EXPERIMENT 10 Preparation of cyclohexene from cyclohexanol. Aim: The objective of this exercise is to prepare cyclohexene from cyclohexanol and determine the efficiency of this conversion. Experimental learning objectives: How to;
set up a distillation apparatus and perform a distillation use a separatory/dropping funnel wash and dry an organic liquid identify the organic phase in an immiscible organic/aqueous mixture synthesise an alkene by dehydration of an alcohol identify the presence of unsaturation in an organic molecule using both chemical reactions and IR spectroscopy
In this experiment an alkene (cyclohexene) will be prepared by dehydration of an alcohol (cyclohexanol) using an acid catalyst such as phosphoric acid. This is one of the most common methods of preparing alkenes.
The crude product is contaminated with water, unreacted alcohol, phosphoric acid and some side products. Washing with water removes most of the impurities. Treatment with sodium carbonate solution removes traces of acid and a final wash with water removes any remaining carbonate. The mechanism of the dehydration of cyclohexanol probably involves the formation of a carbocation.
This carbocation can react in any o fthe ways shown below: 1.
With water to yield cyclohexanol - the starting material. (Note that all the steps
in this reaction are reversible); 2.
by losing a proton to yield cyclohexene;
with cyclohexanol to yield dicyclohexyl ether.
Dicyclohexyl ether then is a probable side product of the dehydration of cyclohexanol. It is immiscible with water is likely to co-distill and may therefore be present in the first distillate. To remove dicylcohexy ether completely a second distillation of the product is usually carried out. Reference: T.W.G. Solomons and C. Fryhle, Organic Chemistry, Chapter 7.7, Dehydration of Alcohols.
Safety Features - CAUTION! 1. You must wear eye protection at all times. 2. Phosphoric acid (85%) can cause severe burns. Wash all spills on the skin with cold water for 15 min. See your demonstrator. 3. Bromine causes severe burns. Keep bromine away from your skin. Do not breathe the vapours. Use only in the FUME HOOD. 4. Cyclohexanol can be irritating to the respiratory system and skin. Do not breathe vapours and prevent contact with skin. 5. Potassium permanganate is a strong oxidizing agent. Handle with care. 6. Cyclohexene has an unpleasant smell. Cover all containers of this compound and do not leave drying agent, glass wool or towels coated with the compound lying on
the bench. Procedure: Pour cyclohexanol (10.0 g, 10.6 mL, b.p. 161°) into a 50 mL round bottom flask (small neck) and cautiously add 85% phosphoric acid (3 mL). Add 3 boiling chips and arrange for a distillation using a cooled 10 mL graduated cylinder as a receiver. (Cool the cylinder by standing it in a beaker of ice and water). Heat the round bottom flask slowly with a small flame. When white fumes appear in the round bottom flask, and about 10 mL of distillate have been collected, discontinue the distillation. Transfer the distillate to a separatory funnel (60 mL) (Check first to ensure that the stopcock of the separatory flask is closed!). The mixture will separate into two layers. Run off the lower layer into a conical flask and set this aside. Add water (10mL) to the liquid in the separatory funnel, stopper the funnel and shake to allow thorough mixing of the liquids. (This exercise is termed "washing the cyclohexene with water". Return the funnel to the clamp, loosen the stopper and allow the layers to separate. Cyclohexene (density 0.81 g cm-3) will separate out from the water. Run off the aqueous layer. (Make sure that it is aqueous by adding a drop of water to the flask in which you have collected it!!) Now wash the cyclohexene (where is it?) with 10% sodium carbonate solution (10 mL). Allow the layers to separat and run off the aqueous layer. Repeat the washing with water (10 mL). Transfer the cyclohexene into a clean dry conical flask and add a spatula full of calcium chloride. (Please remember to quickly cover the bottle of calcium carbonate after use since it is hygroscopic and will soon pick up moisture rendering it useless for the rest of the class). Stopper your conical flask, swirl the mixture and allow to stand. If the mixture is
cloudy you will need to add another spatula full of calcium carbonate. Allow the solution to stand over the drying agent for about fifteen minutes, then, by gravity filtration using cotton wool, filter the cyclohexene into a dried pre-weighed test-tube. Immediately stopper the tube with a cork and discard the drying agent and cotton wool into the waste containers which are in the fume hood. If the liquid is cloudy indicating the presence of moisture, add a few lumps of calcium chloride to the test tube and allow the mixture to stand until clear. Decant the liquid to a clean, dry pre-weighed vial that has been labelled in the usual way. If the liquid is clear, place the sample in a suitably labelled vial. Calculate the determine the yield and percentage yield of cyclohexene. Carry out the bromine/dichloromethane and permanganate tests (see Appendix I on Tests for functional groups) on your product. Have the remainder of your sample available for grading. Note 1. Filtration by gravity will occur much more readily if you place a paper wedge between your funnel and your test tube. Be careful not to let the wedge fall into your solution on removing the funnel. Infrared Spectroscopy In this experiment an alkene has been prepared from an alcohol. Functional group absorptions can be used to assist in the identification of reactants and products. (See Appendix 2). The region shown below is where the O-H and C-H stretches are found and if you ran a sample of your own product it should be possible to determine whether the conversion was successful. Note on your worksheet the value of the O-H absorption of cyclohexanol and the value of the C=C absorption of cyclohexene.
Dehydration Of An Alcohol: Cyclohexene From Cyclohexanol Title: Dehydration Of An Alcohol: Cyclohexene From Cyclohexanol Objective: To produce cyclohexene through the acid catalyzed elimination of water from cyclohexanol. To understand mechanism involved in the reaction. To learn the technique of distillation. Introduction: A secondary alcohol, such as cyclohexanol, undergoes dehydration by an E1 mechanism. The key intermediate in the mechanism is a cyclohexyl cation, which can undergo substitution as well as elimination. To prepare a cyclohexene (olefin)
in good yield, it is necessary to suppress the substitution reaction. In this experiment, the substitution reaction is suppressed by: (1) the use of strong acids with anions that are relatively poor nucleophiles ; (2) a high reaction temperature, which favors elimination; and (3) distillation of cyclohexene from the reaction mixture as it is formed. The dehydration reaction is of paramount importance in the preparation of olefins, which are the raw materials of much of the plastics industry. From the historical point of view it is no less importance, because it has been used time and again in the laboratory in the preparation of important compounds. The first complete synthesis of the alkaloid morphine, for example, involved the use of an olefin intermediate, which was prepared by the dehydration methods. Side Reactions The side products of the dehydration reaction are virtually identical with those encountered in the preparation of n-amyl bromide, the only difference being that the olefin is no longer a side product but is now the desired product. Specifically, the side products are dicyclohexyl ether, polymer, mono and dicyclohexyl sulphate, and degradation products such as carbon, sulphur dioxide and carbon dioxide. The dehydration of cyclohexanol is carried out in such a way that the product, cyclohexene, distils from the reaction mixture as it is formed, the distillation technique serves to remove the olefin from contact with the sulphuric acid before polymerization can set in and it also serves as a first stage in the eventual purification of the olefin. The products and side products fall three categories: (a) gases, composed of sulphur dioxide and carbon dioxide and carbon dioxide, (b) distillate, composed of cyclohexene, un-reacted cyclohexanol, water and traces of sulphurous acid; and (c) residue, composed of high-boiling or non-volatile substances such as dicyclohexyl ether, mono- and dicyclohexyl sulphate, polymer and carbon. Pure cyclohexene is obtained from the crude distillate by the following procedure: Treatment with aqueous sodium carbonate solution to remove sulphurous acid; Addition of calcium chloride, to remove all of the water and part of the cyclohexanol; and Distillation to separate the remainder of the cyclohexanol. The dehydration of an alcohol with phosphoric acid instead of sulphuric acid has two distinct advantages: Very little organic material is lost through oxidation by the acid and The product is not contaminated with volatile decomposition products (e.g. sulphurous acid) Both advantages are attributable to the fact that phosphoric acid, unlike sulphuric acid is not an oxidizing agent. As a result, the yield of olefin is usually higher with phosphoric acid, the workup is simplified, and important from the point of view of the experimenter the labour required to clean the reaction flask is greatly reduced. (Sulphuric acid produces an intractable black tar which adheres tenaciously to the walls of the reaction flask.) Apparatus and Materials: Round-bottomed flask (50 mL), boiling chips, bunsen burner, take-off distillation adapter, condenser, thermometer, cyclohexanol, concentrated (85%) phosphoric acid, anhydrous magnesium sulphate. Experimental Procedure: 10.0 g of cyclohexanol and 2 mL of conc.(85%) phosphoric acid were placed in a 50 mL ST round bottomed flask and the two were mixed by swirling. Several carborundum porcelain or anthracite boiling chips (do not use marble chips) were added, the flask was clamped to a ring stand at Bunsen burner height, and a take-off distillation adapter was attached, a thermometer, a condenser, and a small receiving flask. The reaction mixture was heated so that it boils gently and distillate boiling in the range 85-90 ℃ was obtained. When the distillate was exhausted, the heat was increasing gradually. The same receiver was using; the distillate boiling was collected in the range of 90100℃. The two liquid layers were tested in the receiving flask to see which the aqueous layer was. With the aid of a 9-in disposable pipette, the aqueous layer was drawn off and discarded the aqueous layer. The organic layer remaining in the receiving flask was dried by adding to it 0.1-0.2g of anhydrous magnesium sulphate. The resulting mixture was swirled for a minute or two, and then the drying agent was removed by filtering a mixture through a cotton wool plug wedged into the constricted part of a small funnel. The filtrate was collected in a 50-mL ST round-bottom flask or a small distilling flask. A boiling chip was added to the dried product and it was distilled through a take-off distillation adapter packed with a few small wads of coarse steel wool. The product boiling in the range 3 below to 2 above the boiling point of cyclohexene(83℃) was collected in a tarred bottle. Results and Calculations Weight of round-bottomed flask + beaker 86.15g Weight of round-bottomed flask + beaker + cyclohexene 96.05g
Weight of cyclohexanol started with 9.90 g Weight of conical flask 43.93g Weight of conical flask + cyclohexene 46.38 g Weight of cyclohexene obtained 2.55 g Percent yield: 31.41% From the reaction, 1 mol of cyclohexanol produce 1 mol of cyclohexene. Molecular mass of cyclohexanol is 100 g mol-1. Mole of cyclohexanol = 9.90 g / 100 g mol-1 = 0.099mol Thus, 0.099 mol of cyclohexene was produced. Molecular mass of cyclohexene is 82 g mol-1. Mass of cyclohexene = 0.099 mol X 82 g mol-1 = 8.118g (Theoretical mass) Experimental mass = 2.55g Percentage of yield = experimental yield x 100 % Theoretical yield Percentage of yield = 2.55g x 100 % 8.118g Percentage of yield = 31.41% Question: Dehydration of cyclohexanol gives cyclohexene. Draw mechanism for the reaction. What alkene will be produced when each of the following alcohols is dehydrated? a) t-butyl alcohol CH3 CH3 CH3 – C –OH CH3 – C ═ CH2 + H2O CH3 2-methyl-1-propene b) 3-methylcyclohexanol 80% = 4-methylcyclohexene and 3-methylcyclohexene 20% = 1-methylcyclohexene The dehydration of 3,3-dimethyl-2-butanol yields three different products. Write equations to show how carbonation rearrangements explain two of the products. Elimination step 1 (Secondary carbocation): Product yield is (CH3)3CCH=CH2 (3,3 Dimethyl-1butene). It is a normal elimination product and the least from the amount. Rearrangement of carbocation: Elimination step 2 (Tertiary carbocation): Product yield is 2,3-dimethyl-2-butene. It is the major product. Elimination step 3 (Tertiary carbocation): Product yield is 2,3-dimethyl-1-butene. It is the minor product. Discussion: Elimination reactions involve the loss of a small molecule (H-X) from adjacent carbon atoms, resulting in pi-bond formation. Consequently, elimination reactions are good synthetic methods for producing alkenes or alkynes. These reactions occur through a process called heterolytic bond cleavage. Heterolytic bond cleavage occurs when one atom leaves a compound with both electrons of the original bond, resulting in the formation of ions. For example, elimination of H-X from an organic molecule involves the loss of a proton (H+) and a leaving group (X-). The leaving group departs with both electrons from the original C-X bond. The electrons in the adjacent C-H bond form the new pi bond of the alkene, with the loss of the proton. The elimination of water (H-OH) from alcohols in this experiment is called a dehydration reaction. In many cases, alcohol dehydration is an acid-catalyzed reaction that proceeds by an elimination mechanism called E1. The key intermediate in the mechanism is a cyclohexyl cation, which can undergo substitution as well as elimination. To prepare an alkene in good yield, it is necessary to suppress the substitution reaction. In this experiment, the substitution reaction is suppressed by: (1) the use of strong acids with anions that are relatively poor nucleophiles; (2) a high reaction temperature, which favors elimination. The anion of phosphoric acids in this experiment is a poor nucleophile, and thus substitution reactions are not favored. The first step of dehydration is a proton transfer from the acid catalyst to the oxygen atom of the alcohol. This protonation forms a oxonium ion, the conjugate acid of the alcohol. Weak base are good leaving groups, so changing the leaving group from hydroxide to water favours the reaction. The second step of the dehydration reaction is loss of water from the oxonium ion forming a positively charged secondary carbocation. This step of the mechanism is rate determining. The ease of alcohol dehydration follows the trend 3° > 2° > 1°. The third and final step, a molecule of water deprotonates the carbocation at either of the adjacent carbons. The remaining electrons flow towards the positive charge producing a –bond between the carbons and forming a double bond. From the experiment, only 2.55g of cyclohexene was produced, which is 31.41 % from the theoretical mass. This is due to a significant amount of product left and lost during distillation. Since the connection of the distillation set has been closed fitly, thus it can be sure that some products were left in the flask and in the column. Hence, for recovery of otherwise lost reaction product, a “chaser” solvent e.g. toluene, should be added after the distillation and carry on distillation for second time. Once the toluene distils up the column and reaches the thermometer, most of the cyclohexene and water has been pushed over into the collection vial and maximum yield is ready to be collected. The anhydrous MgSO4 was added due to it is an inorganic drying agent that binds strongly with water and thus removes any traces of water from the solution. Besides that, our group put wrong magnesium sulphate heptahydate to remove the water, this affect the yield that we got. Precaution steps: Phosphoric acids are strong, corrosive acids. If any acid is
splashed on your skin or clothing, wash immediately with copious amounts of water. Cyclohexene and toluene are not particularly dangerous but are highly flammable. Both are quite painful if splashed in the eyes and must be removed by extensive eye washing. Remaining cyclohexene should be disposed of in the fume-hood sink because cyclohexene vapors are heavier than air, they will accumulate in the sink. Conclusion: 2.55 g of cyclohexene was produced, which is 31.41% from the theoretical mass. The loss of water from a cyclohexanol to give a cyclohexene does not occur in just one step; a series of steps are involved in the mechanism of dehydration of alcohols. References: Reference books: T.W.G. Solomons and C. Fryhle, Organic Chemistry, Chapter 7.7, Dehydration of Alcohols. K. L. Williamson, Macroscale and Microscale Organic Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston d. p268 McMurry, J. (2008). Organic Chemistry 7th ed. Brooks/Cole: Thomson Learning. P619-621
Synthesis of Cyclohexene from Cyclohexanol by Elimination
Goal This experiment is designed to demonstrate a simple method for forming an alkene from a secondary alcohol by means of acid-catalyzed dehydration. Specifically, cyclohexanol is heated in the presence of concentrated phosphoric acid to cause an E1 elimination reaction.
Background Elimination reactions. In this experiment, cyclohexanol will be dehydrated (loss of H2O) to form cyclohexene under acidic conditions (see Figure 1).
Figure 1. The overall reaction. The substrate or starting material is cyclohexanol (ROH). Phosphoric acid is present as a catalyst which promotes the
reaction but is not consumed in it. The hydroxyl group in R-OH is a poor-leaving group because it would have to leave as a hydroxide ion (HO-). Therefore, an acid is used to protonate the alcohol (step 1) and form R-OH2+ (see Figure 2). Thus, water (a much better leaving group) is the leaving group in this reaction (step 2) and the product is a secondary carbocation. In the following step (step 3), a molecule of water deprotonates the carbocation at either of the adjacent carbons. The remaining electrons flow towards the positive charge producing a –bond between the carbons and forming a double bond.
Figure 2. The reaction mechanism for this experiment.
Procedure Measure out the cyclohexanol (30 mL) and determine its mass. Place it into a round bottomed flask no less than 100mL volume. Add 8 mL of concentrated H3PO4 and a boiling chip. Cork the flask and gently swirl the mixture. Use a collection flask or beaker that is no less than 50 mL in volume. Gently heat the
mixture to no more than 103 oC degrees and continue heating. Distill the mixture until about 5mL of liquid remains in the round bottom flask. The distillate will be a mixture of water and cyclohexene. To this mixture add NaCl until saturation (no more will dissolve). Next add enough of a 10% solution of NaHCO3 to make the solution basic to pH paper test. Transfer the mixture to aseparatory funnel and remove the aqueous phase. Dry the organic phase using anhydrous sodium sulfate. Transfer the contents of the organic phase to a dry round bottom flask. You may need to add a toluene chaser if your volume is small. Next, redistill bysimple distillation. Be sure that the temperature never exceeds 85 degrees. Record the boiling point, and the mass and volume of the pure product. From this you will determine the density of the distillate. how do the physical properties of the distillate compare to the accepted values for cyclohexene? Transfer approximately 2ml of your product to a dry test tube. Into a second test tube transfer approximately 2ml of cyclohexanol. From the dropping bottle provided, transfer 5 drops of 5% Bromine in 1,2dichlorocyclohexane. Note the color of each solution after addition of the bromine solution. A positive result for the formation of cyclohexene is indicated by a clear colorless solution upon addition of bromine to your product.