INTRODUCTION

TECHNICAL BACKGROUND

Acetone

Acetone is miscible with water, but there is no evidence that it changes the structure or type reactivity of acetone to a large extent. However, if the acetone was diluted enough, it is possible that it will be less reactive. If this was the case, it would show up in the nuclear magnetic resonance (NMR) spectra. Acetone is incompatible with chloroform, the solvent used. This means that when these two compounds are placed in NMR together, they will not react, and the acetone spectra can be seen without reactive interference. Acetone has a relatively high evaporation rate when compared to other organic solvents. This is why the acetone has to be stored at cold temperatures and in tightly sealed containers. Acetone does take part in a free-radical reaction called photolysis when exposed to light, the by-products of that reaction are methane and ethane. There have been many studies to better understand how long the sample would have to be exposed to light in order to break down. The studies have shown that if exposed to light that the half-life of acetone is 22 days. Acetone obtained directly from the supplier has no shelf life or half-life as long as it is stored in dark and tightly sealed containers under sub-zero conditions.

Structures

Figure 1. (a) acetaldehyde structure; (b) acetone structure

The transformation from a ketone to an aldehyde changes the molecular formula from a methyl group to hydrogen. Acetaldehyde (Fig. 1a) is more reactive than acetone (Fig. 1b) because it has less steric hindrance than acetone.

In addition to having more steric hindrance, the methyl groups also decrease the polarization of the molecule, which makes it less reactive as well. Even though this seems like a drastic change, it does not seem to alter the chemistry that takes place during the plastination process. Acetaldehyde has a lower melting point (-120℃) and boiling point (20℃) than acetone (Mp -95℃ and Bp 56℃). Since the acetone portion of the plastination process is under freezing conditions (20 - 30℃), the aldol transformation does not further change the chemistry of the plastination process.

NMR

For the purposes of this experiment, Fourier-Transform Nuclear Magnetic Resonance (FT-NMR) was used to evaluate the acetone samples for signs of acetaldehyde. FT-NMR is considered a pulsed method of analysis and occurs when the sample is pulsed with bands of RF radiation, which allows the nuclei to become excited and spin at different rates based on what else is attached to the molecule. The spectra that are then produced from the experimentation are ‘Absorbance vs ppm’. Nuclear Magnetic Resonance (NMR) essentially works by placing a strong magnetic field on the sample and the nuclei of some atoms behave like small magnets. These nuclei resonate at their specific frequencies (similar to a tuning fork). These frequencies are measured and converted using a Fourier transform into an NMR spectrum to display the frequencies as peaks on a graph. The intensity of the signal, or the height of these peaks, represents the number of nuclei resonating at that specific frequency (more nuclei = higher intensity). The tone, or value, of each frequency, gives information about the surroundings or the neighboring atoms relative to their positions. By examining these peaks, it is possible to determine the 3D structure of the molecule, or, in our case, see if there are any impurities in the sample. The different peaks on the spectra are chemical shifts, which are the small magnetic fields produced when electrons move around the nuclei. An upfield shift correlates with peaks on the lower end of the ppm axis and a downfield shift is the peaks on the higher end of the x-axis. Proton NMR (HNMR) was used because it is fast and produces sharp and narrow chemical shifts. Deshielding occurs when electron density is removed from a molecule, which causes it to experience a downfield shift. Other types of NMR include common isotopes, the most popular of which are H1 and C13. Carbon 13 NMR would not have been helpful in this experiment because the atoms that were thought to have been affected were hydrogen. Also, in CNMR spectra, the aldehyde and acetone peaks are similar, and less distinct than shown in the HNMR spectra.

NMR Analysis

Nuclear Magnetic Resonance was used to analyze the samples for evidence of acetaldehyde. Five samples were run and evaluated using hydrogen NMR. The samples were as follows: pure acetone, acetone after use, acetone after recycling, acetone after molecular sieves, and acetone after being stored for 6 months. The spectra produced showed no evidence of acetaldehyde peaks, only acetone with minimal contaminants. The contaminant was mainly water, which was not shown in the molecular sieve spectra or the acetone sitting after 6 months.

Acetone is a common dehydrant used to remove water and fat from organic tissue. After the dehydration process, the acetone can be distilled and recycled to purify it for further use. The process of recycling and distilling separates acetone and water to 96-98% purity. Molecular sieves can then be used to further purify to a level of 99.5-99.9% by removing excess water (Baptista et al., 2013), though Daniel Seger Pedersen’s blog post “Anhydrous Solvents Part 3: Acetone and Molecular Sieves- Bad idea!” theorized that molecular sieves can cause acetone to change into acetaldehydes and many other compounds. As acetone prices are rising, and as society is moving towards greener chemistry methods, it is important to investigate if molecular sieves are a worthwhile method for the purpose of plastination. These potential side products produced with the use of molecular sieves could potentially contaminate and interfere with the plastination process.

MATERIALS AND METHODS

Acetone Recycling Process

The process of recycling the acetone through the use of an acetone recycler and molecular sieves was as described by Baptista et al. (2013) in their article, “Upgrading Recycled Acetone to 100% with Molecular Sieves”.

Sample Preparation

Each of the samples was prepared according to standard NMR protocol. The acetone samples were stored at -18°C in brown glass containers until use. The samples were prepared at room temperature (25°C). The following solutions were prepared in separate 5 mm NMR tubes: a standard solution of pure acetone of about 6 μl in 1 mL CDCl3 (deuterated chloroform); 6 μl of acetone after use (99% purity) in 1 mL CDCl3, 6 μl of acetone before sieves (99% purity), 6 μl of acetone after recycling Penta (99.8% purity) in 1 mL CDCl3, and 6 μl of acetone after recycling after storage for 6 months (98% purity) in 1 mL CDCl3.

NMR Data Acquisition and Processing

The 1H NMR spectra for each sample were acquired with Varian Unity Inova600 with a Penta probe. The standard proton parameters for each of the samples were applied: relaxation delay of 1.00 sec, Pulse 29.8 degrees, the spectral width of 8000.0 Hz, with 16 repetitions, line broadening 0.6 Hz, FT size 131072, and a total acquisition time of 1 min and 12 seconds.

RESULTS

Results Format

Note that the triplet peak between δH 1.9 to 2.2 ppm corresponds to the protons of the methyl (CH3) groups on acetone. The methyl groups are symmetrically equivalent in the molecule, so this peak is only represented once on the spectra.

Figure 2. Pure 1H NMR (CDCl3, 600 MHz): δ 1.997-2.209 (6H)	Figure 3. 99% after use 1H NMR (CDCl3, 600 MHz): δ 2.015-2.226 (6H)
Figure 4. After recycling before sieves 1H NMR (CDCl3, 600 MHz): δ 1.937-2.246 (6H)	Figure 5. After recycling and sieves 1H NMR (CDCl3, 600 MHz): δ 2.133 (6H)
Figure 6. After storage 1H NMR (CDCl3, 600 MHz): δ 1.998-2.210 (6H)

DISCUSSION

The results shown in the figures above correlate with NMRs for acetone and show no downfield acetaldehyde peak - the typical acetaldehyde peak is shown in Figure 7. The spectra shown above that are prior to desiccation by the molecular sieves show minimal water contaminant peaks around 1 ppm. The typical pure water NMR spectrum is shown in Figure 9. The expected NMR spectrum for pure acetone is then displayed in Figure 8. Comparing these, it is clear that there is a minimal contaminant, and that is why the molecular sieves are a good option to ensure better purification of acetone for the use of plastination.

Figure 7. Typical acetaldehyde NMR peak

Figure 8. Typical acetone NMR peak

Figure 9. Typical water NMR peak

After determining that the use of molecular sieves has no effect on the structure of acetone via NMR, there are other instruments that can be used to verify that there is no conversion to acetaldehyde. Mass spectrometry can be used in the future to ensure that the structure of acetone does not convert. In mass spec, there is an acetaldehyde peak around 44 and an acetone peak around 58. Infrared (IR) spectroscopy can be used as well. IR spectroscopy gives insight into the functional groups associated with the molecule and will show different groupings for the hydrogen on the acetaldehyde and the methyl group on the acetone. The IR spectra should only show the ketone peak and no aldehyde peak. Different samples of acetone could be used from different time periods, to help determine if the sample degrades when it is not used within a certain time period. We plan to test samples that have been stored within 6 months to a year.  From a plastination perspective, it could be determined how pure the acetone needs to be in order to be affected within the process.

CONCLUSION

Molecular sieves do not cause acetone to participate in side-reactions that would cause it to transform into acetaldehyde. As shown in the results, there are not any substantial contaminants in the samples after the dual use of an acetone recycler and molecular sieves. The spectra further support the use of molecular sieves in acetone recycling.