjoy it (something I never thought I'd say!), though this particular problem is making me want to pull my hair out.

 

Understandable. They've certainly not made this easy for you. In the real world, we have 2D sepctra and our peaks are integrated, so don't get too scared away from organic chem. It's a pretty awesome subject, if I do say so myself

[Horza2002]

Good recruitment speach ther hyper

[hypervalent_iodine]

Hey, anything I can do to make people realise that O. Chem is really not as disgusting as undergrad sometimes makes it out to be, I will do to my fullest extent. The world needs more chemists, in my opinion

[tiny529]

Unfortunately, my answer was wrong... The correct answer was 1,3-dichlorobutane. Still doesn't match the C NMR, but it does match the H NMR better than mine does.

 

Thanks for the help guys! I'm sure I will be back soon enough with more annoying undergrad OChem2 questions.

 

Still waiting to hear back from my prof about a problem I think he had wrong... Correct me if I'm wrong, but tert-butylbromide will not show any molecular ion on a mass spec, right? I gave the answer of 2-bromobutane, but he marked it wrong.

[Horza2002]

Right, so it was a simulated NMR then as one of the those (position two) should be a doublet of triplets which it wasnt (unless it just wasnt clear on the spectra provided). And bring oin the questions! O chem ROCKS!!

 

Well that would depend on the ionisation source. If it was ESI, then quiet probably not (as im makng 4-methyl pentan-1-bromide atm and it doesn;t show up in the ESI)...however, if your using EI or CI, then I would expect you to see it yes.

[tiny529]

Right, so it was a simulated NMR then as one of the those (position two) should be a doublet of triplets which it wasnt (unless it just wasnt clear on the spectra provided). And bring oin the questions! O chem ROCKS!!

 

Well that would depend on the ionisation source. If it was ESI, then quiet probably not (as im makng 4-methyl pentan-1-bromide atm and it doesn;t show up in the ESI)...however, if your using EI or CI, then I would expect you to see it yes.

 

I have no idea what any of that is. =b

 

I did check webbook.nist.gov/chemistry and the tert-butylbromide MS doesn't match the quiz... 2-bromobutane does. It's only 1 point, but I'm getting a little anal in my old age. =)

[Horza2002]

Lol might be a little of topic but thyre justdifferrent methods of how MS works. If u want to know, start a new thread and im sure we'll all be happy to fill u in on how they all work!

[Dan_Ny]

Wow, for the next 1000 extremely difficult NMR questions I know myself in good hands now. And I WILL post them. No, seriously, brave job from Horza2002 and hypervalent_iodine!

[mississippichem]

Wow, for the next 1000 extremely difficult NMR questions I know myself in good hands now. And I WILL post them. No, seriously, brave job from Horza2002 and hypervalent_iodine!

 

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