15/05/2008 Chemistry 231 Experiment 11 Lee 1 Cyclohexene from Cyclohexanol Larry Lee Partner: Ichiro Suzuki

Transcription

1 15/05/2008 Chemistry 231 Experiment 11 Lee 1 Cyclohexene from Cyclohexanol Larry Lee Partner: Ichiro Suzuki bjective: The purpose of this experiment is to isolate Cyclohexene from Cyclohexanol by sulphuric acid catalyzed dehydration reaction. Procedure: Please refer to Chemistry 230/231 lab manual, Camosun College, revised July 2005, pages {note: use of superscript to give source of information] Data: Chemical reaction: 2 S 4 heat + 2 Table 1: List of reagents and amounts used for the preparation of Cyclohexene {note: descriptive title} Reagent Molar mass Suggested mass Mass or Mol used Notes/ potential hazards 2 (g/mol) or volume volume used Cyclohexanol g/mol 10.4 ml 11 ml mol Density = g/ml Colourless viscous liquid (bp o C), Flammable, skin irritant Dilute sulphuric acid ~50:50 water 98 g/mol 4 ml 4 ml - 18 M conc sulphuric Add water to acid to dilute. Very good dehydrating agent. Clear colourless and odourless liquid. Very corrosive. {Note: It is not necessary to include reagents used as washes}

2 15/05/2008 Experiment 11 Cyclohexene from Cyclohexanol Lee 2 Table 2: Yield and physical data for Cyclohexene isolated. {note: descriptive title} Product Molar mass (g/mol) Grams obtained % Yield Bp o C Notes 2 Expt. Lit. 2 Cyclohexene 82.1 g/mol 4.15 grams 47.7% Clear colourless and odourless liquid. Some oily droplets was observed in the final isolation. Table 3. Infrared spectral analysis of Cyclohexanol and Cyclohexene Compound Absorption frequency (cm -1 ) Functional group Intensity Literature reference 3 Cyclohexanol cm -1 - Strong, v.broad cm -1 Strong, broad 2929 cm -1 C- alkane Strong, broad cm -1 Cyclohexene 3023 cm -1 =C-, sharp cm cm-1 C- alkane cm -1 Absent C=C Very weak cm -1 Calculations: % Yield calculation: Since mole of Cyclohexanol : mole of Cyclohexene is 1:1, then mol of Cyclohexanol theoretically gives mol of Cyclohexene. Theoretical mass of Cyclohexene = (0.106 mol Cyclohexene)(82.1g/mol Cyclohexene) = 8.70 g % Yield = mass experimental x 100% = 4.15 grams x 100 = 47.7 % mass theoretical 8.70 grams

3 15/05/2008 Experiment 11 Cyclohexene from Cyclohexanol Lee 3 Discussion: The sulphuric acid catalyzed dehydration of cyclohexanol to cyclohexene proceeds by an E1 mechanism. After protonation of the alcohol, formation of a carbocation occurs. This is the slow step or rate determining step of the reaction. The formation of the alkene product occurs by elimination of a hydrogen on a β-carbon to the carbocation. The mechanism is shown below 3 : S S S S The reaction can also proceed in the reverse direction because the carbocation can undergo an S N 1 reaction with water. As a result of this, the reaction is in equilibrium. To force the reaction to proceed towards the E1 product, water and the alkene are collected by distillation. {discuss the reaction and observations}. In this experiment, the initial distillation of the cyclohexene produced a cloudy and acidic product. After neutralization and extraction of the aqueous solution, the crude cyclohexene was redistilled to give a clear colourless liquid, containing a few immiscible droplets which can be attributed to water. {discuss appearance of the product} Table 1 summarizes the reagents and amounts of reagents used in the preparation of cyclohexene from cyclohexanol, while Table 2 gives the yield and physical data for cyclohexene. The yield of cyclohexene was low because the reaction is in equilibrium between the S N 1 and E1 products {discuss yield, side reactions, potential errors that are embedded in the experiment, not personal errors}. n the final distillation of cyclohexene, the product was collected at a boiling range of o C which is very close to the boiling point of pure cyclohexene at 82.8 o C 2 {discuss any physical properties}. Both cyclohexanol and cyclohexene were characterized by IR

4 15/05/2008 Experiment 11 Cyclohexene from Cyclohexanol Lee 4 spectroscopy. {discuss product characterization by instrumental methods} Figure 1 and 2 show the infrared spectrum of cyclohexanol and cyclohexene, respectively, while Table 3 is a summary of the characteristic absorption frequencies of relevant functional groups. f note, the C=C absorption, characteristic of alkenes at about 1620 cm -1, was absent. The reason for this can be attributed to the symmetrical nature of the cyclohexene, and there is no significant dipole change on IR excitation of the molecule. Evidence to support the isolation of cyclohexene was provided by the absence of an alcohol absorption at cm -1 and the presence of =C- absorption characteristic of alkenes at 3023 cm -1. Conclusion: A yield of 4.15 grams and 47.7% of cyclohexene with a boiling range of o C was isolated from sulphuric acid catalyzed dehydration of cyclohexanol. {note: Avoid the use of slang, avoid personalizing the report with I, we, us, our. Use third person passive past tense.} References 1. R.Raap, N.Khalifa. rganic Chemistry Experiments for Chemistry 230 and 231. Camosun College, Department of Chemistry and Geoscience, Victoria, BC, Revised July pages Wikipedia, the free encyclopedia Accessed May 15, McMurry, J. rganic Chemistry, 7 th ed.: Brooks Cole, {Include IR spectrum Labelled figures. Do not include the printed table} {Citation is standard ACS (American Chemical Society) }

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