Introduction: This experiment was a study of protonating a cyclo-alcohol to become a cyclo-ene. In this case 4-methylcyclohexanol was protonated using phosphoric acid to become 4-methylcyclohexene. This demonstrates the loss of an alcohol group by protonation, the loss of a proton from the cyclohexane to form an alkene through elimination. This also demonstrates Le Chatelier’s principle, by using distillation to remove the product as it is formed the equilibrium of the reaction will constantly be in favor of the product. Reaction: Side Reaction:
Due to a 1,3-hydride shift dehydration of 4-methycyclohexanol will produce a small amount of 1-methylcyclohexene. According to research performed by Chris Lamb of Lipscomb University, Nashville, TN (see attatchment) some 1-methylcyclohexene will be produced due to either a 1,3-hydride shift or a series of 1,2-hydride shifts (see image 1).
Image 1 – Hydride shift of methycyclohexene. Compound Structure Formula Mol Wt. (g/mol) Boiling pt (°C) 4-Methylcyclohexanol C7H14O 114. 2 171-173°C Phosphoric acid H3O4P 98. 0 212. 8°C Sulfuric acid H2O4S 98. 1 337°C Saturated Sodium Choloride NaCl NaCl 58. 4 108. 7 °C Anhydrous Sodium Sulfate Na2SO4 142. 0 1429 °C 4-Methylcyclohexene C7H12 96. 2 101-102°C 1-Methylcyclohexene C7H12 96. 2 108-112 °C Table 1 – Chemical properties via www. chemspider. com Experimental: February 12, 2014 First the distillation apparatus was assembled (see image 2), then 1. 5 mL of 4-methylcyclohexanol was placed in a pre-weighed conical vial (CV1) and weighed (see table 2).
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To the conical vial was added approximately 0. 40 mL of 85% phosphoric acid and 6 drops of concentrated sulfuric acid. The solution was mixed thoroughly and a spin vane added.
The conical vial (CV1) with solution was then placed in the distillation apparatus (see image 2) and gradual heat applied while spin vane was activated. Aluminum foil was placed over the conical vial and aluminum heating block to aid the heating of solution. As the solution was distilled the product was removed from the Hickman head via a port on the side and transferred into another conical vial which was promptly capped. Once the solution in the original vial was distilled to a remainder of about 0. 5 mL the heat was turned off and the last of the product was transferred.
Then the Hickman head was rinsed with saturated sodium chloride to obtain any product on the sides and this also transferred to the second vial. The second vial was allowed to sit until two layers formed, the bottom aqueous layer was removed and discarded; the remaining organic product was transferred to a test tube and dried over anhydrous sodium sulfate for 10 to 15 minutes while capped. The remaining product was carefully transferred to another pre-weighed conical vial (CV2) and weighed to determine mass of product obtained (see table 2).
Product was sealed in a conical vial with silicon seal and stored at room temperature. Image 2 – Distillation apparatus. February 17, 2014 Conical vial with product removed from drawer, vial attached to condenser without water water-flow applied and placed on heat to determine boiling point. Results and Discussion: Vial Mass Empty (g) Mass Combined (g) Mass of Solution (g) CV1 27. 185 28. 788 1. 603g C7H14O CV2 21. 615 22. 535 0. 920g C7H12 Table 2 – Mass measurements. In this experiment 4-methycyclohexanol was dehydrated using phosphoric acid to yield 4-methylcyclohexene.
The experiment began with 1. 603g of 4-methycyclohexanol and resulted in a yield of 0. 920g of product. Since the theoretical yield is 1. 35g this leads to a percent yield by mass of 68. 1% (see calculations).
The boiling point of the product obtained was found to be 104. 4°C which is higher than the expected 101-102°C (see table 1); this could also be due to the side reaction product whose boiling point is 108-112 °C (see table 1) causing the boiling point found experimentally to be a combination of the two products.
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The side reaction, however, should not affect the percent yield since both products have the same MW. Conclusions: In the process of this experiment phosphoric acid was used to catalyze the dehydration of 4-methycyclohexanol to 4-methylcyclohexene and possibly 1-methycyclohexene as explained above. Being a volatile solution keeping the solutions contained whenever possible was an important consideration. The results obtained in this experiment deviated only slightly from what was expected, however since results from similar experiments could not be found the percent yield could not be compared to other’s results.
The boiling point found experimentally was only 2. 4°C higher than the accepted value of the product expected (4-methylcyclohexene).
However, as stated above, this error could be due to the fact that some of the product was 1-methylcyclohexene but this would not explain the yield since the MW of both products are the same. Error in the yield could be due to the aforementioned volatility of the product, especially since the solution required multiple transfers exposing it to air and allowing evaporation whenever it was uncapped.
Further error would be due to having accidentally left the cap off of the vial while washing the Hickman head with saturated sodium chloride allowing more evaporation than would be expected in this experiment. Purity and Quantity: With a percent yield of 68% and a boiling point that was very close to the accepted it is believed the expected products were obtained with fair purity. Calculations: Mass of 4-methylcyclohexanol = vialfull – vialempty = 28. 788g – 27. 185g = 1. 603g Mass of product = vialfull – vialempty = 22. 535g – 21. 615g = 0. 920g Theoretical yield: Percent yield:
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