Preparation+of+Cyclohexene

=Introduction=

In this lab, the synthesis of cyclohexene from cyclohexanol will be performed by the dehydration of an alcohol. This is an elimination reaction, in which two groups, or atoms, will be removed from the starting molecule. Elimination reactions can occur as either a one-step (E2) or a two-step (E1) mechanism. Due to steric hindrance, this reaction will proceed primarily via E1. The synthesis of cyclohexene occurs via the following E1 elimination reaction.



Dehydrations are typically catalyzed by a strong acid. In the reaction shown above, it can be seen that the excess of H + protonates the alcohol. This protonation lowers the leaving groups pka value of 16 (H 2 O) down to≈ 0 ( H 3 O + ). This lowered pka promotes the leaving groups departure and the formation of a 2⁰ carbocation intermediate. A water molecule then removes a beta hydrogen and an alkene is formed. In the dehydration of a liquid alcohol, no additional solvent is needed. Historically, Sulfuric acid (H 2 SO 4 ) has been used to catalyze the dehydration of an alcohol; however, to emphasize green chemistry, concentrated (85%) phosphoric acid (H 3 PO 4 ) will be used in its place. Atom economy, or atom efficiency, is the conversion efficiency of a chemical process and an important concept of green chemistry. It is a calculation of desired product yielded from all of the reactants, expressed as a percent. In the synthesis of cyclohexene, the formation of water, a 'by product', reduces the atom economy. However, its effects are benign and a drying agent will be used to remove it from the final product.

As the reaction takes place, fractional distillation will be used to separate the product from the starting materials. Fractional distillation is a physical separation process from a boiling mixture that uses vaporization and condensation cycles to separate a mixture. It is primarily used when trying to separate substances that have similar boiling points. It will be used for this lab because cyclohexene has a boiling point of approximately 80⁰C and the byproduct of the reaction, water, has a boiling point of 100⁰C (NIST). Copper turning will be placed into the fractionating column in order to increase the surface area the product must encounter as it is distilled. The increase in surface area causes the distillate to condense and vaporize many more times than in a simple distillation and results in a purer distillate.

After the distillate is separated from the initial mixture, an infrared spectrum will be obtained to aid in the identification of the product. An infrared spectrum shows the absorption patterns in the bonds between the functional groups of a compound. The absorption patterns on the spectra are indicative of the molecular structure of the compound. Therefore, the results can be used to identify a compound.

GOOD JOB ON THE INTRODUCTION. Thorough, accurate, __nice.__

=Procedure=

The procedure for this experiment can be found here (Doxsee, 2004). The second distillation was omitted from this experiment.

Relevant Compounds
(Retrieved Via NIST Chemistry WebBook)


 * Name:** Cyclohexanol
 * CAS #:** 108-93-0
 * Molecular Formula:** C 6 H 12 O
 * Molecular Weight:** 100.1589 grams/mole


 * Name:** Cyclohexene
 * CAS #:** 110-83-8
 * Molecular Formula:** C 6 H 10
 * Molecular Weight:** 82.1436 grams/mole


 * Name:** Phosphoric acid
 * CAS #:** 7664-38-2
 * Molecular Formula:** H 3 PO 4


 * Name:** Sodium Sulfate
 * CAS #:** 7757-82-6
 * Molecular Formula:** Na 2 SO 4

=Data=

Reaction
7.420 + / - .001g of cyclohexanol was mixed with 1.75 + / - .05ml of 85% H 3 PO 4 and 3 small boiling stones in a 50ml round-bottom flask. The mixture was swirled lightly to mix layers. The mixture was clear and began to develop a strong odor shortly after mixing the components. The mixture was heated at reflux for approximately an hour and a half until the reaction was complete. At t=1 hour, a small amount of separation in the collection flask was noted. Substance remaining in the 50ml round bottom flask after distillation was milky white in color. Below is the resulting distillation curve of temperature (℃ versus time. 0 hours on the graph represents the time at which condensation was noticed approximately 1 / 2 " below junction in distillation adapter.



Workup
The contents of collection flask were transferred were transferred to a separatory funnel and washed with approximately 5ml of water. The mixture was allowed to separate. Water, being more dense ( 1g / cm 3 ) than cyclohexene ( .811g / cm 3 ) sank to the bottom of the mixture. The water was emptied into a 50ml flask and discarded while the cyclohexene was put into a 25ml Erlenmeyer flask. In total, 3 small scoops of sodium sulfate were used to remove any unwanted water that may have been in the product. The remaining product was then transferred to a clean, dry, pre-weighed sample vial via pasteur pipette.

Characterization
The empty sample vial was weighed at 5.702 + / - .001g. The vial with sample was weighed at 8.257 + / - .001g. The purified product was clear in color. Due to a lack of lab time, the sample vial was given to the lab instructor for an infrared spectrum analysis. Below is the IR spectrograph with an accompanying peak finding result table.



=Analysis=

Reaction
Figure 1 shows that the reaction and subsequent distillation took approximately 72 minutes to complete and had a temperature range of 24⁰C. Once distillate began to form, the temperature was not very consistent. This may be indicative of product impurities. .

Characterization

 * Percent yield:**



calculation is accurate but you missed on the sig figs. You could have given me four digits (41.98%).


 * Product identification:**

Below is an IR Spectrum of the starting material cyclohexanol retrieved via the National Institute of Standards and Technologies (NIST) database.



For the purpose of this experiment, the most interesting features of this IR spectrograph happen >2800 wavenumbers. The most important being the broad and strong alcohol "swoosh" produced at 3,200-3,400 wavenumbers. This feature represents the sigma bond between oxygen and hydrogen in an alcohol. The second feature is the strong spikes <3,000 wavenumbers. These spikes represent C sp3 -H bonds found in various alkyls.

Below is the IR Spectrograph of the sample recovered from the distillation via instructor assistance.



Again, for the purpose of this experiment, features >2800 wavenumbers were analyzed to determine if reaction took place. By looking for the two features described in Figure 4, it can be seen that the alcohol "swoosh" at 3,200-3,400 wavenumbers is not present in this sample. The strong spikes <3,000 wavenumbers are still present, but there has been an additional spike >3,000 wavenumbers. This spike represents the C sp2 -H bonds found in alkenes.

The sample spectrograph was then compared to the IR spectrograph for cyclohexene retrieved from the NIST database shown below.

The sample spectrograph and the cyclohexene spectrograph retrieved from NIST show a remarkable similarity. These similarities not only present themselves >2,800 wavenumbers where the most notable changes in the reaction take place, but extend down into the fingerprint region as well.

very nice. =Conclusion=

The purpose of this lab was to perform the two-step elimination (E1) reaction of cyclohexene from cyclohexanol by the dehydration of an alcohol. In this reaction, phosphoric acid was used in the dehydration of cyclohexanol to emphasize green chemistry. Figure 1 shows the distillation data for the reaction; the reaction began at 53° C and was complete at 77° C. In the distillation graph, inconsistent spikes and drops in the temperature were observed. One possible cause could be condensation forming and dropping off of the temperature probe, placed at the top of the fractionating column. Another possible cause could be due to product impurities. I wish I could have more analysis from you on the distillation temps. You mention the variability of temps during distillation, but what do you think about the overall temp range?

The infrared spectrum of the final product is shown in figure 2 and 4. The absorption patterns on the spectra can be used to identify the type of bond between functional groups of the compound, and therefore aid in determining the molecular structure of the compound. Most notably, the peak at the frequency 3013 falls in the absorption range of an alkene, and represents a C sp2 -H bond. This combined with the absence of an alcohol "swoosh" at 3,200-3,400 wavenumbers signifies that the desired product was synthesized. The peak at the frequency 2916 represents a C sp3 -H bond and is found in both cyclohexanol and cyclohexene.

The calculated percent yield for this reaction was 42%. The low percent yield could be due to possible errors or limitations that occurred during the lab. One possible error was impurities caused by drying the glassware with compressed air prior to starting the lab procedure. There was also a substantial loss of product during the separation of layers in step 4 of the procedure, also contributing significantly to the lower percent yield.

Good job.

=References=

Doxsee, K. M.; Hutchison, J. E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004; pp 129-134.

(n.d.). //Experiment 5 alcohol dehydration of cyclohexanol to cyclohexene//. [Web Graphic]. Retrieved from http://jan.ucc.nau.edu/~jkn/235L5-Dehydration.htm

P.J. Linstrom and W.G. Mallard, Eds., NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg MD, 20899, http://webbook.nist.gov, (retrieved February 15, 2012).