Introduction: Cyclohexanol is mainly used in the production of caprolactam and adipic acid that is a raw material of nylon 6 (Zhang, et al, 2002). Cyclohexanol can be produce through several methods, which include the oxidation of cyclohexane, the hydration of cyclohexene, or the hydrogenation of phenol (Zhang, et al, 2002). Problem with oxidation of cyclohexene is poor selectivity, extremely large recycles and explosion hazards. (Suresh, Sridhar, Potter, 1988). The purpose of this experiment was to synthesis cyclohexanol by hydration of cyclohexene using concentrated sulphuric acid as an acidic catalyst.
In hydration reaction, C=C B bond is replaced by hydroxyl group (Hornback, 2006). Overall reaction: (McFadden, 2012) In the first step, the mixture of water, concentrated sulphuric acid, and cyclohexene was shaken vigorously until it became a homogenous solution. Followed by, the addition of water, and the distillation process lead to the hydrolysis of the alkene. Finally, addition of diethyl ether to the mixture then distillation took place to be purified and to obtain the final product, cyclohexanol (McFadden, 2012).
Diethyl ether was used to extract alcohol from salt-water mixture because diethyl ethers solubility in water is lower than cyclohexanol which helps remove alcohol from the salt-water mixture (Merzougui, A, et al. , 2011). (McFadden, 2012) Cyclohexene is added to water-acid solution, which formed two liquid phases were due to insolubility of cyclohexene in water-acid solution. It is very important that mixture is mixed well to make a homogeneous solution and allow reaction to complete. Cyclohexene was reacted with water and with sulfuric acid to form protonated cyclohexanol and cyclohexyl hydrogen sulfate.
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Protonated cyclohexanol and cyclohexyl hydrogen sulfate are soluble in water-acid solution (McFadden, 2012). When this mixture is heated, cyclohexyl hydrogen sulfate converts to protonated cyclohexanol which is equilibrium with cyclohexanol (McFadden, 2012). Moreover, distillation technique is used to separate components of a liquid mixture, where liquid is boiled to vaporize and then condensed back into liquid called distillate. Distillate components are collected with a lowest-boiling point to highest-boiling point.
In this experiment, distillation is used to separate the organic compound from sulphuric acid solution; azeotrope of cyclohexanol and water is formed as distillate, it also contains some cyclohexene components. Azeotrope has a fix boiling point like a pure compound. Cyclohexanol is considerably soluble in water, so poor yield is expected (McFadden, 2012). Addition of sodium chloride to distillate solution improves the final product yield because it reduces the solubility of organic compound, and extracts cyclohexanol from aqueous phase. It is due to salt’s higher solubility than organic compound.
Also, anhydrous potassium carbonate is used to absorb water and to neutralize any trace of acid. During distillation of pure cyclohexanol, most of the product should be collected as temperature reaches 161? C (Weast, 1988). This experiment also introduces determination of the refractive index; it is one of the most convenient methods used to determine purity of liquid. It is a ratio of the sine of the angle formed when light ray is bent when passing from an air medium into a liquid medium; in other words it is a ratios of the speed of light in a vacuum to the speed of light in the liquid medium.
Refrective index (n) = C_vaccume/V_medium (McFadden, 2012). At 20°C, the expected refractive index of cyclohexanol is 1. 4641 (Weast, 1988). Procedure: The experiment was done in two parts. Part-A was hydration of the alkene, and Part-B was isolation and purification of the cyclohexanol. To perform hydration of alkene, 7. 0mL of water and then 14mL of concentrated sulphuric acid were added to a 125mL Erlenmeyer flask. After that, flask was placed in ice-bath until it was cold to touch. 16. 4g of cyclohexene was added to weighed 250mL round-bottom flask.
In the same round-bottom flask, the chilled water-acid mixture was added. In order to mix the solution, the flask was vigorously shaken for 20 minutes. While shaking, regularly stopper was released to prevent any build-up of vapour pressure. After flask was vented and allowed it to stand without disturbing for about 5 minutes. Because two distinct layers appeared, continued shaking for extra 10 minutes until solution was homogeneous. Next, an additional 120mL of distilled water were added in homogeneous solution with addition of 3 boiling chips.
The same round-bottom flask was then set onto a distillation apparatus, and started distillation. Distillate was collected into an 125mL Erlenmeyer flask, and boiling point range of azeotrope was noted. Subsequently, 25g NaCl was dissolved to the distillate, but not all salt was dissolved. After that, flask was covered with parafilm (McFadden, 2012). In the next lab, distillate mixture was transferred to a separatory funnel. Distillation receiver flask was washed by 20mL diethyl ether, that diethyl ether was then transferred into the separatory funnel.
Mixture was allowed to delayer for 3 minutes. Afterwards, bottom aqueous layer was drained into a aqueous waste beaker; and top ether layer was transferred in an another 50mL Erlenmeyer flask which contained 3g of anhydrous potassium carbonate, swirled and allowed the mixture to stand for 15 minutes. Next, no potassium carbonate but only liquid mixture was transferred to round-bottom flask for the distillation. Diethyl ether and cyclohexene were distilled and collected into a flask until it reached 120°C, and later discarded in an organic waste.
As temperature reached 120°C, a clean, dry and pre-weighed flask was replaced to collect final product, cyclohexanol. As soon as flask was replaced, cold water was turned-off and hot water was turned on. Continued to distil until there was no liquid in the distillation flask, and boiling chips started to change colour. Small amount of residue was kept in distillation flask to prevent it from breaking. Finally, cyclohexanol was weighted and from small sample the refractive index was determined (McFadden, 2012). Result: Amount of cyclohexene used = 16. 40g = 16. 40g of C_6 H_10? 1/(82. 143 g/mol)=0. 9965=0. 1997mol of C_6 H_10 Limiting reagent is: Cyclohexene Bp range of azeotrope mixture: 85-95. 4°C Literature bp range of azeotrope mixture: 97. 8°C; edition: 53rd; page: D-16 Bp range of diethyl ether: 34. 6-41. 6°C
Literature bp range of diethyl ether: 34. 51°C; edition: 53rd; page: Bp range of cyclohexene: 82. 8-90°C Literature bp range of cyclohexene: 82-98°C; edition: 53rd; page: C-259 Bp range of pure cyclohexanol: 157-161. 0°C Literature bp range of cyclohexanol: 161. 1°C; edition: 53rd; page: C-257 Weight of pure cyclohexanol = 7. 1g Percent yield = (actual yield (g))/(theoretical yield (g))? 00%=7. 1g/20. 00g? 100%=35. 5% Theoretical yield= (0. 1997mol of C_6 H_10)/? (1 mol of C_6 H_12 O)/(1 mol of C_6 H_10 )? (100. 158 g)/(1 mol of C_6 H_12 O)=20. 00g Refrective index of pure cyclohexanol: raw 1. 4643 at 21. 2°C Corrected 1. 4658 at 20°C Correcting refractive index: n_D^20=n_D^21+[0. 00045°C^(-1)? (21. 2-20°C)] =n_D^21+[0. 00045°C^(-1)? (1. 2°C) =1. 4643+[0. 00054] =1. 46484=1. 4648 Refractive Percent yield error: ((1. 4648-1. 4641))/1. 4641? 100%=0. 05% Literature refrective index of cyclohexanol: 1. 4641 at 20°C Edition of CRC: 53rd; page: C-257 Discussion:
Before reaching a concrete conclusion, it is very important to interpret the result that was obtained in this experiment. In this experiment, cyclohexene was hydrated to produce cyclohexanol; because the direct hydration of cyclohexene is very slow, concentrated sulphuric acid as an acidic catalyst is used to speed up the reaction (McFadden, 2012). When cyclohexene was reacted with water and concentrated sulphuric acid, dark homogenous solution was formed from colourless heterogeneous mixture. The reaction was cyclohexene ? protonated cyclohexanol + cyclohexyl hydrogen sulfate. Both of hese products were soluble in water-acid solution, therefore reaction could go to completion (McFadden, 2012). Moreover, azeotrope of cyclohexanol and water was a positive azeotrope which means boiling point of azeotrope was less than the boiling point of cyclohexanol and water. Moreover, distillation process can prevent side reactions and by removing the products it shifts equilibrium on right hand side to increase percent yield; however, it is not enough to improve percent yield. Cyclohexanol is soluble in water; so addition of NaCl forces cyclohexanol to leave aqueous phase into organic phase.
By reducing its solubility in water, NaCl molecules were holding water molecules. It is due to NaCl’s stronger attraction to water than cyclohexanol; solubility of NaCl in water is 360g/L, and solubility of cyclohexanol in water is 36. 0g/L (Weast, 1988). It is a great way to separate azeotrope into different components. However, enough salt is necessary to make solution saturated to separate all cyclohexanol from aqueous solution; for example, 45. 72g of NaCl is needed to make solution saturated in 127mL of water. Required salt can be calculated by multiplying solubility of salt in water with given volume.
In part B, there was cyclohexanol found in the condenser because temperature of water running in the condenser was low compare to melting point of cyclohexanol, which is 24°C; so some of the cyclohexanol was stuck on the inner-surface of the condenser. Cold-water was turned off, and hot-water turned on when cyclohexanol was collecting during distillation process to remove the cyclohexanol from the surface of condenser and used in the final product. The result shows that the percent yield is 35. 5%. As expected percent yield is low because the strong acidic conditions and solubility of cyclohexanol in water (Hornback, 2006).
Observed reflective index is very close to literature value of reflective index of cyclohexanol, which tells that product is pure but there is still some impurities. The result is also due to inefficient experiment procedures. This experiment required precise measurements of data in order to obtain accurate results. But, there are many possible sources of experimental error when performing this experiment. Firstly, if water-acid solution was not cooled enough to add cyclohexene, then some of the cyclohexene have evaporated.
Cyclohexene is a limiting reagent meaning it will affect the overall weight of cyclohexanol by reducing the amount. Secondly, not using properly clean and dried equipments may affect on reactants’ activities, such as a flask, beaker, graduated cylinders. Thirdly, solution was not homogeneous; in other words, failing to mix properly for the reaction to go to completion. It was hard to judge due to very dark colour of the solution. If reaction was not fully reacted means not all cyclohexene were reacted to form protonated cyclohexanol and cyclohexyl hydrogen sulfate.
Fourthly, some potassium carbonate may have entered in distillation flask which resulted in reverse reaction causing to lose more cyclohexanol. Fifthly, a small amount of product might be lost when transferring from one container to another. Sixthly, some cyclohexanol was left in round-bottom flask in order to prevent the round-bottom flask from breaking due to over-drying or over-heating. There are a few methods that would improve the accuracy of the experiments, if considered and followed with care. While recording the volume from the graduated cylinder the goal is to look for at the curve on the top of a standing body of liquid.
Before experiment takes place make sure to clean all equipments that are going to be used throughout experiment to avoid any beside reactions in the experiment that may affect the final result. When transferring from one flask to other, sometimes filter paper would be a better solution to prevent unwanted product from entering into a reaction flask, and to prevent any reverse reaction to occur, such as potassium carbonate. Conclusion: In this paper, distillation process for the indirect hydration of cyclohexene to cyclohexanol using sulphuric acid as an acidic catalyst is demonstrated.
In the hydration process, double bond of cyclohexene is replaced by the hydroxyl group to form alcohol. Obtain reflective index of cyclohexanol is 1. 4648, and the literature value of reflective index of cyclohexanol is 1. 4641at 20°C; which shows that final product was very pure. The result also showed that the percent yield is only 35. 5%, it is due to the strongly acidic conditions and solubility of cyclohexanol.
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