In this experiment, sodium hypochlorite (NaOCl) in acetic acid mixture was used to oxidize emdo-borneol (an alcohol) to camphor (a ketone). The product would then be purified by sublimation and then be analyzed by Infrared spectroscopy and melting point test.
Procedure and ObservationsPlease refer to the lab manual and the carbon copy attached.
Data and CalculationsWeight of endo-borneol = 0.2013gNo of moles of endo-borneol = 0.2013g/154.25g mol-1 = 1. 305mmolSince one mole of endo-borneol should give one mole of camphorTheoretical yield of camphor = (1.305 mmol )( 152.23g mol-1) = 0. 1987gTheoretical melting point of camphor = 179.75 oCActual Yield of the product (camphor) = 9. 5360 – 9. 4873 = 0. 0487 g% Yield of the product = (100)* Actual yield/ Theoretical yield= (100)* 0.0487g/0.1987g =24.5%Melting point of product tested = 136oCConclusions and Answers to QuestionsIn this experiment, aqueous sodium hypochlorite (NaOCl), or common household bleach was used as oxidizing agent and it was generally made from chlorine reacting with sodium hydroxide:Acetic acid, which acted as a solvent, was then added into the NaOCl to generate hypochlorous acid (HOCl) via acid base reaction:
Since OH- was a bad leaving group, HOCl was not likely to be ionized to produce positive chlorine species. The possible source of positively charged chlorine (Cl+) was likely to be from heterolytic bond cleavage of chlorine:In the first step of reaction mechanism of hypochlorite oxidation of endo-borneol to camphor, a hydroxyl proton was replaced by the positive chlorine. In the second step, HCl was eliminated via E2 elimination from the alkyl hypochlorite to form ketone. In the first step, Cl+ was added to the alcohol but in the following step Cl- was lost. The net change was a reduction of two electrons and the endo-borneol that provided the electrons was therefore oxidized.
To make sure there was an excess of hypochlorite oxidizing agent so that the reaction could go to complete, starch-iodide test paper was used at regular time intervals. In acidic solution, iodide ions should be oxidized by the hypochlorite ions to iodine and therefore gave a purple black color.
The result of this experiment was not very satisfactory, although the reaction was carefully controlled so that the temperature was kept more or less at 40oC. First of all, the yield of the product (24.5%) was quite low. This was mainly because of the fact that there was significant loss of materials during transfer of crystals and because of incomplete conversion of endo-borneol to camphor. It was observed that during swirling of endo-borneol to help it dissolve in acetic acid, some of them got stuck on the wall of the flask, and therefore was not being oxidized by the hypochlorite.
For improvement, it might be a good idea not to add all the acetic acid at one time, but left a little bit of it to wash down the endo-borneol stuck at the wall. The loss of crystals during transfer could be minimized by more careful handling. However, some loss seemed to be inevitable as it was impossible to get all the tiny crystals out from one flask to another.
The melting point was much lower than expected. It turned out be 136oC, instead of the theoretical value of nearly 180oC. This great melting point depression proved that there were impurities present in our product. The impurities were likely to be the unreacted alcohol present in the product mixture, which was further confirmed by the analysis of infrared spectrum. From our IR spectrum, a stretching absorption at around 3500cm-1 was observed and this corresponded to the hydroxyl stretching vibration (from 3200 cm-1 -3600 cm-1). This proved the presence of OH functional group, that could only arised from alcohol, but not ketone. Also, a peak at around 800 cm-1 should be completely absent in a pure ketone product.
The fact that it did not disappear also implied that there was unreacted alcohol. However, a large amount of desired product (camphor) was indeed present in the sample, as there was a strong peak at around 1750 cm-1 (between 1600 cm-1 -1800 cm-1), which proved the presence of carbonyl group. To improve this experiment, the time of the oxidation reaction could be prolonged to ensure complete reaction and the sublimation experiment could be carried out in a larger scale and controlled a more careful manner.
Answer to the questions1)Stepwise reaction mechanism of hypochlorite oxidation of endo-borneol to camphor:2) To determine the amount of endo-borneol present in a sample of camphor, one possible way to look for the intensity of the peak at a specific absorption frequency that was characteristic of endo-borneol but absent in camphor. In this case, the choice of frequency could be 800 cm-1 or 3200 cm-1 -3600 cm-1 which was the hydroxyl stretching absorption. The greater was the intensity of the peak at these specific frequencies, the greater was the amount of endo-borneol. To conclude, the amount of endo-borneol present in a sample of camphor could be quantified by measuring the peak intensity at well-defined absorption frequencies.
Williamson, Macroscale and Microscale Organic Experiments, 4th Edition, 2003, P. 120-1222)Solomons and Fryhle, Organic Chemistry, 8th edition, 2004, Sec. 12.4 P. 546-550
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