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Experiment 2: Oxidation of (-)-Borneol to (-)-Camphor using green chemistry

Aim:

This experiment focuses on the synthesis of (-)-Camphor from (-)-Borneol via oxidation. This was done by using ClO^- as the oxidizing agent formed from Cl^- . Oxone as a key oxidizing agent used to form ClO^- from Cl^- . The crude (-)-Camphor were obtained after extraction and was purified via sublimation. Pure (-)-Camphor were analyzed by Infrared (IR) spectroscopy in order to obtain the spectrum of the product. The spectrum will be studied to analyze the purity of the pure product obtained.

Results and discussion:

Synthesis of (-)-Camphor equations:

KHSO₅ + Cl^- + H⁺ → KHSO₄ + HOCl

Calculations:

Amount of (-)-Borneol used:  =  6.48 x 10^-3 mol

Amount of Oxone used: (Amount of (-)-Borneol used ) x 1.2

                                        = 6.48 x 10^-3 x 1.2 = 7.78x10^-3 mol

Hence, (-)-Borneol is the limiting reagent.

Amount of NaCl used for 1^st addition: (Amount of (-)-Borneol used) x 0.2

                                                              = 6.48 x 10^-3 x 0.2 =1.30x10^-3 mol

Amount of NaCl used for 2^nd addition: (Amount of (-)-Borneol used) x 0.08

                                                              = 6.48 x 10^-3 x 0.08 =5.18x10^-4 mol

Mass of Oxone needed: (Amount of Oxone used x MR of Oxone)

                                       = 7.78x10^-3 x 307.38 = 2.39g

Mass of NaCl needed for 1^st addition: (Amount of NaCl used for 1^st addition x MR of NaCl)

                                                              = 1.30x10^-3 x 58.44= 0.08g

Mass of NaCl needed for 2^nd addition: (Amount of NaCl used for 2^nd addition x MR of NaCl)

                                                              = 5.18x10^-4 x 58.44= 0.03g

Actual mass of crude (-)-Camphor: 0.870g

Actual mass of pure (-)-Camphor: 0.806g

Theoretical mass of (-)-Camphor: (Theoretical amount of (-)-Camphor) x (MR of (-)-Camphor)

                                                      =6.48 x 10^-3 x 152.23= 0.986g

Percentage yield of pure (-)-Camphor:  x 100 =81.7%

Percentage yield from crude (-)-Camphor:  x 100 = 92.6%

The percentage yield for pure (-)-Camphor was 81.7%, which was moderately good. However, there were still loss of yields due to several factors. (-)-Camphor has a relatively high vapor pressure of 1mmHg at 42^oC which makes it volatile ^I. Hence, there was loss of product to the surrounding atmosphere as the experiment goes along. Solute vaporization may occur during the use of rotary evaporator as pressure is high in the system, causing vaporization of (-)-Camphor together with the solvent ethyl acetate as (-)-Camphor is highly volatile ^II. Furthermore, ethyl acetate requires more energy for evaporation than other solvents such as diethyl ether. This will cause more vaporization of (-)-Camphor in the rotary evaporator ^III. Moreover, there were some loss of products throughout the experiment when the product was transferred from one containment to another. All of these factors may contribute to the loss in the products.

Table 1: Assignment of peaks for the IR spectrum of purified (-)-Camphor

Wavenumber range (cm-1)	Intensity of Peaks	Assignment
1743.41	Strong	C=O Stretching
2960.66, 2874.58	Medium	Sp3 C-H Stretching
3470.55	Weak	O-H Stretching

The sharp peak at 1743.41 cm^-1 can be interpreted as the stretching of the carbonyl functional group C=O as it’s near to 1740 cm^-1and is a rather strong peak. This indicate the presence of the ketone functional group that is present in (-)-Camphor. The sharp peaks 2960.66 cm^-1 and 2874.58 cm^-1 can be interpreted as sp³ C-H stretching. This could be due to various groups C-H present in (-)-Camphor. Hence, this shows that (-)-Camphor was indeed present.

However, there is a broad peak at 3470.55 cm^-1, which corresponds to O-H stretching. There are two possible explanation for this. Firstly, the O-H stretching may be due to leftover (-)-Borneol. Since (-)-Borneol have similar structure as (-)-Camphor, they have similar volatility and may vaporize together. There was no proper temperature control during purification to facilitate sublimation. Furthermore, no fractional column was used during sublimation to properly separate these two substances. Secondly, the peak could be due to water that was still present. Although there are many steps throughout the experiment to remove water, (-)-Camphor is hygroscopic and it difficult prevent it from having contact with the air moisture. (use column chromatography)

Even so, the signal intensity at 3470.55 cm^-1 was very weak. This shows that the end product was of high purity and can be used for direct characterization of (-)-Camphor or use in a subsequent reaction, such as NaBH₄ reduction to form more useful substance such as (+)-Isoborneol ^IV.

Observations:

(-)-Borneol was first dissolved in the solvent ethyl acetate. Although ethyl acetate requires more energy for evaporation in later process, it is a good choice of solvent in the organic chemistry teaching laboratories as it poses less fire hazard as compared to diethyl ether and is less corrosive compared to solvents such as glacial acetic acid, which is a commonly used solvent with bleach in green oxidations ^III. Oxone and NaCl were added but was unable to dissolve in 1.5ml of water. Oxone is a rather good choice of oxidizing agent here. It is an inexpensive reagent that is comparable to H₂O₂ and bleach, some of the common oxidizing agents around ^V. Furthermore, Oxone will be quenched with sodium bisulfite in later steps to form a mixture of non-hazardous sulfate salts in water. These byproducts are clean and green and unlike oxidizing agents like chromium trioxide and bleach, it does not emit pungent vapors which pose the risks of inhalation ^VI. However, Oxone have poor atom economy as only one of the triple salts was the actual oxidizing agent.

NaCl was added at a catalytic amount and react accordingly to the equations shown above. Hence, the catalytic nature of the chloride ions can be seen here. Yellowish green color observed 5minutes into stirring and color persisted throughout the reaction. This is due to the production of Cl₂ as a byproduct via comproportionation if Cl^- and ClO^-.

HOCl + NaCl Cl₂ + NaOH^VII

Due to this reaction, it is possible to lose some Cl^- as Cl₂ gas throughout the reaction. Hence, there is a need to add 0.08 eq of NaCl in later steps.

The solution remained cloudy throughout the reaction. The remaining salts that are unable to dissolve in 1.5ml of water will cause the solution to be cloudy. A precaution here was to add water in limiting amount to prevent excessive Oxone salt from dissolving during the reaction process. After 15ml of water was added, all the salts dissolved and form non-hazardous salts. Afterwards, sodium bisulfite was added to quench the reaction by reducing all the oxidizing agents presented. Starch-iodide paper was used to indicate if the oxidant is present. The following reactions took place if oxidizing agent was present

HOCl + 2I→ I₂ + Cl^- + OH^-

I₂ + I^- → I₃^-

Hence, if any oxidizing agent was present I^- will be oxidized to I₂, which will further react to form I₃^-. I₃^- would then get stuck within the starch to form a blue-black. If there is no more oxidizing agent present, I^- would remain and no color change of the starch-iodide paper would be observed. This also shows that all trace oxidizing agents have been efficiently reduced by sodium bisulfite.

The mixture was extracted using 3 x 15ml of ethyl acetate and 2 clearly immiscible layers can be seen. After the extraction process, the crude product was an organic layer with a slight yellowish color. Brine was added to remove water present as concentrated salt solution wants to become more dilute and because salts have a stronger attraction to water than to organic solvents ^VIII. White flakes were obtained as crude products after ethyl acetate was removed through the use of rotary evaporator.

The crude products were then purified via a simple sublimation. As mentioned above, there was no proper temperature control during sublimation and no fractional column was used. The yields could be improved by using a better sublimation setup shown in diagram 1.

Diagram 1: Better sublimation set-up ^VIIII

(-)-Camphor forms white crystals on the condenser. Before the crystals starts falling back to the bottom of the apparatus, stop the heating and remove the condenser out to scrape the (-)-Camphor crystals into a container. Cover the apparatus with a watch glass when doing this to prevent (-)-Camphor vapors from escaping into the environment.

Another precaution using the petri dish sublimation technic was to constantly remove the pure product obtained on the lid of the petri dish to another containment. By doing so, the sites for deposition will always be easily available for more purified (-)-Camphor to be formed. The purified products obtained after sublimation were also white flakes. Some brown impurities were observed after the purification process which indicates that impurities were separated from the pure (-)-Camphor.

                                                  

Conclusion

In the current industry, the challenge for chemists is to develop products and processes in a sustainable manner to not only outplay industrial competitions, but also to help maintain the natural environment. There were many ways to oxidize (-)-Borneol to (-)-Camphor like bleach-acetic acid oxidation or using chromium based oxidizing agents. However, the use of Oxone and catalytic amount of NaCl was an efficient and clean method to oxidize (-)-Borneol to (-)-Camphor and fufilled many of the criteria of the 12 Principles of Green Chemistry ^X.  These includes the use of non-toxic reagents (Oxone and NaCl), environmentally friendly and safer solvents (ethyl acetate), catalytic reagent (NaCl), reduce hazardous waste products produced (Oxone removed as harmless salt) and energy efficient (reaction conducted at room temperature). Though (1S)-camphor is likely not a compound needed, it can undergo further reactions to form more useful compound such as (+)-Isoborneol ^IV.

Reference

·         ^I  D.Pavia & G. Lampman & G.Kriz & R. Engel, 2011, A Small Scale Approach to Organic Laboratory Techniques third edition, M. Finch, pg 766, accessed on 3 September 2014

·         ^II D.Parriott , 1993, A Practical Guide to HPLC Detection, pg 259, accessed on 3 September 2014

·         ^III Anne E. Marteel-Parrish & Martin A. Abraham , Green Chemistry and Engineering: A Pathway to Sustainability  , pg 927-934, accessed on 3 September 2014

·         ^IV McMaster University Chem2006 Lab Manual , Experiment 7- Isomerization of an Alcohol by Oxidation-Reduction: Borneol, Camphor, and Isoborneol accessed on 3 September 2014

·         ^V Aldrich Chemical Company, 2000, Catalog Handbook of Fine Chemicals, pg 1258, accessed on 3 September 2014

·         ^VI M. Sundar,D & Easwaramoorthy,S & Kutti Rani & M. Palanichamy, Journal of Solution Chemistry 2007,36 Mechanistic Investigation of the Oxidation of Lysine by Oxone  pg 1129–1137 accessed on 3 September 2014

·         ^VII Wikipiedia, Disproportionation ,http://en.wikipedia.org/wiki/Disproportionation , accessed on 3 September 2014

·         ^VIII University of Colorado at Boulder, Department of Chemistry and Biochemistry, Drying Organic Solutions, accessed on 3 September 2014

·         ^VIIII BUTE - Department of Inorganic and Analytical Chemistry, Purification of Camphor by Sublimation  accessed on 3 September 2014

·         ^X American Chemical Society (ACS), 12 Principles of Green Chemistry, http://www.acs.org/content/acs/en/greenchemistry/what-is-green-chemistry/principles/12-principles-of-green-chemistry.html , accessed on 3 September 2014