Synthesizing_cyclohexene_from_cyclohexan (1).docx

Synthesizing Cyclohexene from Cyclohexanol by Dehydration Abstract Cyclohexene was synthesized from cyclohexanol by dehydration in the presence of a strong acid, in this case, phosphoric acid. The cyclohexene product was isolated from cyclohexanol through distillation, and then further isolated from water by pipetting and the use of drying salts. The percent yield of cyclohexene obtained was 68.104%, which is lower than the expected percent yield of 80%. This is assumed to be due to human errors while carrying out this experiment. Introduction A common method to synthesize alkenes is by alcohol dehydration. Dehydration is the process of removing water, and it is a commonly used process to produce carbon-carbon double bonds (Brown et al, 1923). Heating alcohols at high temperatures in the presence of a strong acid will cause the alcohols to undergo elimination reactions. Since the product exists in equilibrium of both cyclohexanol and cyclohexene, the production of cyclohexene was increased by applying Le Chatelier’s Principle, which states that any disturbance in an equilibrium will shift to minimize that disturbance (Pahlavan, 2012). Figure 1 shows the dehydration of cyclohexanol.

Figure 1: Dehydration of Alcohol (Kho et al, 2010) In this experiment, cyclohexanol was dehydrated in the presence of phosphoric acid, which is a strong acid. The product that resulted from this process was then weighed, and then

used to calculated its percent yield with the following equation: percent yield = (actual yield / theoretical yield) * 100. The expected percent yield of cyclohexene is 80% and above because as the substances are going through the dehydration process, it is being optimized, resulting in a high percent yield. The mechanism of cyclohexene being synthesized with cyclohexanol, begins with the protonation of the alcoholic hydroxyl group by phosphoric acid, making it a good leaving group. The phosphoric acid also acts as the catalyst, in which it promotes the reaction, but does not consume it. This causes the water to be removed, resulting in the unstable carbonium ion intermediate. The carbonium ion intermediate then quickly loses a proton to water to form the alkene (Kho et al, 2010). The mechanism is shown in figure 2. Because water is being removed from the cyclohexanol to form cyclohexene, this is an elimination reaction, specifically E1 (Falcone, 2007).

Figure 2: Mechanism of Cyclohexanol Dehydration (Kho et al, 2010)

Table 1: Physical Properties of Substances Used

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Cyclohexanol

Melting Point: 25°C Boiling Point: 161°C Density: 0.96 g/mL Molecular Weight: 100.16 g/mol

Cyclohexene

Melting Point: 104°C Boiling Point: 83°C Density: 0.81 g/mL Molecular Weight: 82.14 g/mol

Phosphoric Acid

Melting Point: 42.35°C Boiling Point: 158°C Density: 1.885 g/mL Molecular Weight: 98.0 g/mol

Water

Melting Point: 0°C Boiling Point: 100°C Density: 1 g/mL Molecular Weight: 18.01 g/mol

Results and Discussion After completing the distillation process, the clear liquid, which is the cyclohexene obtained was 1.0723 g. The theoretical amount of 2 mL of cyclohexene is 1.5745 g. The percent yield calculated was 68.104%, which is well below the expected yield of 80%. A large factor of this low percent yield is human error, where certain steps were not carefully carried out. One particular step that could’ve skewed the results was the separation of the pure cyclohexene from the drying salts. Not all of the pure cyclohexene was removed from the tube, which lowers the actual amount of pure cyclohexene present in the end result.

Experimental 3

To begin this experiment, a Hickman Still was set up (figure 3). This is the distillation apparatus used to dehydrate cyclohexanol to form cyclohexene. In a 5 mL rounded bottom flask, 0.3 mL of water was added, followed by 0.4 mL of 85% phosphoric acid, 2 mL of cyclohexanol, and a few pieces of boiling chips. The flask was then attached to the Hickman Still, making sure that it does not touch the heating mantle below it. The heating mantle was filled with sand, and the mixture was set to heat. The mixture was heated or distilled until the remaining volume is about 5 mL; this resulted in the mixture turning a distinct yellow color. The mechanism of the Hickman Still is due to the different boiling points of the substances. Since water (b.p.: 100°C) and cyclohexene (b.p.: 83°C) has lower boiling points than cyclohexanol (b.p.: 161.84°C) and phosphoric acid (b.p.: 158°C), it distills out, and collects in the Hickman distilling head after the condenser cooled it (Pahlavan, 2012). The completed distillate mixture was then transferred into a centrifuge tube, and then set standing upright for the mixture to separate. Since water has a greater density than cyclohexene, the bottom layer of the mixture, which is the water, was removed. To ensure that all impurities were removed, 40 mg of magnesium sulfate (drying salt) was added to the centrifuge, and then gently inverted. Once the drying salts settles, the cyclohexene liquid was transferred into a pre-weighed vial with a cap. The product was then weighed, and the percent yield was calculated.

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Figure 3: Hickman Still Distillation Apparatus To calculate the percent yield, the moles of cyclohexene must be calculated from 2 mL of cyclohexanol, assuming 1:1 mol ratio. With the moles of cyclohexene, its theoretical yield was calculated. The percent yield was then determined with equation 1 from above. During the execution of the lab, the following equipment and supplies were used: a Hickman Still distillation apparatus by Kontes Glass Co., A hot plate by Laboratory Craftsmen Inc, and a balance scale by O’haus from the Adventure Pro series. The cyclohexanol used was provided by Fisher Scientific, the magnesium sulfate was provided by J.T. Baker Chemical Co., and the phosphoric acid, water, boiling chips, and sand were provided by the UH Hilo Chemical Stockroom.

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References Brown, A. B., Emniet Reid, E., 1923. The Catalytic Dehydration of Alcohols. J. Phys. Chem. pp. 1079-1080. Falcone, Joe. Keystone College, 2007, Synthesis of Cyclohexene from Cyclohexanol by Acid Catalyzed (E1) Elimination. http://academic.keystone.edu/JFalcone/SynthesisCyclohexene.htm, accessed December 2014. Kho E. and Vaduvescu S. Dehydration of Cyclohexanol to Cyclohexene. - Hilo : University of Hawaii, 2010. - pp. 1-2. Pahlavan C. Preperation of Cyclohexene from Cyclohexanol. - Houston : Houston Community College System, 2012. - pp. 1-6.

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