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 1st 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 2nd 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 1st addition: (Amount of NaCl used for 1st
addition x MR of NaCl)
= 1.30x10-3 x 58.44= 0.08g
Mass
of NaCl needed for 2nd addition: (Amount of NaCl used for 2nd
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 42oC 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 + NaOHVII
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
·
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
