EXPERIMENT 5

ALCOHOL DEHYDRATION OF CYCLOHEXANOL TO CYCLOHEXENE

OBJECTIVE: Synthesize cyclohexene from cyclohexanol via an alcohol dehydration reaction. Cyclohexanol and phosphoric acid are reacted at reflux and the products, cyclohexene and water, are separated from the unreacted cyclohexanol by fractional distillation as the reaction progresses. The cyclohexene is then separated from the water and purified by fractional distillation. The identity and purity of the product is evaluated using boiling point (BP), infrared spectroscopy (IR), and gas chromatography (GC).

INTRODUCTION:

Alcohol dehydration is an acid catalyzed reaction, which can be performed by strong, concentrated mineral acids such as phosphoric acid.

The acid protonates the alcoholic hydroxyl group, (the unprotonated alcohol serves as the reaction solvent), permitting it to dissociate as water to give the unstable carbonium ion intermediate;

which quickly loses a proton to water or the conjugate acid to give the alkene and regenerate the catalytic acid.

Since the cyclohexene and cyclohexanol exist in equilibrium, it is difficult to convert all the cyclohexanol to cyclohexene. We will employ Le Chatelier's principle which states that the equilibrium which exists between reactants and products will shift in the direction favoring the products (alkene and water in this experiment) if a product, such as water, is removed.

ALCOHOL <=> ALKENE + WATER

Cyclohexene and water are removed from the distilling flask as an azeotrope (see appendix) by fractional distillation as the reaction progresses. After the fractional distillation, we will determine the effectiveness of the separation of products from reactants as well as the purity of the two fractions. The purity and composition of the cyclohexene will be determined by infrared spectroscopy and gas chromatography analysis.

EXPERIMENTAL PROCEDURE:

Prelab Preparation

Reading topics:

Preparation of alkene by dehydration of an alcohol (lecture text)

Refluxing and fractional distillation (appendix)

Gas chromatography (appendix); infrared spectroscopy (appendix)

Answer the prelab questions found at the end of the experiment.

Glassware and Materials:

Fractional Distillation: 25, 50 (preweighed with stopper for product), 100 and 250mL round bottom flasks, 3-way connecting tube, West condenser, distilling condenser, vacuum connecting tube, straight tube adaptor, thermometer, thermometer holder, two or three ring stands, 250mL heating mantel and voltage controller, 3-4 clamps and clamp holders, ring support, boiling chips, ice, large beaker for ice.

Other Equipment: separatory funnel with glass stopper, pH paper, gas chromatograph.

CAUTION: Phosphoric acid (2023) is corrosive to tissue. Wash immediately upon contact. Cyclohexanol (1212) and cyclohexene (1323) are flammable so take care to contain any fumes. All work must be done in a fume hood. Never remove glassware containing cyclohexene from the hood unless they are stoppered. Make sure all joints on the distillation apparatus are sealed or you will lose product. Also, be sure to rinse all glassware that contained cyclohexene with acetone in the prep hoods before washing in sink. Cyclohexene can also form peroxides over time if exposed to oxygen in air and light over an extended period.

Procedure:

Set up a fractional distilling apparatus: Set up the fractional distillation apparatus as shown in figure 2 on page A-2 of the appendix (do not place steel wool in the condenser), lightly grease and seal glassware joints, correctly place the thermometer, and keep the 100 mL receiving flask cold in an ice-water bath to prevent loss of the product, cyclohexene, by evaporation. The apparatus does not need to be completely dry. Cool the horizontal column connected to the receiving flask with cold water.

Mix reactants: Add about 35 mL (record to the nearest 0.2 mL) of cyclohexanol, 10 mL of 85% phosphoric acid, and about 4-8 boiling chips to a clean, dry 250 mL round bottom flask. Reconnect the 250 mL round bottom flask to the apparatus and get the TA or instructor to approve your setup before beginning the reflux/distillation.

Begin the distillation/reflux: Use a heating mantle with the controller initially set at 6 and increase as needed to gently boil the reaction mixture so that the vapors ascend slowly up the fractional column, begin to condense, and then collect in the receiver flask at a rate of about 1 to 2 drops per second. Adjust the controller down or up if the rate of distillation is too fast or too slow. Observe and record the temperature of the first drop of distillate, the temperature range over which the bulk of the distillate is collected, and the temperature at which the heat source is removed. Remove the heat source when you observe white vapors and foam in the distilling flask or column - this will occur when most of the starting volume has reacted and distilled. The white vapor may be phosphoric acid. Remove the collection flask and keep a stopper on the flask containing the cyclohexene at all times to avoid loss due to evaporation. Cyclohexene is very volatile and it will evaporate very rapidly if left open to the air.

Isolation and purification of product: Neutralize the distillate which contains some phosphoric acid (some phosphoric acid unavoidably distills with the products) by

WARNING: THE ADDITION OF 5% SODIUM BICARBONATE TO THE ORGANIC DISTILLATE MAY EVOLVE A LARGE AMOUNT OF CARBON DIOXIDE GAS. ADD IT SLOWLY AND SWIRL THE SOLUTION AS THE NaHCO₃ IS ADDED.

transferring the distillate to a clean separatory funnel and adding about 10 mL of 5% NaHCO₃ product solution. Swirl the solution slowly at first and then shake vigorously to neutralize the phosphoric acid. Allow the layers to separate, drain the aqueous layer into an Erlenmeyer flask, and test the pH of the aqueous layer (bottom layer). Repeat the neutralization if the pH is below 5, or continue to the next step if the pH is 5 or greater. Stopper the flask with the used aqueous bicarbonate solution when transporting it to the waste container labeled “Waste 5% Sodium Bicarbonate” in the fume hood while saving the top layer.

Drain the top layer, the cyclohexene product, into a clean, preweighed 50 mL round bottom flask (weigh with stopper). Reweigh the flask with the product to get the mass of cyclohexene that you made. If the period is almost over, add 0.1 grams of sodium sulfate per mL of product obtained, swirl, and stopper with lightly grease glass stopper until the next lab period. This will help remove water from the product and prevent evaporation. Label the flask with your name and section letter and place the flask in a labeled 150 or 250 mL beaker. Give this to your TA to store in a refrigerator until next week.

Second Week

Continue drying the organic solution by draining it through a small amount of anhydrous sodium sulfate layered above a plug of glass wool in a glass filter funnel. Collect the dry organic layer in a clean, dry, preweighed 250 mL round bottom flask after it passes through the funnel. Weigh the flask to obtain the mass of the product.

Fractional Distillation: Take advantage of cyclohexene's relatively Iow boiling point and purify the organic solution by fractional distillation. Since cyclohexene has a Iow boiling range, it should distill and be collected first followed by cyclohexanol. If small amounts of water, phosphoric acid, acetone, or hexane are present other distillation fractions could be collected.

Set up the fractional distillation apparatus so that the 250 mL round bottom will be in the heating mantle. Use preweighed clean, dry 25, 50, and 100 mL round bottom flasks to collect the purified product. The 25 mL flask will be used to collect the first 5-10 drops of distillate call the forerun. The 50 mL flask will be to collect the bulk of your product. If the temperature increases significantly (or if it drops and you have to increase the heating mantle setting) during the distillation, switch to the 100 mL flask. Keep the collection flasks cool during the distillation by keeping them in an ice-water bath. Make sure each of the flasks will fit in your ice bath. Obtain permission from your TA or instructor before beginning the distillation.

Start distillation: The temperature controller for the heating mantle can initially be set at "6" and increased to "7 or 8" if needed as the distillation progresses. As before, observe and record: the temperature of the first drop of distillate, the temperature range over which the distillate is collected in each flask, and the temperature at which you turn off the heating mantle. Switch collection flasks if the temperature rapidly increases or you stop distilling product and have to increase the temperature. Do not distill to dryness. Record the mass of each flask and determine the mass of each fraction and the temperature range it was collected over.

Obtain an infrared spectrum (IR) for the middle fraction. Load the cell and run the IR immediately since cyclohexene evaporates so quickly. Finally have the TA take a gas chromatogram (GC) of the purified product (middle fraction in 50 mL flask). The IR of your purified product must be run and shown to your TA before they can run a GC and collect your product. Obtain a GC and/or IR of the other fractions and the sample taken before the second distillation if time allows.

Report Requirements: No report is required for this experiment. However, be sure to thoroughly evaluate your IR and GC spectra, as well as all of your boiling points in your lab notebook. Determine the theoretical and percent yield of cyclohexene and clearly explain your findings and any possible error in the lab notebook also.

Experiment 5 Study Guide - Prelab Questions

These questions are designed to help you understand the experiments, but in order to be helpful, you must read the laboratory manual, study the appendix and complete the study guide before the class in which you plan to begin the experiment. On the day of the experiment, the quiz will cover this material and the prelab lecture.

1. What is the primary dehydration product of 1-methylcyclohexanol? Be able to predict the primary dehydration product of any alcohol.

2. The products, cyclohexene and water, are removed from the reaction as an azeotrope. What is an azeotrope?

3. What is Le Chatelier's principle? Why is it difficult to convert 100% of the cyclohexanol to cyclohexene? What experimental techniques are performed in this experiment to "push" the reaction to completion?

4. Be able to explain why it is necessary to neutralize, separate, and dry the product before the 2nd fractional distillation.

5. Describe three ways to prevent fumes of cyclohexene from escaping into the lab.

6. Define distillate and distilland. In what order will cyclohexanol, cyclohexene, and water be collected when conducting a fractional distillation?

7. Identify the limiting reagent and catalyst in this experiment. Of these two volumes, which must be known with more accuracy? Be able to explain the answer! ·

8. How many moles are in 35.0 mL of cyclohexanol?

9. What is the theoretical yield in this experiment? If a student made 2.43 g of product what is the percent yield?

10. Consider the structures of cyclohexanol and cyclohexene: Are they both water soluble? Explain! Which one has the higher boiling point? Relate the differences in boiling points and solubility in water to intermolecular forces.

11. Will cyclohexene or cyclohexanol be the first to exit or be detected by a gas chromatograph if the separation is based on boiling point? Explain why you think this is the case.

12. Risk Assessment: What are the safety hazards and precautions for this experiment?