NMR Help!

[soccerchick747]

---

Members don't see this ad.

 

[QofQuimica]

I have to do a presentation tomorrow, and I'm having trouble interpreting my NMRs. I can't manage to find my NMR sheet from sophomore year, so can someone (PLEASE) post what they would expect the approximate ppm values would be for each hydrogen (and whether its singlet, doublet, or triplet, or quad)?? This would help me SO much.

I promise, you're not doing my homework for me, I just know that I'm off, and I have no way of telling how off I am.

The molecules are attached, and the hydrogens already labeled by letter.

I can't look at your picture and type a reply at the same time, so I'm not going to go by the letters. The protons next to the carbonyl will have ppms around 2.5. The one on the double bond will come around 5.5-6. The others will come around 1. Do you have an organic textbook? It will have a table with ranges that you can use to check where the different types of protons come; my estimates are just average values. If you don't have a book either, try finding a table on google.

You can determine the splitting using the n+1 rule, where n = the number of neighboring protons. (ex. if proton A has two neighbors, it will show up as a triplet.) The protons that have more than one set of neighbors will probably show up as multiplets on a real NMR, or you might possibly see them show up as a split of a split if your shimming is really good. (ex. if proton B is next to one proton A and one proton C, it will be a doublet of a doublet, 4 lines total.)

Good luck with your presentation.

 

[daysleeprx]

A = 0.96
B = 2.21
C = 1.16
D = 2.47
E = 4.75
F = 1.71
G = 1.80
H = 1.76

I'm not gonna work out the splitting, but you should really be able to do that yourself, the molecule doesn't look too bad.

 

[soccerchick747]

thank you both so much! The reason I'm having a prob with the NMR is because there are two isomers in the sample, so I'm seeing two of everything.

 

[QofQuimica]

There are two chiral centers, right? So you could have as many as four possible stereoisomers, and you will see any diastereomers individually in the NMR. Are they present in equal amounts? If not, you can use the integrations to gauge which peaks belong to each diastereomer. If so, you will have a harder time figuring that out, short of separating the diastereomers and taking separate NMRs of each one.