[bri3432]

I have to identify the structure of an unknown compound and I am having a lot of trouble figuring out what it is. I have determined that the compound is an amino acid, I have the IR spectrum, the H NMR, and the decomposition point, which is somewhere between 220-240 °C. If anyone could provide some insight that would be greatly appreciated!

[kriggy]

Why didnt you post the NMR spectra? Should be rather easy to solve the structure compared to IR.

How did you conclude it is amino acid?

Decomposition point? You mean melting point? If yes then the range is way too wide to give any information.

Show us your line of thought so far and we can help you

[wildfyr]

This sample is either full of water or the IR was backgrounded poorly. I wouldnt trust anything except the 3 peaks near 1650. It looks like it isnt an aryl containing amino acid. And NMR would be by far the better analytical method

[bri3432]

Here is the HNMR. The compound tested positive in both the ninhydrin and the copper complex tests. I gave a broad range for the decomposition point to include all possible compounds.

[bri3432]

That wasn't the M.P. range, The first time I took the Melting point I got a range between 220-223 °C. I then retook it and it was about 10 °C higher around 231-234 °C. That is why I gave such a broad range of possible compounds.

[wildfyr]

I got it. 99% sure. I think the NMR IDs it on its own. The MP could be worthless (and low) because all these amino acids pick up water. The IR bears out the sample being quite wet. The characteristic peak of the functional group is something that requires an eagle eye in the fingerprint region, the so IR wouldn't have helped anyways.

There is only 1 amino acid that gives 1:2:2:2 (the 4 is just two similar signals on top of one another). I checked on SDBS (http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi) and it matches. Look at https://upload.wikimedia.org/wikipedia/commons/a/a9/Amino_Acids.svg for a list of the aminoacids and you should be able to tease it out.

Its unfair to give students such a wet sample but that's how the cookie crumbles.

Give it a little more work with this extra data, and if you still can't get it I'll try to help you along. I would ignore trying to include any exchangeable protons like on amines/carboxylic acids/guanidine/thiol when looking at the NMR.

[bri3432]

I have been trying to narrow down the list of compounds, and I still haven't been able to figure out which one should give a 1:2:2:2. I have also been trying to cross reference the NMR's of each amino acid with the NMR spectra I have, and I cant find any that are similar...

[wildfyr]

Which amino acids would have 4 distinct sets of protons, 3 sets of 2, and one set of 1? All are in the alkyl region. Only 2 amino acid have this characteristic. This tells you that you can rule out anything that has a CH3 group right away. It also tells you that the 3 sets of two are almost certainly all in sequence in a linear chain. An alkyl ring containing material would have more signals since the top and bottom of the ring have different electronic environments since you are working with a chiral compound (I assume the L version of the amino acid right?). That narrows it down to 1. I checked and the MP also doesn't match one of two most obvious suspects as well if water is depressing the MP.

Also, I assume this NMR was in D2O? Don't forget to discount the peak for water in D2O.

If you are brute forcing this by comparing your NMR to all possible amino acids via the SDBS data base I'm not sure why you haven't found it yet. I hope you don't take this pathway, its not very useful for actually learning anything.

[bri3432]

Okay thank you. I have started analyzing the compounds more instead of looking at the NMR's of amino acids and realize that the only possible compoud would be L-Arginine. It has 3 CH2 carbons in a chain and a C-H, which gives the singlet. Is this correct?

[wildfyr]

I agree  . and SDBS does too.

[bri3432]

Thanks for all of the help again. I appreciate you taking the time to assist me. I have a Lab final exam coming up which will have lots of NMR and spectral analysis. What would you suggest as the best way to really get it down well in the next week or so?

[wildfyr]

Find more practice problems with unknowns. See if this books http://www.wiley.com/WileyCDA/WileyTitle/productCd-EHEP001779.html is in your library (any edition). it has tons of practice problems in this realm, though some will be out of your league.