Kayla Schulz
Kelly Fegles
02/10/2011

Introduction:
In this experiment, cyclohexene was synthesized from cyclohexanol via an acid-catalyzed dehydration reaction; and was then separated from the reaction mixture using fractional distillation. Cyclohexene is clear colorless liquid, with a strong odor. It is used as a solvent as well as "an intermediate in many industrial processes"(Wikipedia). This acid catalyzed preparation is very common in the organic chemistry laboratory. In this procedure a greener version of the reaction was practiced. Instead of using H2SO4- , which is a very strong and corrosive acid, concentrated phosphoric acid was used. This dehydration takes place via an Elimination (E1) reaction. In the first step, an acid base reaction takes place when the phosphoric acid donates a hydrogen to the hydroxyl (OH) group on the cyclohexanol. This creates a good leaving group (H2O), and water leaves the cylohexane ring to form a cyclohexane- carbocation. In the second step water then acts as a base and strips a proton from a beta-hydrogen, leaving the electrons available to form a Pi bond in the ring, thus producing cyclohexene. After the first step, and the formation of the carbocation, there are other reactions that can takes place. The reaction can proceed in reverse to reform the starting material, or the cation can react with the cyclohexanol to form dicyclohexyl ether (UWI MONA). In order to purify and separate the cyclohexene from the reaction mixture, fractional distillation was practiced. Fractional distillation is a very similar setup and procedure as simple distillation, except there is the addition of an extra condenser column, which contained copper. This extra column allows for fractionation of the distillate. Because of the difference in boiling points of the desired product and starting materials, the product is therefore evaporated into its gaseous form, and then re-condensed and collected.
I like this intro, and I like the fact that you have shown me the reaction here. Good!
The Reaction:
lab4_001.jpg

Procedure:

See GEM website:
http://greenchem.uoregon.edu/Pages/Overview.php?CategoryIDString=&FullTextSearchKeywords=cyclohexene&CategoriesToSearch=&NumberOfMainCategories=7&AnyAll=Any&ID=70
Sources: Doxsee, K. M.; Hutchison, J. E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004; pp 129-134.

Data:
Before adding cyclohexanol to 1.75 mL of 85% H3PO4, 0.074 moles of cyclohexane was converted into grams:

Molar Mass of C6H12O= 100 g/mol
0.074 moles C6H12O (100 g/mol / 1 mole)= 7.4 g C6H12O

After being mixed together, the reaction mixture was heated. After 19 minutes the mixture began to rapidly boil. The fractioning distillation column was checked periodically, and it was approximately 26 minutes after boiling that the vapor began to ascend into the column. At this point the temperature of the distillation was monitored, recorded and graphed. The first drop of the distillate was collected 3.5 minutes later at 64.5 degrees Celsius. The temperature increased significantly more during the distillation process ( to 75.3 degrees Celsius) and was not consistent. The last drop was collected at 12 minutes and 63.8 degrees Celsius.The following are a table and a graph of this data. The temperature range of the distillation was 64 degrees C to 75 degrees C.graph_redo_001.jpg
chart_001.jpg


The molar mass C6H10 = 82 g/mol
final mass = 3.100 g * (1 mol / 82 g) = .038 moles
After the product was collected, tested using IR spectroscopy. This type of spectroscopy makes it possible to identify important features and functional groups of a compound. The following graph was obtained for the distilled product. The important features that can be identified are both sp2 and sp3 hybridized carbon hydrogen stretches. Another important thing to note is the lack of a peak corresponding to a hydroxyl group.



graph_IR_001.jpg



Discussion:

When evaluating the success of this procedure there are a couple things that need to be taken into consideration. The first is percent yield. The amount of product recovered in the experiment was 3.100g or 0.038 moles of cyclohexene. The amount of the starting limiting reagent, cyclohexanol was 7.400g or 0.074 moles. The percent recovery was therefore 51% (.038/.074= .51 *100). The second thing that must be taken into consideration is the purity of the product. The desired product was cyclohexene however, in order to be sure that it is the product obtained, further analysis must be done. One way to access the purity is by looking at the distillation temperature. The temperature of the distilled vapors corresponds to the boiling point of the distillate. By monitoring the temperature of the distillation vapors and comparing them with the known boiling point of the desired product, it can be determined if the product is impure. The temperature range over which the product distilled was 64 degrees C to 75 degrees C. The reported boiling point for cyclohexene is 82.98 degrees C (Wikipedia). This indicates that the distillate collected was not in fact pure cyclohexene because it boiled not only lower than the reported BP, but over a much wider range as well. After the distillate was collected Infrared Spectroscopy was used to analyze the product. This is an important test because IR identifies functional groups of the structure. According to the IR graph obtained for the product, both sp2 and sp3 hybridized carbon hydrogen stretch was identified. This indicated that there is the presence of a pi bond. Also the IR graph lacked a hydroxyl group peak. This strong, broad peak is an indicator that an alcohol is present, and its absence indicated that the product did not contain any OH groups.
From all of this information it can be concluded that the collected product was not pure cyclohexene, however the elimination of the hydroxyl group from cyclohexane was successful.
Nice analysis! You have hit all the important consideration, and you have spoken clearly about each one.

Error:

One possible error that may have occurred during the distillation process was that in using the extra condenser column with the copper, some of the product being evaporated may have been left behind on the copper, therefore leading to a lower percentage yield. This is a common frustration in fractional distillations. The volume is said to be "held up" in the column. Another error may be inherent in the equipment used; the Lab-quest used to record the temperature and may have not been as accurate as the equipment used to test the boiling point of cyclohexene in the reported value. Another error to consider would be any initial contamination in the reaction mixture that may have distilled out with the desired product, leading to its impurity. I find that our recorded temperatures are often significantly lower than literature values. Part of the explanation lies with barometric pressure, which is reduced at the elevation of our laboratory.

Post Lab Question:
Correct: good.
1) Calculate the atom economy for this reaction. Atom economy is defined on Wikipedia, but essentially is the mass of the desired product divided by the mass of all reactants (don't include catalysts or solvents in our reaction).
Atom economy is a very important aspect in green chemistry. It involves the conservation of materials in a chemical reaction, so that individual atoms do not get wasted as byproducts in a reaction. Atom economy can be defined as the conversion efficiency in a chemical process in terms of all atoms involved (Wikipedia). A synthesis with a high atom economy will have a large proportion of atoms used in reactants found in the desired product.
The atom economy can be calculated by dividing the molecular mass of the desired product by the molecular mass of the reactants. The molecular mass of cyclohexanol is 100.16 g/mol. The molecular mass of Cyclohexene is 82.14 g/mol.

(82.14 g/mol C6H10) / (100.16 g/mol (CH2)5CHOH)= 0.8201*100= 82.0%

Works Cited:
UWI. Experiment 10. "Preparation of Cyclohexene from Cyclohexanol". Online. [10-Feb 2001]
http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt8.html

Wikipedia. "Cyclohexene". Online. [10-Feb 2011]
http://en.wikipedia.org/wiki/Cyclohexene

Wikipedia. "Atom economy". Online [10-Feb 2011]
http://en.wikipedia.org/wiki/Atom_economy

This lab earned the following score: 14. Need I explain?
Nice work!!