Organic chem ques: interpretation of NMR

[Sonya]

Hi,

I'm having consistently trouble with these kinds of question, and absolutely can not find info on it. (my o-chem text book doesn't seem to cover it, and the berkeley review book makes absolutely no sense).

What i am asking for is a link to sources you may know explaining this stuff.
Q: What besides the basics can we interpret from an NMR?

"The basics" (what i already understand):

The value of the shift indicates the functional groups.
The number of peaks = sum (# of hydrogens on all adjacent carbons) + 1

There is something more i think we need to know... though i'm not sure what (because I problems demanding it). Specifically, there is something about a cis vs. trans bond and the difference in the splitting. I think it is related to the distance between peaks in a single group of peaks. I think, somehow, from an NMR I should be able to tell if a double bond is cis or trans.... i have no idea how.

More generally, what does the distance between peaks mean.

I'm doubting this is general expected organic knowledge, because it seems so esoteric. but, it keeps coming up as mistake for me... so I suppose i should learn it.

Again, links, or explanations appreciated.

Sonya

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[Macken]

This may not be the answer you're looking for, but I gotta tell you that you'll probably only see one (at the VERY most, two) question related to spectral analysis on the MCAT.

Spectrometry was always a terrible weak point for me. I have to confess that I never got it, wasn't interested in it, and didn't study it. You won't see enough of it on the MCAT to get stressed about it.

Hope this helps.

 

[deleted9493]

The distance between the peaks is the coupling constant (J). In the proton NMR this is useful in determining the connectivities of atoms in a molecule. This is the case because hydrogens which split each other (are adjacent) have equivalent coupling constants. In other words, if you have a triplet adjacent to a quartet....their coupling constants will be equal. So...for example...
CH3-CH2-X The methyl hydrogens will be represented by a triplet (3 peaks....given by 1 + # of hydrogens on adjacent carbon) and the methylene hydrogens by a quartet (4 peaks). The spacing between the individual peaks of the quartet and the individual peaks of the triplet will all be equal.

Yeah, cis hydrogens do have different coupling constants than trans hydrogens do. I'm sorry, I don't remember the exact numbers...I think it's like 12-18 Hz for trans....and 5-12Hz for cis. I could be wrong though and have those switched around.....

Sorry, I don't have any web resources that I know of that would help you here.

 

[woolie]

I love Organic Chemistry and so for me it was a real passion but my sense is that the MCAT asks for basic things about identifying the alkanes (methylenes), alcohols, aldehydes, carboxylic acids, etc and the big ticket items for the IR too like alcohols, carboxylic peaks. I do not think they would ask for amine peaks on IR, or something really rough.

What helped me learning this stuff is that I worked through so many spectroscopic images that they became very intuitive for me.

If it's something you feel you have time for, can you spend some time on a good website looking at tons of examples?

I didn't think the MCAT asked about the cis/trans splitting but I could be wrong ...

I think Columbia's Chemistry Dept has good links for this stuff.

 

[deleted9493]

The stuff about coupling constants that I wrote about in the previous passage is beyond mcat organic. I'm quite sure that basic knowledge of splitting patterns and integration sketches (the number of hydrogens...based on relative areas under the peaks) is necessary but that's probably about it for proton NMR.

 

[Sonya]

ok, sounds good. I suspected that stuff was way out there for mcat. but, maybe other people's introductory organic was different.

I don't think it's practice i need, i had a ton of practice with NMR in organic , and was pretty good with it. I can look up the coupling constant of cis/trans, and leave it at that. (I suspect learning something complicated will be a waste and probably get me more confused). anyway, soon enough i'll get to the AAMC test, and I can see if it still comes up.

Thanks,
Sonya

 

[Mudd]

The fact that you have that question shows that you have a baseline amount of knowledge that cis and trans alkenes generate different NMR spectra. That's plenty.

The only time I've heard of something like that showing up was the very first year I taught/tutored. They had a passage on trans-retinal and cis-retinal and wanted to know what 1HNMR feature could distinguishing the two structures. The vinylic J-values was the best answer (I think... based on the choices my tutee remembered).

If you have any extra room in your memory, trans coupling constants (J-values) are larger than cis coupling constants (J-values). That's probably more than you'll need to know.

If I remember right, they had a question on why a singlet had no J-value, or something like that. Other than that, I don't recall anyone ever asking me (after the MCAT anyway) what a J-value was.