[aspiringphysician]

Hello,

In lab, I performed an NMR spectroscopy of a solution containing an unknown compound. The possible molecular formulas are: C₂H₃Cl₃, C₄H₁₀O, C₄H₈O₂, or C₄H₈O. Below is the data:

Carbon-13 NMR:

Peaks at 206.6, 78.0, 59.2, and 26.2 ppm.

Proton NMR:

4.0 ppm - singlet - 2 hydrogens
3.4 ppm - singlet - 3 hydrogens
2.1 ppm - singlet - 3 hydrogens

Based on this data, I narrowed the molecular formulas down to either C₄H₈O₂ or C₄H₈O. I know that there is one degree of unsaturation, and since there’s a carbon peak close to 200 ppm, there is probably a carbon-oxygen double bond. Am I correct so far?

Now I am having a difficult time figuring out which formula corresponds to the spectra, and how to draw its structure. I would appreciate any *delete me* 
 

[aspiringphysician]

What on earth!? Is "help" a bad word now? 

[orgopete]

You should use the Tinker Toys method. Draw the pieces the are present from the data and try to connect them together. The proton spectra tells you that three carbons have hydrogens attached. If you have a carbon with three hydrogens attached, what is it? Those three carbons are not connected to each other. The carbon spectrum tells you that you have four carbons. Therefore, one carbon does not have any hydrogens attached. You have two oxygen atoms. How many ways can these atoms be connected together?

[Wald_ron]

Are you sure you have given us the correct information?

[fledarmus]

You should use the Tinker Toys method. Draw the pieces the are present from the data and try to connect them together. The proton spectra tells you that three carbons have hydrogens attached. If you have a carbon with three hydrogens attached, what is it? Those three carbons are not connected to each other. The carbon spectrum tells you that you have four carbons. Therefore, one carbon does not have any hydrogens attached. You have two oxygen atoms. How many ways can these atoms be connected together?

And you have a carbonyl group from your C-13.

[aspiringphysician]

Are you sure you have given us the correct information?

I shared all of the data I obtained from the NMR spectra; whether or not it is correct, I'm not sure. There could have been an error in the graph...

You should use the Tinker Toys method. Draw the pieces the are present from the data and try to connect them together. The proton spectra tells you that three carbons have hydrogens attached. If you have a carbon with three hydrogens attached, what is it? Those three carbons are not connected to each other. The carbon spectrum tells you that you have four carbons. Therefore, one carbon does not have any hydrogens attached. You have two oxygen atoms. How many ways can these atoms be connected together?

Okay...well, I tried this method, and I came up with the structure of ethyl acetate...does that seem right? I am still confused as to why they all showed up as singlets...

[fledarmus]

Nope, that's not right.

Since none of your protons are split, none of the carbons with protons attached can be next to another carbon with protons attached.

Do like Orgopete said. How many pieces do you have? How many of them have hydrogens on them? How many ways can they be arranged so that no two carbons with hydrogen are attached to each other?

[aspiringphysician]

Nope, that's not right.

Since none of your protons are split, none of the carbons with protons attached can be next to another carbon with protons attached.

Do like Orgopete said. How many pieces do you have? How many of them have hydrogens on them? How many ways can they be arranged so that no two carbons with hydrogen are attached to each other?

Oh, I see...I didn't think that seemed right!  Okay, so I have two methyl groups, a CH2, a carbonyl, and an oxygen atom, right? If I put them together so no splitting occurs, it will look like this, right? (see attached pic)

[fledarmus]

Bingo! 

[aspiringphysician]

Yay! Thank you so much! I understand it now  

[orgopete]

The essence of the Tinker Toys approach is that it can enable generating a correct structure despite what you might "think". It should have been clear that if you have two CH3 groups, a C=O, a CH2, and an O, that simplifies to three choices, or which group will be in the center. It is sometimes (always?) easier to look at those results and determine which must be the correct structure than to try to think of the one structure than must match the data.

Another advantage of this trial and error type of thinking is that it can lead to partial solutions. If you had a more complicated structure, but one in which you had not determined the final structure, if you saw some fragments that matched the spectral data, this may simplify the remaining options. In the problem in question, there were only three possibilities. That should not be too difficult.