Synthesis of Cyclohexene from Cyclohexanol

ABSTRACT

The synthesis of cyclohexene from cyclohexanol is an example of elimination reaction. Cyclohexanol, a secondary unsaturated alcohol, undergoes dehydration reaction to form a good leaving group which is H20 because the OH group of an alcohol is a very strong base making it a poor leaving group. The reaction will then be followed by the obstruction of a hydrogen atom to form a carbon double bond or an alkene which in this case is cyclohexene. Cyclohexene is an unsaturated hydrocarbon which is very reactive due to its negative center (Ault, 1973; Williamson, 2013; Eagleson, 1994). Gas chromatography-mass spectrometry is an instrument which is used to separate gaseous substances and it functions as an analyzer for the compound. This instrument may be able to provide the molecular weight, formula and structure of an unknown compound. Synthesis was done via simple distillation since distillation gives a relatively pure yield (Karesek-Clement, 1988). Phosphoric acid was added to cyclohexanol in a round bottomed flask to have the dehydration reaction which would yield to the cyclohexene. It was then distilled and the group was able to get 3 ml of yield. The distillate was observed using the mass spectrometer and it showed that the yield had an 81.98 molecular weight which is very close to the theoretical molecular weight of cyclohexene which is 82. Fragments were also observed with the result obtained and some can be considered as impurities. The product was affirmed to be cyclohexene based from the results obtained from the mass spectra. We can say that the gas chromatography-mass spectrometry is a great tool in analyzing either an unknown or for comparative reasons of a compound since it gives accurate results. Also, it is recommended to analyze at once the product for it not to evaporate.

INTRODUCTION

Alcohols undergo elimination reaction in the presence of a strong acid to form an alkene. Strong acids such as Sulfuric acid (H2SO4) and Phosphoric acid (H3PO4) are used in the dehydration reaction of alcohols. The acid needs to protonate the –OH group because -OH is a strong base making it a poor leaving group. Once the –OH has been protonated to H20, it can leave and the nucleophile will obstruct a beta hydrogen to form a carbon double bond or an alkene (Williamson, 2013). An example of this reaction was observed in this experiment with the synthesis of cyclohexanol to cyclohexene via distillation and extraction. Cyclohexanol is a secondary saturated alcohol with boiling point of 100.16 celcius which undergoes elimination reaction to form cyclohexene product

with respect to their interaction with the column (stationary phase) and the gas/helium (mobile phase). A mass spectrum may be able to give the following information: molecular weight, molecular formula and molecular structure of the substance (Ault, 1973)..

The objectives of this experiment are to isolate cyclohexene from cyclohexanol through acid-catalyzed elimination of water and to be able to determine the identity of the distillate product through Gas Chromatography.

II. METHODOLOGY

In this experiment, Cyclohexanol was synthesized to cyclohexene via distillation and extraction. In a 50 ml round bottomed flask, 5.0 grams of cyclohexanol, 1 ml of 85% phosphoric acid and boiling chips were added. After the flask was swirled to mix the contents, it was attached to a fractionating column which was fitted with a distilling adapter, thermometer and a simple condenser. The flask was heated using an oil bath with boiling chips for 5 minutes. The distillation process was done until the residue reduced to 1 to 15 ml. The group was able to collect only 3 ml of distillate. The receiver was placed in an ice bath for the residue not to evaporate. The distillate in the receiver was then transferred to a separatory funnel. Furthermore, 5 ml of water was added and was stoppered and mixed through shaking. The lower aqueous layer was discarded while the upper organic layer was decanted to an Erlenmeyer flask. Enough amount of Anhydrous sodium sulfate were placed in the Erlenmeyer flask to absorb water residues as it was swirled occasionally for 10 minutes. The contents of the Erlenmeyer flask was then transferred to a test tube and the next distillation process was not done since the yield of the distilled product was too few. The product was then analyzed using a gas chromatography-mass spectrometry apparatus in the instrument room.

III.RESULTS AND DISCUSSIONS

For this experiment, the yield was analyzed using the gas chromatography-mass spectrometry apparatus. The graph shows the relationship between the relative abundance (y-axis) to M/Z ratio (x-axis). The result of the gas chromatography-mass spectrometry of the product can be seen in fig 1 at the last page, the parent ion or the molecular ion denoted by M+ has a value of

Distillation helps in obtaining purer substances. Also, an advanced instrument such as the gas chromatography-mass spectrometry apparatus gives a very accurate result or molecular weight of the product and other fragments.

There can still be further modifications for better results. First, the instruments to be used should always be dried properly to make sure that there will be no impurities such as water. Second, another round of distillation should also be done to have purer product to avoid seeing impurities. Lastly, it is greatly recommended for the yield to be analyzed at once since it was observed that for some, the distillate evaporated.

V.REFERENCES

Bernard, M., Chandler, Z. The-Mach. Elimination reactions; cyclohexene from cyclohexanol. http://the-mach.wikispaces.com/Elimination+reaction%3B+cyclohexene+from+cyclohexanol (accessed June 12, 2014)

Khan Academy. E1 elimination: Carbocation rearrangements. https://www.khanacademy.org/science/organic-chemistry/substitution-elimination-reactions/e1-e2-tutorial/v/e1-elimination–carbocation-rearrangements (accessed June 12, 2014)

Ault, A. Techniques and Experiments for Organic Chemistry, 6th ed.; Waveland Press Incorporated: Illinois, 1973.

Eagleson, M. Concise Encyclopedia Chemistry; Walter De Gruyter Inc: Berlin, 1994.

Masters, K., Williamson, K. Macroscale and Microscale Oorganic Experiments, 6th ed.; Cengage Learning: Stamford, USA, 2010

Clement, R.E., Karasek, F.W. Basic Gas Chromatography-Mass Spectrometry: Principles and Techniques; Elsevier, 1988.

University of Bristol. Gas Chromatography Mass Spectrometry (GC/MS). http://www.bris.ac.uk/nerclsmsf/techniques/gcms.html (accessed June 20, 2014)

Baklajian, Alex (May 2012). Introduction to mass spectrometry.

and water which co-distill (Eagleson, 1994). The distillate product is cyclohexene which is unsaturated and has the boiling point of 83 celcius. Synthesis was done via distillation since it helps in obtaining purer substances (Bernard-Chandler, n,d).

Gas chromatography-Mass Spectroscopy is a physical method of separating a compound which are volatile and thermally stable. This instrument can separate, identify and quantify compounds. The two phases that involves the process are the stationary phase which is the column and the mobile phase which is the carrier gas like helium (Karesek-Clement, 1988). The vaporized sample will be attacked by beam of electrons which is called the ionization process in which the positively charged ionic fragments are produced. This process involves the removal of electrons since the beam of electrons knock off one electron from a molecule which forms the parent ion or molecular ion. Fragmentation happens with

the product of ionization to give smaller charged and neutral pieces. A magnetic field would force the circular flow of the ions and the separation will occur since they will follow different path of radius

81.98 which gives us the relative formula mass of the molecule (University of Bristol, n.d; Ault, 1973). Also, it can be seen that there is a presence of an isotope since there is a peak that is close in

value with the M+ to the right. This is because carbon has an isotope which is 13C. The natural abundance of Carbon 12 is much higher than the natural abundance of Carbon 13 but since the mass spectrometer gives very accurate results, it may be able to detect the isotope. The parent ion M+ has the highest mass among the peaks. Any peaks lower than the M+ are just considered as fragment readings in the spectrum while the isotope, denoted by M+1, is the peak which is to the right of the parent ion which is lower in intensity (chem.ucla.edu, n.d; Baklajian, 2012). Other peaks can be considered as just fragments. These fragments are due to the breaking down of the unstable positive ions (chemguide.co.uk, n.d; Ault, 1973). On the other hand, the base peak is the one with the 56 molecular weight indicated by its 100 reading in abundance. The base peak always has the highest abundance among all the peaks (Karesek-Clement, 1988). The peak with the 100.013812 reading can be considered as an impurity. Even if it has the highest mass reading, it was not considered as the molecular ion peak since the compound being talked about here is cyclohexene. To sum it all up, the group was able to collect cyclohexene via distillation of cyclohexanol. The obtained results from the mass spectrum showed the presence of an impurity and also an isotope peak. The yield’s molecular weight (81.98) was indeed very close to the theoretical molecular weight which is 82.

IV.CONCLUSION AND RECOMMENDATION

It was observed that in this experiment on synthesis of cyclohexene from cyclohexanol, the group was able to produce cyclohexene as can be seen in the mass spectra which indicates that the product obtained by the group had an 81.9265 molecular weight which is relatively close to the theoretical molecular weight of cyclohexene which is 82. The group was able to perform all of the objectives for this experiment which are to synthesize cyclohexene from cyclohexanol and obtain a mass spectra of the product via gas chromatography-mass spectrometry. In conclusion, the synthesis of cyclohexene from cyclohexanol can be done in many ways and distillation is one of its examples.