[Silver-Midnight]

Okay, one of the problems that I was given was to look at the H1-NMR, C-13-NMR, IR, and MS data and determine the structure as well as IUPAC name. We are given the molecular formula. For the particular problem that I'm looking at, I'm having trouble deciphering the H1-NMR data. Some of it, I do understand. Some of it, I don't. It's basically as follows:

3H ---> triplet
6H -----> doublet
3H -----> doublet
2H -----> triplet
2H -----> multiplet
1H ------> multiplet
1H ------> multiplet

Here's what I've concluded so far. Based on the molecular formula, C8H18, which, to me, looks like octane, along with the IR Spec. data, this is only composed of carbons and hydrogens. It doesn't even have double bonds, from what I can tell anyway.

I know that C-13 NMR doesn't split but H-NMR does. And it splits by the N + 1 rule, n equals the number of bydrogens on the adjacent carbon. So, for example, if something is a quartet, the adjacent carbon has three hydrogens (n + 1= 3 +1 = 4).

Now, what's confusing me about the problem is the fact that there is one with 6 hydrogens, especially given that carbon can only bond to four different atoms.

Could someone please help me?

Thanks.   

[discodermolide]

Perhaps the 6H doublet is actually two overlapping 3H doublets?

[Dan]

Now, what's confusing me about the problem is the fact that there is one with 6 hydrogens, especially given that carbon can only bond to four different atoms.

Look up chemical equivalence (chemically equivalent nuclei). Think about symmetry.

[Silver-Midnight]

Perhaps the 6H doublet is actually two overlapping 3H doublets?

But if that's the case, how would the structure of the compound look still?

Because I'm seeing it as:

CH3-CH3-CH3


Which I don't think is possible because there are too many bonds around carbon still, right?

[Silver-Midnight]

Now, what's confusing me about the problem is the fact that there is one with 6 hydrogens, especially given that carbon can only bond to four different atoms.

Look up chemical equivalence (chemically equivalent nuclei). Think about symmetry.

Well,  chemical equivalence is discussing how the NMR only shows the nuclei(or in this case, hydrogens) that are chemically different from one another, meaning not the same. However, the way that I'm seeing to solve this problem, going by my notes, it's like the intensities were added up and then divided by the amount of hydrogens (I think). Then that number was divided into each individual intensity at each signal. That said the number of hydrogens at that particular one.

So, is it instead saying that it's 3 equivalent CH2 groups instead? 

[Dan]

So, is it instead saying that it's 3 equivalent CH2 groups instead?

On the face of it, it could be 3 equivalent CH₂s (3 x 2 = 6), yes, but if you try to draw structures that have this feature and fit the rest of the data you will get stuck. Remember that 2 x 3 = 6 as well....

Does this make sense to you:

The NMR of propane is:

6H, triplet
2H, multiplet (septuplet)

Consider the symmetry of propane.

[Silver-Midnight]

So, is it instead saying that it's 3 equivalent CH2 groups instead?

On the face of it, it could be 3 equivalent CH₂s (3 x 2 = 6), yes, but if you try to draw structures that have this feature and fit the rest of the data you will get stuck. Remember that 2 x 3 = 6 as well....

Does this make sense to you:

The NMR of propane is:

6H, triplet
2H, multiplet (septuplet)

Consider the symmetry of propane.

Well, propane is CH3-CH2-CH3, right?

Yeah, I'm still not getting it.    I'm sorry. 

[Dan]

You should review the basic concepts of NMR. Can you tell me what you think the spectrum of propane would look like with your current understanding, and we can go from there?

As I said, look up chemical equivalence. Try to get to grips with a very simple case like propane before attempting the question you have posted.

Example

[Silver-Midnight]

You should review the basic concepts of NMR. Can you tell me what you think the spectrum of propane would look like with your current understanding, and we can go from there?

As I said, look up chemical equivalence. Try to get to grips with a very simple case like propane before attempting the question you have posted.

Example

Well, after reading the link and based off the link I'm about to post, here's what I think.

http://www.chem.ucla.edu/harding/30C/30C_w03/NMRsupp.pdf


Okay, using Propane, in the example that link gave, they used 2-chloropropane I believe. In that example, chlorine was on the second carbon, and the CH in the middle was surrounded by 2 CH3 groups on each side. The 2 CH3 groups each produced a triplet I think. (They should, right?). And the 1 CH group in the middle would make a septet/multiplet. Why? Because the 2 CH3 are chemically in the same environment and are both next to the CH group. The signals from both of them combine into one.

So, looking at regular propane, which is CH3-CH2-CH3, the CH3 groups would both show up as triplets ( 2 +1 = 3), and the CH2 group would show up as a septet/multiplet (3 +3 +1 = 7). Right? 

[Dan]

So, looking at regular propane, which is CH3-CH2-CH3, the CH3 groups would both show up as triplets ( 2 +1 = 3), and the CH2 group would show up as a septet/multiplet (3 +3 +1 = 7). Right?

You are almost there, now you need to consider chemical equivalence. Please read the link I provided before.

E.g. The spectrum of benzene is just one singlet. Can you see why?

[Silver-Midnight]

So, looking at regular propane, which is CH3-CH2-CH3, the CH3 groups would both show up as triplets ( 2 +1 = 3), and the CH2 group would show up as a septet/multiplet (3 +3 +1 = 7). Right?

You are almost there, now you need to consider chemical equivalence. Please read the link I provided before.

E.g. The spectrum of benzene is just one singlet. Can you see why?

Well, they're chemically equivalent. They all have the same chemical envoronment; all the hydrogens are bonded to a C that is bonded to another C by single bond and another C by double bond.

However, I'm still not understanding. I do feel like I have to really close though.

So, the one with 6 H doesn't mean that the C is bonded to 6 Hs but that those H s are chemically equivalent, right? So, the 6 H could actually be two CH3 groups, right? Then the 6 H was a doublet, which means it has to be next to something with one H. Then there's the 2H doublet, 2H multiplet, and 3H doublet left. I keep missing something. But I can't figure out what.

[Dan]

So, the one with 6 H doesn't mean that the C is bonded to 6 Hs but that those H s are chemically equivalent, right?

Correct

So, the 6 H could actually be two CH3 groups, right?

Exactly

Then the 6 H was a doublet, which means it has to be next to something with one H.

Yes

Then there's the 2H doublet, 2H multiplet, and 3H doublet left. I keep missing something. But I can't figure out what.

You have all the pieces of the puzzle, now you need to try to fit them together.

[tomek]

Also it is useful to examine chemical shifts values as alkyl substitution will shift your ppm downfield (this is especially good with CNMR) . Beside that, inspect coupling constants which will tell you a lot about mutually coupled protons (assuming you have first-order spectrum).