These are the instructions as given:
1. Begin by using the melting point of your purified product to help determine the major product and, more importantly, the halobenzene starting material. At this point, you may be able to compare your crude NMR spectrum to the given NMR spectra and confirm the identification from the MP data, but the following steps will solidify your identification. You will also have to complete the following exercise to calculate isomer and product percentages.
2. The dinitrated product will contain the most deshielded proton of all the possible products. This highly deshielded proton is ortho to both of the nitro groups (HA in the above example) and will have the highest chemical shift. Your chemical shift value for this peak, if present, should agree to within 0.05 ppm of the given spectra. It appears as a doublet of doublets meaning that HA is split by both HB and HC, but split differently by each. Protons HA and HB are meta to each other, while HA and HC are para to each other. This difference in relative regiochemistry generates the complex splitting pattern because the coupling constants (J) are different for meta (J = 1-3 Hz) and para (J = 0-1 Hz). The splitting tree below shows how different coupling constants can generate complex splitting, a little more clearly (hopefully). Do not confuse a doublet of doublets with a quartet, they are very different, and make sure you aren't lazy with your terminology…just because there are four peaks does not make it a quartet. The para coupling constant is very small and may not be observed at all. If this is the case, JA-C = 0 Hz and the peak appears as only a doublet. Record the integral value for HA. Referring back to the given spectrum of the 2,4-dinitro compound, find and mark the peaks for HB and HC in your spectrum. Their integrals should be the same as HA (1:1:1). Again, if a signal is dominated and overlapped by another it cannot be marked very accurately.
3. The next step is to seek and mark the downfield (higher ppm) signal for the para[1]halonitrobenzene. This signal represents the two H atoms closest to the nitro group and will appear as another doublet of doublets. The protons will be strongly coupled/split by their ortho neighbors, but will also be split by the non-equivalent para proton. Again, this para coupling is very weak and may not be observed, in which case the peak appears as a widely spaced doublet. The identification and chemical shift of this signal should be checked against the given spectra. Record the integral value for this peak, but realize it represents two equivalent protons, unlike the integrals from step 2, which only Nitration of Halobenzenes 6 represented one proton each. You will account for this ratio disparity later. From this doublet (or doublet of doublets), it is easy to spot the signal for the other pair of equivalent protons. Since the coupling is the same, the peak looks like a mirror image, only more upfield because the protons are not as deshielded. However, for para[1]iodonitrobenzene, only one peak appears because all four protons are almost exactly at the same chemical shift.
4. Identifying and marking the signals for protons in the ortho-halonitrobenzene isomer is more difficult, but it is often the last molecule to be taken into account. The most downfield proton, representing the proton ortho to the nitro group can be located with the given spectra as a guide. It appears as another doublet of doublets. Record the integral value for this peak. This proton is likely to be the only clearly discernible peak for ortho[1]halonitrobenzene. The others are more upfield and are often overlapped with each other and with signals from other molecules. Notice for ortho-iodonitrobenzene, how the doublet of doublets begins to overlap, giving the appearance of a split triplet.
5. For spectra having signals for the halobenzene starting material, signals for the 2,4-dinitro product are not likely to be found. Because the halobenzene has no nitro groups, all the signals will appear upfield from the ortho-nitro proton of step 4. Again the given spectra will be important in locating the signals for the unreacted halobenzene starting material. Unfortunately, these signals are usually overlapped on each other.
Here is my NMR:
- Please Help Me Connect My Compound Nitro Iodobenzene Both Para And Ortho Should Be Present To My Proton Nmr From My Ni 1 (60.43 KiB) Viewed 51 times