An experiment was carried out to synthesize cyclohexene by the dehydration of cyclohexanol using concentrated acid as a dehydrating catalyst. Subsequently the product was separated from the reaction mixture (solution instead) by distillation. Analytical techniques like Infrared Spectroscopy and Gas Chromatography were used for identifying and quantifying the different constituents in the reaction mixture. Procedure: The reaction was carried out by mixing 7. 3 ml of cyclohexanol and 1. 75 ml of 85 % H3PO4 in a 50 ml round bottom flask.
An apparatus for dehydration of cyclohexanol was set up, with a fractionating column, a distillation adapter, a thermometer, a condenser, and a vacuum adapter. A rubber septum was used to provide a seal between the thermometer and the glassware. The mixture was then heated at a gentle reflux for 5 min, and then at a much stronger reflux to speed up the distillation process. It was distilled till the volume in the distillation flask had been reduced to 1 ml. The distillate was then transferred to a separatory funnel, and washed with 5 ml of water.
The layers are separated and the organic layer transferred in an Erlenmeyer flask. Sodium sulfate anhydrous was added to the solution to reduce the water content. The liquid was pipetted out, and placed in a round bottom flask for the distillation process. The distillation apparatus was set up the same as the fractional distillation, except for the fractionated Vigreux column. Boling point was then determined during distillation. The distilled product was then transferred to an empty vial, and used for IR (Infrared Spectroscopy) & GC (Gas Chromatography).
IR Procedure: In this procedure the cyclohexanol, cyclohexene, and product was tested. A few drops of each of the solutions was then spread between two sodium chloride plates, and placed in the machine. Sodium chloride was used as it does not absorb strongly in the infrared region whereas glass does, however it does dissolve readily in water. A record of the IR was obtained and printed out for analysis. GC Procedure: Gas chromatography involved the distilled product, vaporized and injected onto the head of the chromatographic column.
The sample was then transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid. The start button initiated the procedure and the data was recorded and printed out. Data: The data from the Gas Chromatography is presented below: Distilled mass of purified product: 1. 49 gram Final mass of cyclohexene : 2. 02 gram Results & Discussion: The product was a colorless liquid. Mass of the purified product was 1.
49 gram and the final mass of cyclohexene was 2. 02 gram. Theoretical yield of the product (cyclohexene) is 5. 75 gram. Boling point observed during initial distillation was 83. 1 oC, while it was 82. 8 oC during final distillation. The value of boiling point during initial distillation is more than the actual boiling point of cyclohexene. This is because of interaction of the solute (cyclohexene) with the solvent (cyclohexanol). H3PO4 i. e. ortho-phosphoric acid used in this experiment was a concentrated strong acid.
A concentrated strong acid is known dehydrating agent and very commonly used for dehydrating alcohols into ethers and alkenes. Because it was not consumed during the reaction rather was just helping the reaction by participating in its mechanism and getting regenerated, therefore, its role was that of a catalyst. Infrared (IR) spectrum shows characteristic absorption peaks of different bonds in the molecule (http://en. wikipedia. org/wiki/Infrared_Spectroscopy). In the IR spectrum of Cyclohexanol, there is a very strong absorption peak at 3335 cm-1.
This peak is characteristic of O-H bond and therefore this is absent in the IR spectrum of Cyclohexene. The absorption peak at 1067. 31 cm-1 is characteristic of C-O bond and therefore, this is also absent in the IR spectrum of Cyclohexene. Absorption peaks near 3000 cm-1 are characteristic of C-H bond and therefore these peaks are present in the IR spectrum of both Cyclohexanol as well as cyclohexene. Similarly, the absorption peaks corresponding to C-C bonds in 800 – 1500 cm-1 are present in the IR spectrum of both Cyclohexanol as well as Cyclohexene. The absorption peak corresponding to C=C at 1652.
45 cm-1 is present in the IR spectrum of Cyclohexene only and not in the IR spectrum of Cyclohexanol. Area under Gas Chromatography (GC) response curve corresponds to the number of moles of the species being detecting. Therefore dividing the area under Cyclohexene GC curve divided by area under the GC curve of Cyclohexene + Cyclohexanol; gives the proportion of Cyclohexene in the total mix. This ratio is ~ 0. 39. This is very close to the actual yield of Cyclohexene that was obtained in this experiment. Atom economy is defined as the molecular weight of the desired product(s) (here 82.
14 gm per mol for Cyclohexene) divided by that of the reactant(s) (here 100. 14 gm per mol for Cyclohexanol). Therefore, in this experiment the atom economy is ~82%. In this experiment, 7. 01 gm Cyclohexanol was used. This corresponds to 0. 07 moles of Cyclohexanol. As one mole of Cyclohexanol can theoretically yield 1 mole of Cyclohexene; therefore, in this experiment the theoretical yield of Cyclohexene is 0. 07 mole, which is 5. 75 grams. Hence theoretical yield of Cyclohexene is 5. 75 gram. REFERENCES: 1. http://en. wikipedia. org/wiki/Infrared_Spectroscopy
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