Identification of Unknowns
The purpose of this laboratory exercise is to allow you to use what you have learned, both in class and in laboratory, about the chemistry and properties of organic compounds. We will be solving the puzzle of identifying an unknown from among the following functional groups: alcohols, alkenes, alkynes, nitriles, aldehydes, ketones, acids, and esters. You must also indicate whether the compound is aromatic or not. In the past, functional groups were identified using chemical tests, and specific assignments of unknown compounds were made by comparison of physical properties such as mp and bp to known standards. Also, compounds could be derivatized and those derivatives compared to known standards. These tests and derivatizations, while historically interesting, are no longer commonly used for identification purposes. The modern organic chemist relies heavily on spectroscopy for assignment of structure. The most useful types of spectroscopy for the organic chemist are NMR, IR and MS. Of these, NMR gives the most information, providing a “map” of the molecule as well as information about functional groups. IR allows assignment of functional groups, and MS allows confirmation of structural assignment as well as providing information about regioisomers.
Overview of Procedure:
You will be assigned an unknown. Spectral information (IR, NMR and mass spec) can be found in the information for spectroscopy folder on D2L for your unknown. You should then be able to assign the structure of the molecules. Table 1 gives a list of compounds and all of the unknowns can be found on that list.
Report and Grading
A large percentage of your grade will depend upon the process of how you arrived at identification of each of your unique unknowns. All possible compounds are listed in Table 1, which is arranged according to functional groups. The rest of the grade will come from your approach to the problem: did you identify the correct functional group(s), did you assign signals correctly in the NMR, does the MS match? You should turn in all spectra, with relevant absorbances identified in the IR, all peaks assigned in the NMR, and the molecular ion in the MS. REMEMBER : assign specific signals for your proposed compound , not just generic ranges of absorbances. For example if you think there is a methoxy group (OCH3), do not just write “CHX” next to the signal. You need to draw the structure on the spectrum, and CLEARLY identify which H’s belong to which signal (or C’s for the 13C spectrum) . In the IR, you should be able to clearly label at least two absorbances for YOUR structure. In the mass spectrum, you should be able to identify the molecular ion, and hence the MW of the compound.
Table 1. Possible Unknowns and Boiling or Melting points
|Compound Name||Boiling Point (oC)||Meltng Point (oC)|
|NITRO and Amides|
|trans chrysanthemic acid||54|
|trans cinnamic acid||133-134|
|acetyl salicylic acid||136-140|
NMR Interpretation Hints
The value on the x axis (chemical shift) for each peak or group of peaks is a good indicator of the proton environment in a molecule. Apply the usual rules to interpreting 1H and 13C spectra (see textbook for a review).
Don’t forget that NMR solvents are deuterated but have a small percentage of extra protons that show up in NMR as well as TMS (reference at 0 ppm). For example in 1H spectra, residual protons in the solvent show up at 7.26 ppm (singlet, CHCl3), 2.49 ppm (septet, DMSO) and 3.5 (singlet, H2O). For 13C spectra, deuterium splits the CDCl3 carbon into a triplet at 77.00 ppm. Occasionally, peaks will be partially cut off on the high ppm (downfield) side of the spectrum in proton or carbon (your TA should be alert to this if it happens to you).
Your spectra will be integrated by DELTA, however you may need to manually figure out how many hydrogens are responsible for each signal. You can do this with a ruler, by measuring the height of the smallest integral, and setting that to one hydrogen for instance 5 mm may be the smallest height; then measure remaining integrals and divide by that number (in the previous example a signal with an integral that is 15 mm would equal three hydrogens, 15/5 = 3).
If the splitting (s, d, t, m, etc.) is not clear in your 1H spectrum, ask for instructions on how to expand the file. As you recall, multiplicity is useful for determining connectivity (n+1 rule). However, your COSY spectrum is also helpful in determining connectivity, since the off-diagonal peaks point out which protons are on adjacent carbons.
Count the number of unique carbons in the 13C NMR spectrum and get an idea about the environment of the carbons (e.g. aromatic, carbonyl, etc.). Remember that the DEPT spectrum will have NO quaternary carbons, CH/CH3 will point up and CH2 will point down. NOTE: the instrument doesn’t really “know” which signals are CH/CH3 and which are CH2 so it automatically phases the largest signal “up”. SOMETIMES the largest signal is actually a CH2, so the DEPT will be inverted. This usually occurs when there are long alkyl chains. Think about what makes sense…for instance if you see one “down” signal and many “up” signals in the alkyl region, you may have an inverted DEPT. Check with your instructor if you’re not sure.