12: Experiment The Magtrieve Oxidation Of 4-Chlorobenzyl Alcohol, A Solvent-Free Reaction Purpose: To oxidize 4-chlorobenzyl alcohol(primary) to an aldehyde with out over-oxidation to a carboxylic acid using Magtrieve. Discussion: From the H NMR spectra of the starting material, the hydrogens on the carbons adjacent to the chlorine are on the far left (7.220-7.304) due to the deshielding by chlorine. These peaks are split because of coupling with the other hydrogens on the ring. Those remaining two hydorgens form peaks at 4.557. These peaks are not as deshielded because of the electron density. On the right of the spectrum are peaks at 2.510 signaling the hydrogens on the carbon between the ring and alcohol group. From the 13C NMR of the 4-chlorobenzyl alcohol, the peak at 139.21 is the carbon connected to the chlorine. To the right of that, the peak at 133.29 is the carbons flanking the carbon with the chlorine substituent. At 128.25 and 128.62 are the remaining two chlorines without any substituent. The peaks at 76.81-77.32 signify the remaining carbon on the benzyl ring with the methanol. Finally, the last peak on the right is the carbon adjacent to the alcohol group. In the spectrum for 1H of 4-chlorobenzaldehyde, the peaks at 9.986 signal the hydrogen on the aldehyde group. The peaks on the far right represent the hydrogens on the carbons on both sides of the chlorine. The middle set of peaks at 7.619-7.835 is the hydrogens on the two remaining carbons on the benzene ring. In the 13C NMR of the aldehyde product, the peak at 190.87 marks the carbonyl carbon. The peak at 140.97 represents the carbon with the chlorine substituent. The following peak at 134.73 is the carbon on the ring with the attached carbonyl group. While the peaks on the right are the carbons flanking the chlorine (130.92) and the remaining carbons (129.47).
In this experiment, Magtrieve and 4-chlorobenzyl alcohol was mixed while they were in solid form. Upon heating in a sand bath, the alcohol melts and liquefies as it is oxidized by Magtrieve to form 4-chlorobenzaldehyde. Because the aldehyde has a low melting point and high vapor pressure, it evaporates and becomes a gas. However, upon contact with the cold watch glass, the gaseous aldehyde solidifies and forms crystals. The aldehyde crystals were found to weigh 0.113g (percent yield of 58.33%) and have a melting point of 43.7 - 46.1 C. Only 168mg of Magtrieve were needed in this experiment. However, an excess of 527.02 mg (6.275mmol) were used (0.695 used total). An excess of the reactant was necessary because only the surface layer of the CrO2 in each Magtrieve particle can react with the alcohol. Conclusion: By using an excess of Magtrieve with a primary alcohol, the oxidation will stop at the aldehyde and the desired product is easily separated due to its low melting point and high vapor pressure. Questions: 1. Explain the relative Rf values for the starting material and the product. When performing TLC with the starting and material product in a 60:40 hexane: ethylacetate solvent, two spots were visualized by UV light. The starting material had an Rf value of 0.688 while the product had an Rf value of 0.563. This is because the solvent is slightly more non-polar. Therefore, the less polar product did not travel as far as the more polar starting material. 2. Write the structure of the product that would be obtained if potassium dichromate in aqueous acid were used for oxidation.
3. Propose a simple experimental method that would determine which scenario is correct. To determine which scenario is correct, one could perform the same experiment, but instead, apply a vacuum to make the product evaporate more quickly. If making the products evaporate allows more crystals to form without continued heating, then one can conclude that the product had not completely evaporated form the reaction mixture after 30 minutes. However, if there is no difference, then, not all of the starting material had been oxidized after 30 monutes. Or to determine which scenario is correct, one can simply make use of TLC. 4. Explain why the vapor pressure of 4-chlorobenzaldehyde is greater than that of the alcohol. 4-chlorobenzaldehyde has the higher vapor pressure because it is less polar than the alcohol. The hydrogen bonding between the alcohol molecules creates a strong dipole that makes it more polar than the aldehyde. Because the aldehyde has weaker intermolecular forces, it tends to be in the vapor phase and had a greater vapor pressure. Compare and contrast the environmental consequences of the experimental procedure from the literature with the one in this experiment using Magtrieve. Title: Aldehydes from Primary Alcohols by Oxidation with Chromium Trioxide: Heptanal Authors: J. C. Collins and W. W. Hess Name: Organic Syntheses Journal Year: 1972 Volume:52 Page: 5 In this experiment, chromium trioxide is used as a non-acidic reagent for alcohol oxidations to prepare aldehydes. 5.
First, the chromium trioxide is mixed with pyridine to create dipyridine chromium (VI) oxide. This compound in anhydrous is mixed with 1- heptanal. However, the experimenters noted that the reaction of chromium trioxide with pyridine is extremely exothermic; the preparation should be conducted in a hood. Also the dipyridine chromium (VI) oxide should not contact the atmosphere because it is very hygroscopic. They also noted that pyridine added to chromium trioxide spontaneously ignites. However, unlike the laboratory manual, the article failed to mention the harmful affects of chromium (VI) reagents on humans if inhaled, and their inability to be recycled. Fortunately, the Magtrieve used in the experiment is a recyclable chromium oxidant and is not a suspected carcinogen. In addition, Magtrieve can be retrieved from the reaction using a magnet. Furthermore, unlike the experiment using chromium trioxide, Magtrieve does not need to be used with dichloromethan making it even more environmentally-friendly.