Preparation of Cyclohexene from Cyclohexanol Using Exothermic Reactions - Essay Example

Preparation of Cyclohexene from Cyclohexanol using exothermic reactions Lab 9 Chemistry Paper [Department] Cyclohexanol, an alcohol, mixes with acids such as sulfuric acid and phosphoric acid to undergo elimination of El. This involves a unimolecular two-step process of conversion through mechanism cyclohexene. The product of end result structure after experimentation usual confirmation is through Refractive Index, FTIR, and HNMR analysis. Introduction Cyclohexanol is a secondary alcohol that undergoes dehydration by an E1 mechanism. This molecule is related to cyclohexane with replacement of one hydrogen atom of a hydroxyl group. The major intermediate of this mechanism is a cyclohexyl cation, which can undergo both substitution and elimination (Lister & Janet, pp 167-245). The substitution reaction always competes with elimination reactions so to prepare Cyclohexanol in good amount, it is essential to suppress the substitution reaction. It is achievable using strong acids such as sulphuric acid whose conjugate base is a poor nucleophiles and running at high temperature to favors elimination. Cyclohexene is a hydrocarbon of formula C6H10. It is as an intermediate in various industrial processes useful in synthesizing other desired molecules. It also helps in the synthesis of maleic acid and it stabilizes high-octane gasoline (Williamson & Katherine, pp123-178). Cyclohexene is not very stable when stored for long exposed to light and air because it readily forms peroxides. Results and Discussion Dehydration is the elimination reaction of alcohol; a unimolecular elimination (E1) reaction of an alcohol. This elimination reaction involves the loss of a hydroxyl group (OH‑) from one carbon and hydrogen (H) from the adjacent carbon. The overall effect of this reaction is the loss of a water molecule, resulting in the formation of a ∏-bond of an alkene or an alkyne. Dehydration is therefore the loss of water molecule (Williamson & Katherine, pp173-212). In most cases, dehydration of an alcohol requires the use of an acid catalyst and high temperature. Phosphoric acid (H3PO4) and sulfuric acid (H2SO4) are the most commonly used acid catalysts. When more than one elimination product can be formed, the more substituted alkene becomes the major product, which is obtained by removing a proton from the adjacent carbon that has fewer hydrogens (Schlosser, pp153-189). The more substituted alkene is the major product because it is the more stable alkene; therefore, it has the more stable transition state leading to its formation. Alkenes can also hydrate; that is the addition of water in the presence of an acid catalyst (Williamson & Katherine, pp123-178). The process of hydration of an alkene is the reverse of the acid catalyzed dehydration of an alcohol. This is illustrated below: Dehydration RCH2CHR RCH=CHR + H2O | OH hydration As the alkene forms in the dehydration reaction, to prevent it from reforming back to alcohol product is removed by simple distillation takes place, as it forms. The alkene distils first as it has a lower boiling point than the alcohol. Removing the product displaces the reaction to the right in agreement with Le Chatelier’s principle (Wirth, pp212-235). Since the OH group exists in a very poor leaving group, an alcohol is able to undergo dehydration only if OH group converts into a better leaving group. Hydroxyl ions protonating converts it hydroxyl group into a good leaving group thus during the first step of dehydration reaction (Lister & Janet, pp 167-245). Protonation processes changes the very poor leaving group –OH into a good leaving group –OH2+. During the second step, water departs and leaves behind a carbocation. During the third step, the base HSO4- removes a proton from the carbon adjacent to the positively charged carbon, forming an alkene and regenerating the acid catalyst H2SO4 (Schlosser, pp153-189). Since the cyclohexene has a lower boiling point than the Cyclohexanol, the cyclohexene distils as it forms as depicted in figure (1) below. HNMR data in figure (2) confirms the products identity after the distillation process. The data gathered also shows three significant peaks at 1.00 (ppm), 2.03(ppm) and 2.04(ppm) implying proper placement of hydrogen environments on the cyclohexene molecule. Another strong peak at 2.04 (ppm) was detected showing the hydrogen environment was next to a carbon double bond. In addition, detection of small peak shows the presence of an aromatic ring (Williamson & Katherine, pp113-135). The evidence of typical 1 HNMR chemical shifts of cyclohexene also confirms that of the products identity (Lister & Janet, pp 167-245). Conclusion Based on the data formulated from the experiment, the group successfully prepared cyclohexene from cyclohexanol. The NMR and IR graphs displays similar readings as expected of cyclohexene. Although the identity of the product is confirmed the percentage yield of 23.43% is lower than expected. Some of the product could have been lost during distillation. Experimental Section Chemical reagents for the experiment include; Cyclohexanol, phosphoric acid (85%), 10% NaCO3, Br2/ CCl4, 0.5% KMno4, Drying agent (CaCl2). Purpose of the experiment is to prepare an alkene by dehydration (elimination of water) of an alcohol in the presence of an acid catalyst, calculation of percentage recovery of product and test for purity and identification of alkenes Place 6.25 ml of cyclohexanol in a round bottomed flask and allow it to cool in a mixture of water and ice. Add 2.0 ml of sulfuric acid (Schlosser, pp153-189). Thoroughly shake the mixture to mix the two solutions completely. Place the round bottomed flask in a simple distillation apparatus whilst maintaining the temperatures below 900 C. The distillation was carried out slowly at a rate of about 1.2 drops per minute until about 25ml of the distillate is collected The distillation must not reach dryness. Remove the source of heat and then cool the apparatus to room temperature. Transfer the distillate in the small round bottomed flask to a separatory funnel. Wash the organic layer with several 5.6ml aliquots of aqueous saturated sodium bicarbonate solution (Williamson & Katherine, pp123-178). Dry the product for 15 minutes using sodium sulfate. 1.54g of the product collects which translates to a percentage yield of 23.43%. Determine a target of 0.060 moles of cyclohexene prior to the experiment. A calculated amount of 6.24 mL of cyclohexanol was prepared with 2.33 mL of H2S04 mixed with I.22 mL of H3P04 (Wirth, pp212-235). During careful distillation of the combined alcohol and acids, reach a boiling point of II 0° C before the cylcohexene compound begins to collect in a separate round bottom flask. Cooling of the compound occurs with the addition of sodium bicarbonate to remove excess acid that remains in the process of distillation. Cyclohexene is insoluble in water and thus is not lost during the washing with aqueous sodium carbonate solution (Williamson & Katherine, pp123-178). Again, add anhydrous Na2S04 to the solution for about 15 minutes to remove excess water since it is a salt that forms a hydrate. Draining of the contents takes place while putting it in a sample vile for a Refractive Index recording. Perform the analysis of FTIR and E1 HNMR. The actual yield of cyclohexene after distillation records at 0.66 grams. This was determined to be about I3% yield of cyclohexene. Calculate the Refractive Index results of the distillate at 1.4437 n20D, which relates to the identified Rf of cyclohexene at I.4461 n20D. The IR spectral analysis of the distillate compound confirms the product formation showing the proper placement of C-H bonds at (2836 cm-1) and (2857 cm-1) with a strong peak at (2924 cm-1). Evidence shows that a =C-H stretches at (302I cm-1) in reference to an alkene (Wirth, pp212-235). Little evidence showing any remaining alcohol groups with a slight hump in the alcohol ranges between (3150 cm-1 and 3350cm-1). These ranges give a slight contamination of un-distilled OH groups remaining. Typical IR spectral data also confirms the product results. Appendices Figure 1: Structure of cyclohexene Figure 2: Typical HNMR chemical shifts for cyclohexene Figure 3: The spectra for the NMR Work Cited Williamson, Kenneth L, & Katherine M. Masters. Macroscale and Microscale Organic Experiments. Belmont, CA: Brooks/Cole, 2011. Print. Schlosser, M. Organometallics in Synthesis: Third Manual. , 2013. Wirth, Thomas. Microreactors in Organic Synthesis and Catalysis. Weinheim: Wiley-VCH, 2013 Lister, Ted & Janet Renshaw. Essential As Chemistry for Ocr. Cheltenham: Nelson Thornes, 2004. Print. Read More