D2O shake

Menthol NMR - Glenn A. Facey, University of Ottawa NMR Facility BLOG, October 3, 2007. http //u-of-o-nmr-facility.blogspot. ca/2007/10/proton-nmr-assignment-tools-d2o-shake.html... 

Just to recap on the procedure, add a couple of drops of D2O to your solution, and shake vigorously for a few seconds. Note that with CDCI3 solutions, the best results are obtained by passing the resultant solution through an anhydrous sodium sulfate filter to remove as much emulsified D20 as possible. (Note... 

An unknown compound shows a weak molecular ion at m/z 87 in the mass spectrum, and the only large peak is at m/z 30. The IR spectrum follows. The NMR spectrum shows only three singlets one of area 9 at 8 0.9, one of area 2 at 81.0, and one of area 2 at 52.4. The singlet at 81.0 disappears on shaking with D2O. Determine the stmcture of the compound, and show the favorable fragmentation that accounts for the ion at m/z 30. 

The next spectrum, shown in Figure 9.12, is of a substance C3H7OCI. The spectrum consists of peaks at 53.79 (2H, triplet), 3.68 (2H, triplet), 2.85 (IH, singlet) and 2.01 (2H, quintet). The peak at 52.85 disappears on shaking the sample with D2O, so the hydrogen is probably attached to the oxygen atom. Thus we have... 

The next spectrum is the proton NMR after shaking phenol with acidic D2O. Most of the peaks have almost disappeared because the H atoms have been replaced with D. Only one signal remains the same size, and even that is simplified because it has lost any coupling to adjacent protons it may have had previously. 

Because the chemical shifts of O—H and N—H protons depend on the concentration and the solvent, it is often difficult to tell whether or not a given peak corresponds to one of these types of protons. We can use proton exchange to identify their NMR signals by shaking the sample with an excess of deuterium oxide, D2O. Any... 

When a second NMR spectrum is recorded (after shaking with D2O), the signals from any exchangeable protons are either absent or much less intense. 

Since we cannot be certain that the 52.4 absorption is actually a hydroxyl group, we might consider shaking the sample with D2O. If the 2.4 ppm absorption represents a hydroxyl group, it will shrink or vanish after shaking with D2O.)... 

A student found an old bottle labeled thymol on the stockroom shelf. After noticing a pleasant odor, she obtained the following mass, IR, and NMR spectra. The NMR peak at 54.8 disappears on shaking with D2O. Propose a structure for thymol, and show how your structure is consistent with the spectra. Propose a fragmentation to explain the MS peak at tn/z 135, and show why the resulting ion is relatively stable. 

Another similarity between O—H and N — H protons is their failure, in many cases, to show spin spin splitting. In some samples, N—H protons exchange from one molecule to another at a rate that is faster than the time scale of the NMR experiment, and the N—H protons fail to show magnetic coupling. Sometimes the N — H protons of a very pure amine will show clean splitting, but these cases are rare. More commonly, the N—H protons appear as broad peaks. A broad peak should arouse suspicion of N — H protons. As with O—H protons, an absorption of N—H protons decreases or disappears after shaking the sample with D2O. 

The following spectra for A and B correspond to two structural isomers. The NMR singlet at 51.16 in spectrum A disappears when the sample is shaken with DiO. The singlet at S0.6 ppm in the spectrum of B disappears on shaking with D2O. Propose structures for these isomers, and show how your structures correspond to the spectra Show what cleavage is responsible for the base peak at m/z 44 in the mass spectrum of A and the prominent peak at m/z 58 in the mass spectrum of B. 

On occasion, one may use the rapid exchange of an alcohol as a method for identifying the —OH absorption. In this method, a drop of D2O is placed in the NMR tube containing the alcohol solution. After shaking the sample and sitting for a few minutes, the —OH hydrogen is replaced by deuterium, causing it to disappear from the spectrum (or to have its intensity reduced). 

Note any broad single resonances, which are evidence of labile protons from alcohols, phenols, acids and amines that can undergo slow exchange with other labile protons. Comparison of the spectrum with another after shaking the sample with a few drops of D2O will confirm the presence of an exchangeable proton by the disappearance of its resonance signal and the appearance of another at 4.7 d/ppm due to HOD. 

The CH2 protons in propanamide (Fig. 7) are deshielded by the adjacent carbonyl group and couple with the CHj protons to give an A Xj quartet and triplet. The resonance of the amine protons at 6.4 5/ppm is broadened by proton-proton exchange and could be removed by shaking with D2O. The integral ratios are 2 2 3. 

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