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Solubility book (3rd Edn)
Proton NMR Integral Ratio Comparision using 4-Chlorobenzaldehyde in Chloroform-
To evaluate the integral ratio precision using of proton NMR for solubility determination in non-aqueous solvents using chloroform-d with shimming, 60 sec relaxation time, and 65 536 data points.
Materials and Instrumentation
A JEOL 300-MHz high resolution 5-mm probe NMR was used with no spin and no lock at 22.4 °C.
Shimming was performed followed by three NMR runs while the spin was temporarily working at 12-13 Hz with
2.0600 g reagent grade non-deuterated chloroform in an NMR tube.
Mass determinations were performed using a 0.1 mg accuracy Sortorius balance. Shimming was performed followed by three NMR runs while the spin was temporarily working at 12-13 Hz with
2.0600 g reagent grade non-deuterated chloroform in an NMR tube
. Sonication was performed with a Branson 50/60 Hz Ultrasonic Cleaner.
Chloroform (1.9228 g; 1.2992 mL; 16.1 mmol) and 4-chlorobenzaldehyde (14.7 mg; 9.9 μL; 105 μmol) were added in various increments to a 5 mm NMR tube until solute precipitated and mixed by 30 min of sonication. This solution was then analyzed by 1H NMR.
Identification of Peaks on Spectrum 297:
Raw JCAMP-DX for Spectrum 295
Raw JCAMP-DX for Spectrum 297
Printed Spectra Integrated with Hd set as norm=1:
Table 1: Integral Comparison for 4-Chlorobenzaldehyde
While JEOL was able to integrate the spectra, the exported JDX files were also processed in ACD/NMR Processor 12.01 and converted in JSpecView to XY format to be integrated by Dr. Lang's intXY.php. The area beneath the signals from protons Ha, Hb, and Hc of 4-chlorobenzaldehyde were compared using these two methods of integration. Table 1 shows that JEOL performed more consistent integration within JEOL (1.4525% Average RSD) than the intXY.php performed on the converted exported spectra (2.4776% Average RSD). Overall, both methods consistently integrated the spectra of 4-chlorobenzaldehyde.
Since 4-chlorobenzaldehyde was added until precipitation, the solution never seemed to reach saturation even after sonication. The most saturated solution reached was less than 0.08
. This is even less than the 0.81
solution formed in
clearly shows the identity of the compound to be 4-chlorobenzaldehyde. However, the solid resin at the bottom of the solute bottle may indicate some form of polymerization that would leave the majority of the 4-chlorobenzaldehyde that was added insoluble. Maybe a fractionation procedure could be preformed to purify the soluble monomers if this was the case.
Note: When converting spectra using ACD/NMR Processor 12.01, export as JDX with Fixed Length to preserve shifts.
JEOL integrated the original spectra with 1.45 Average %RSD and intXY.php integrated the converted spectra with 2.48 Average %RSD. The insolubility of 4-chlorobenzaldehyde may be attributed to some form of polymerization impurities in the solute.
15.40 - Photos taken of equipment and reagents.
16.15 - Weighed tube.
16.20 - Added chloroform-
16.30 - Shimmed sample.
17.00 - NMR Run
17.45 - Weighed sample.
17.49 - Added 4-chlorobenzaldehyde and weighed.
17.50 - Sonicated for 30 min (sample felt warm afterwards with condensation inside of tube).
NMR Run 295
18.56 - Weighed sample.
18.57 - Added 4-chlorobenzaldehyde and weighed.
19.00 - Sonicated for 30 min.
19.10 - Shimmed sample.
NMR Run 296
19.57 - Weighed sample.
20.00 - Added 4-chlorobenzaldehyde and weighed.
20.03 - Sonicated for 30 min.
20.33 - Shimmed sample.
NMR Run 297
16.00 - Solid residue observed in NMR tube.
20.30 - Photo of solid residue taken (
Lin, M., Tesconi, M., Tischler, M., Use of H NMR to Facilitate Solubility Measurement for Drug Discovery Compounds, International Journal of Pharmaceutics (2008), doi:10.1016/j.ijpharm.2008.10.038
The author wishes to thank Dr. Robert Stewart for his advice throughout the course of the experiment and for the use of the NMR and other equipment, Dr. Jean-Claude Bradley for setting up the server for the interactive NMR spectra, Dr. Antony Williams for his advice on improving the H NMR peaks and providing the reference listed, Dr. Hal Reed for the use of the camera, and Dr. Andrew Lang for recommending the posting of pictures of the experiment.
It was expected that the integral ratios of the last two samples would be close to equal. With only two samples, it was found that there is a 95% chance of future samples to have intregral ratios of (2.36
0.39) * 10^(-3). Solid residue noticed after 68 hours (
The 95% chance of future samples to have intregral ratios of
(2.36 ± 0.39)_*_10^(-3)
was much better than the
(1.04 ± 0.27) * 10^(-2)
[Can you explain this a little more explicitly? Are you saying that one of the aromatic H's is integrating 2.36:1 to the aldehyde? If so which peaks specifically? JCB]
The ratio was calculated as follows: (grams solute/(integral of Ha/integrate of Hd(1))). Ha and Hd refer to the corresponding peaks in the
. The number of samples needs to be increased to confirm the integral ratios. Increasing the number of scans may further increase the precision. Sonication might have warmed the sample after 30 min. A thermometer needs to be placed in the sonicator to determine how much the temperature increased. Condensation was observed in the tube and the sample lost
after sonication. As the boiling point of chloroform-
, the sonication most likely did not heat the sample to boiling. As the solubility would change, any increase in heat during sonication needs to be avoided. Adding cool water to the warm water in the sonicator at regular intervals may counter the heating. Vortexing in a separate container rather than sonicating could be another way to avoid the heating. However, unless a syringe is used, it is likely that some of the volatile solvent will evaporate during the transfer to the NMR tube.
[You can use a Pasteur pipette to transfer chloroform but you must be quick. JCB]
The 4-chlorobenzaldehyde coming out of solution does not seem to be attributed to not enough time sonicating, since it had been sonicated for a total of 90 minutes. At the low concentration used, it was expected to dissolve quickly. It might have dissolved and oxidized forming the residue.
[If it had oxidized how would that change the NMR? JCB]
, the 4-chlorobenzaldehyde came out of solution less than an hour after sonication.
The oxidation product would most likely be 4-chlorobenzoic acid. The problem is that the
for 4-chlorobenzoic acid is pretty much the same as 4-chlorobenzaldehyde without the aldehyde peak at ~10 ppm. Both show a doublet of doublets between 7.5 and 8.0 ppm. Also, the chloroform peak's downfield lorenzian tail is in that region. Since there is an aldehyde peak at ~10 ppm, some 4-chlorobenzaldehyde was present. However, the 4-chlorobenzoic acid could change the solubility.
The deuterated chloroform, shimming, and increased relaxation time and number of data points increased the precision, as the relative standard deviation fell from
. However, until solubility determinations are performed, accuracy cannot be acounted for.
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