Preparation of Cyclohexene from Cyclohexanol

Experiment 5 Preparation of Cyclohexene

In this experiment we will prepare cyclohexene from cyclohexanol using an acid catalyzed dehydration reaction. We will use the cyclohexanol that we purified in our previous experiment. The reaction involved is shown in Figure 5.1.

Figure 5.1 Preparation of cyclohexene O H + H2 SO4 sulfuric acid

+

H2 O

cyclohexanol Mechanism H O H O + H O S O OH CH CH H O

cyclohexene H2 O O H + OS O OH

H C

H C

H + H 2O

H C H3 O+ + H C

This is an acid-catalyzed dehydration reaction. The first step is the reversible protonation of the oxygen atom of the alcohol. The lone pair of the oxygen, acting as the base, attacks the proton of the sulfuric acid to give the protonated species. The protonated alcohol is now a good leaving group. Loss of H2O occurs to give a carbocation, followed by loss of an adjacent proton. This is an elimination reaction. Note that H3O+ is regenerated in the final step so that the H3O+ is not used up in the reaction. As you see in Figure 5.1, all steps in this conversion are reversible: cyclohexene, water and acid react to give cyclohexanol. The procedure works only because the pot temperature is about 160 °C. As soon as cyclohexene and water are formed, they evaporate and are removed from the acid solution. Sulfuric acid has a very high boiling point. Physical Constants Compound Mol. Wt (g/mol) Density (g/mL) b.p. (°C) m.p. (°C) Cyclohexanol 100.16 0.96 161.5 25.5 Cyclohexene 82.15 0.811 83 -104 Sulfuric acid 98.08 1.840 330 Experimental Procedure and Set-up The set-up for the apparatus is shown in Figure 5.2. First, add 10 mL cyclohexanol to a clean, dry 100 mL round bottom flask. Add 2-3 boiling chips and then add about 0.5 mL concentrated sulfuric acid (about 10 drops). Swirl thoroughly to mix the contents. At this point it is normal to see a bit of yellow color. Clamp the 100 mL round bottom flask in place as shown in Figure 5.2. Add your distilling column, the three-way adapter, your...