ORGANIC cHEMISTRY - v
MSCM313
Experiment 5
Aim of the Experiment
To identify the structure of organic compounds using given spectral data.
Principle
There are a number of possible approaches that you may follow while solving the problems of spectral organic structures. There are no “right” ways of solving them. In general, however, you should first try to gain an overall impression by looking at the gross features of the spectra provided in the problem. As you do so, you will observe evidence for pieces of the structure. Once you have identified pieces, you can assemble them and test against each of the spectra the validity of the structure you have assembled.
1. Mass Spectrum. You should be able to use the mass spectrum to obtain a molecular formula by performing the Rule of Thirteen calculation on the molecular ion peak (M) labeled on the spectrum. In most cases, you will need to convert the hydrocarbon formula to one containing a functional group. For example, you may observe a carbonyl group in the infrared spectrum or 13C spectrum. Make appropriate adjustments to the hydrocarbon formula so that it fits the spectroscopic evidence. When the mass spectrum is not provided in the problem, you will be given the molecular formula. Some of the labeled fragment peaks may provide excellent evidence for the presence of a particular feature in the compound being analyzed.
2. Infrared Spectrum. The infrared spectrum provides some idea of the functional group or groups that are present or absent. Look first at the left-hand side of the spectrum to identify functional groups such as O-H, N-H, CN, C=O, NO2, and aromatic rings. Ignore C-H stretching bands during this first “glance” at the spectrum as well as the right hand side of the spectrum. Determine the type of C=O group you have and also check to see if there is conjugation with a double bond or aromatic ring. Remember that you can often determine the substitution patterns on alkenes and aromatic rings by using the out-of-plane bending bands. A complete analysis of the infrared spectrum is seldom necessary.
3. Proton NMR Spectrum. The proton (1H) NMR spectrum gives information on the numbers and types of hydrogen atoms attached to the carbon skeleton.. You will need to determine the integral ratios for the protons by using the integral traces shown. In most cases, it is not easy to see the splitting patterns of multiplets in the full 300-MHz spectrum. So to simplify this, multiplicities of peaks as doublet, triplet, quartet, quintet, and sextet are mentioned on the full spectrum. Singlets are usually easy to see, and they have not been labeled. Many problems have been provided with proton expansions. When expansions are provided, Hertz values have been shown so that you can calculate the coupling constants. Often, the magnitude of the proton coupling constants will help you to assign structural features to the compound such as the relative position of hydrogen atoms in alkenes (cis/trans isomers).
4. Carbon NMR Spectra. The carbon (13C) NMR spectrum indicates the total number of nonequivalent carbon atoms in the molecule. In some cases, because of symmetry, carbon atoms may have identical chemical shifts. In this case, the total number of carbons is less than that found in the molecular formula. Commonly, sp carbon atoms appear to the upfield (right) side of the CDCl3 solvent peak, while the sp carbon atoms in an alkene or in an aromatic ring appear to the left of the solvent peak. Carbon atoms in a C=O group appear furthest to the left in a carbon spectrum. You should first look on the left-hand side of the carbon spectrum to see if you can identify potential carbonyl groups.
5. DEPT-135 and DEPT-90 Spectra. In some cases, the problems list information that can provide valuable information on the types of carbon atoms present in the unknown compound. Positive and negative values of DEPT spectra are very important in determining the type of carbon atoms. (Quaternary, CH2 etc)
6. Ultraviolet/Visible Spectrum. The ultraviolet spectrum becomes useful when unsaturation is present in a molecule.
7. Determining a Final Structure. A complete analysis of the information provided in the problems should lead to a unique structure for the unknown compound.
Materials required
Spectral Chart
Procedure
In each of these problems you are given the IR, NMR, and molecular formula. Using this information, your task is to determine the structure of the compound. The best approach for spectroscopy problems is the following steps:
Calculate the degree of unsaturation to limit the number of possible structures. Remember, each degree of unsaturation is a ring or pi bond (likely an alkene or carbonyl). An alkyne has two degrees of unsaturation (2 pi bonds), and an aromatic ring has four (3 pi bonds plus a ring.) Although there's no guarantee, if your structure has more than four degrees of unsaturation it's quite likely to have an aromatic ring.
Look at the IR absorption bands at wave numbers above 1500 cm-1 to determine what functional groups are likely in the compound. Remember that these functional groups must be consistent with the degree of unsaturation.
Look at the NMR to determine the connectivity of the compound. If you can't figure out the entire structure at once, it helps to come up with fragments of the molecule that you can stick together into larger and larger groups until you have the entire structure.
Approach this as a puzzle - it can be fun!
Result
The structure of of organic compound is.....
Questions
What is HDI?
Developed by
Dr. Parin Kanaiya & Dr. Gourav Upadhyay,
Assistant Professor, Chemistry
gourav.upadhyay@gsfcuniversity.ac.in